It's been almost 2 weeks since I put up the multi-band inverted vee, so now is a good time to summarize my impressions of its performance. To briefly recap, this is 4-band inverted vee with parallel wires for 30, 20, 17 and 15 meters, and that also resonates on 10 and 6 meters.
The apex is up 14 meters. The heights of the ends depends on the end and the antenna length. The tie-down points are up 6 meters (north end) and 8 meters (south-southwest end). The angles from horizontal are about 40° and 23°, respectively.
The comparison antenna is a tri-band dipole (my so-called TH1vn) up 9 meters during this test. It is oriented to favour NE and SW directions, roughly perpendicular to the inverted vee. With both antennas there are no azimuth gaps in coverage, and in many directions they have similar azimuth gain. The only significant difference is their respective heights.
All contacts were made with 10 watts from my KX3 transceiver. QRP is a good way to show up antenna under-performance. Adding +10 (or +20) db to transmitter power will cover up many antenna problems!
I'll get 6 and 10 out of the way first. I have listened on both bands but did not make any contacts. Nothing heard on 6 meters recently except for ground-wave beacons. On 10 meters there have been openings but these have been in the morning when I had little to no opportunity to operate. Most DX is at very low elevation angles on 10 meters, which should favour the inverted vee (in directions where their azimuth gains are about the same). What I found is that there was no predictable pattern for azimuth and path length: sometimes one one better, yet the other might be better on the same path the next day.
I have some ideas about why this might be happening, pertaining to the likely presence of multiple azimuth lobes in the pattern. Until I model the antenna with EZNEC this will have to remain a guess. That's my best approach since neither antenna is rotatable.
On 15 meters the inverted vee definitely shows an advantage on long paths, such as to KH, ZL and 3D2. The vee's pattern has a notch toward the south and north, and that is noticable in antenna comparisons. Therefore the dipole favours Central America and East/Southeast Asia.
There is no comparison antenna for 17 meters. Even so it works well and has garnered a number of contacts in Europe, Asia and Central and South America. I could heard the South Pacific but not well. The same is true of the Middle East and Africa. This is partly due to conditions.
Apart from azimuth pattern differences, on 20 meters there wasn't too much difference between the antenna. The greater height of the inverted vee made little discernible difference.
I was particularly interested in how it would perform on 30 meters since this is the first proper antenna I have had for this band. As with 17 meters, there is no comparison antenna for 30 meters.
I was reasonably pleased with how it did on 30 meters. The best DX was the omnipresent 3B8CF on Mauritius. Europe, the Caribbean and South America were also easy shots, despite the pattern null toward the south. Since my operating was confined to the evenings I did not test its performance toward the Pacific and Asia. Those are in any case hard nuts to crack on 30 with only 10 watts.
Some stations on 30, strong though they were, were not workable. Either they are alligators -- all mouth and no ears -- or the QRN and QRM at their end defeated my QRP signal.
Overall impressions
This multi-band inverted vee up 14 meters works just as if it were...a multi-band inverted vee up 14 meters. In other words, it works but it's still just an inverted vee. There will be no miracles.
Because the Site-B mast on which it is supported is house-bracketed it is noisier than the dipole on the Site-C tower. This is likely due to all the computers and other electronic appliances in my house and the adjacent house. When the antennas are close in performance I choose the dipole. Atmospheric noise covers up the digital noise on bands below 20 meters, so this is not a factor on 30 meters.
There were many stations on the higher bands -- 17, 15 and 10 meters -- that were barely copyable but not workable. Yet others had no difficulty. These were longer paths, including JY, 9M6, 3D2, HL, DU, KH0 among others. There is no magic sauce that can be added to an inverted vee and QRP that will convert these into QSOs. Even with no one else calling they didn't hear me or copied only a letter or two of my call.
I will have to accept these limitations due to my new status as a little gun or I must contemplate a bigger station. For the present and well into 2014 I will remain a little gun. Afterwards...we'll see.
What this means
I plan to keep this antenna for the winter season. This requires that I do a few things to weather harden it so that it survives the winter, everything from sealing joints to spreader stabilization.
The antenna fills pattern gaps in the dipole, an important consideration when one does not have rotatable antennas. It will also allow me to position the dipole perpendicular to the yet-to-be-installed 40 meters delta loop. I want them perpendicular to avoid potential interactions even though my modelling experiments indicate that interactions are low when they're parallel.
I did not bother adding 40 (or 80) meters to the multi-band inverted vee since my experience is that DX performance will be poor at this modest height. There remains some uncertainty regarding 30 meters, so I am contemplating putting a delta loop on the tower, nested within the 40 meters delta loop. This is added work and mechanical complexity that I'd like to avoid, plus my preliminary models show heavy interaction between the 30 meters loop and the tower. This does not occur to an appreciable degree on 40 meters.
I will start by loading the 40 delta loop on 30 with a tuner to determine whether there is something to be gained with this additional antenna work. Otherwise the inverted vee will be my only 30 meters antenna for the next while.
Further antenna work may be delayed until Thanksgiving (second weekend of October) due to my schedule. That's when I plan to try and turn my tri-band dipole into a quad-band dipole (addition of 17 meters), modify the antenna mount and then build the extension mast to support the 40 meters delta loop.
The weather is great for the present. It won't last much longer so I have to get cracking. Winter is coming.
Sunday, September 29, 2013
Wednesday, September 25, 2013
Raindrops Keep Falling On My Antenna
This past weekend, shortly after erecting and tuning the new multi-band inverted vee, it rained. A lot. Rain affects antennas, sometimes in surprising ways. I'll discuss that and a few more details about the construction of this antenna.
Most antennas don't show much change when it rains. The effects are small enough to escape notice or, if noticed, are not of much concern. Careless design can amplify the effects of rain, resulting in performance problems. The latter is true of my inverted vee.
To understand what can occur we need to talk about velocity factor (VF). That is, the velocity of electromagnetic (EM) radiation is less than c (the speed of light) in any medium other than a vacuum. This matters since the propagation of EM along a conductor is partly in the field surrounding the conductor rather than fully within the conductor.
The VF of air is close enough to 1 that in ham-related applications its effect can be ignored. In coaxial transmission lines the dielectric material filling the space between conductors determines the VF. The fields associated with RF current in both inner and outer conductors is within that material. For example, the most common material, solid polyethylene, causes a VF of about 0.66. That must be accounted for when constructing coaxial transformers.
The same effect occurs with typical commercial ladder line since the wires are encased in plastic and are held apart with plastic spacers. The VF is higher than in coax (in the range of 0.95 to 0.98) since most of the fields travel in air, not plastic.
Antenna wires work the same way. That is why antennas made of insulated wire must be made shorter by ~1% to 3% than indicated by standard length formulas. EZNEC makes this easy to include in models and I always do so for wire antennas I plan to construct.
When it rains there will be some water clinging to the antenna. The EM fields must travel through this water. Therefore when it rains the antenna becomes electrically longer. This is true whether the antenna is made of bare metal or insulated. In most cases the effect is small and can be ignored. Snow has a lesser effect but stays longer.
Freezing rain can have effects on antenna resonance of several percent. However, we are usually more concerned with the antenna staying up and in one piece in this last instance. That tends to distract our attention from high SWR and other performance impacts. If you live in a climate similar to Ottawa where freezing rain is common you'll likely already know what I'm talking about.
In my case the rain caused none of the above effects to any significant degree. It did however cause a major resonance impact on every band. While I did not take the trouble to measure and test, I know what the rain did, and indeed I knew before I raised the antenna. It has to do with wire termination.
Any good antenna book recommends high-quality terminations for wire antennas. A ceramic insulator is best. The ends of antennas are high-impedance (low current, high voltage) points, and are prone to coupling with other conductors in the vicinity. The problem is worse with wires than tubes. With high power, a pointy wire and low humidity you can even get corona discharges to the air itself.
I did not use high-quality terminations for the parallel inverted vee antenna. I would need 8 of them which I did not have and, importantly, would add undesirable weight (and sag). Instead I created end loops out of the wires themselves by tying the bare end back on to the wire. These loops partly compensate for the wire's narrow diameter (lower the terminal impedance). However the problem lay elsewhere: in the nylon ropes.
Ropes contain a lot of air space between the strands. When it rains water fills those spaces. Synthetics such as the nylon rope I used shed water more readily than natural fibres. But that only comes into play after the rain stops, with the nylon drying faster than natural fibre rope. While fresh water is not a great conductor, electrolytes are usually present from collected debris. The high voltage at the ends of the wires amplifies the coupling.
The result in my case is that in the rain the antenna's resonant frequency drops quite a lot on all bands. It averages about 200 kHz. Since my antennas are tuned to favour CW band segments this raised the SWR on much of the phone segments, sometimes well above 3. Since I primarily operate CW this didn't slow me down, at least not after the initial surprise. Some loss due to coupling to the wet rope is also to be expected.
The pattern of a dipole or inverted vee is not appreciably affected by rain, just the resonance and impedance. If you are using a wire array such as a yagi the pattern will be highly distorted. Not only is the driven element affected, so are the parasitic elements. Drop the resonant frequency of a director or reflector by 200 kHz and your front-to-back may entirely vanish. Gain will also drop, though by a smaller factor.
As an example of this effect I've included here the azimuth patterns of a 3-element NBS yagi for 20 meters in free space. One at resonance on 14.150 MHz and one when resonance is lowered by 200 kHz. The latter is simulated by setting the test frequency to 14.350 MHz. While not shown here, the SWR would also rise sharply since the radiation resistance is 2x higher and there is substantial reactance.
As a general rule the tolerances of multi-element arrays are always tighter than in any single element antenna. For this reason I always used ceramic insulators on the wire yagis I built in the past.
The second and last point about the multi-band inverted vee I want to mention is the arrangement of the wires and spreaders. Recall that there are 4 inverted vees in the shape of a virtual cylinder, cut for 30, 20, 17 and 15 meters.
The outermost spreader is placed just inside the termination of the 20 meters antenna. That spreader (and its mate on the other side of the antenna) support wires for 20 and 30 meters, and the ropes coming from the ends of the 17 and 15 meters antennas. To assure the stability of this spreader it is important that the wires for the two lowest bands be positioned opposite each other. Since the nylon ropes run nearly frictionless through the spreader notches (necessary for tension adjustment) the spreader would tip over if those wires were adjacent rather than opposite. The same reason dictates positioning the spreader inside the termination loop of the 20 meters antenna.
Most antennas don't show much change when it rains. The effects are small enough to escape notice or, if noticed, are not of much concern. Careless design can amplify the effects of rain, resulting in performance problems. The latter is true of my inverted vee.
To understand what can occur we need to talk about velocity factor (VF). That is, the velocity of electromagnetic (EM) radiation is less than c (the speed of light) in any medium other than a vacuum. This matters since the propagation of EM along a conductor is partly in the field surrounding the conductor rather than fully within the conductor.
The VF of air is close enough to 1 that in ham-related applications its effect can be ignored. In coaxial transmission lines the dielectric material filling the space between conductors determines the VF. The fields associated with RF current in both inner and outer conductors is within that material. For example, the most common material, solid polyethylene, causes a VF of about 0.66. That must be accounted for when constructing coaxial transformers.
The same effect occurs with typical commercial ladder line since the wires are encased in plastic and are held apart with plastic spacers. The VF is higher than in coax (in the range of 0.95 to 0.98) since most of the fields travel in air, not plastic.
Antenna wires work the same way. That is why antennas made of insulated wire must be made shorter by ~1% to 3% than indicated by standard length formulas. EZNEC makes this easy to include in models and I always do so for wire antennas I plan to construct.
When it rains there will be some water clinging to the antenna. The EM fields must travel through this water. Therefore when it rains the antenna becomes electrically longer. This is true whether the antenna is made of bare metal or insulated. In most cases the effect is small and can be ignored. Snow has a lesser effect but stays longer.
Freezing rain can have effects on antenna resonance of several percent. However, we are usually more concerned with the antenna staying up and in one piece in this last instance. That tends to distract our attention from high SWR and other performance impacts. If you live in a climate similar to Ottawa where freezing rain is common you'll likely already know what I'm talking about.
In my case the rain caused none of the above effects to any significant degree. It did however cause a major resonance impact on every band. While I did not take the trouble to measure and test, I know what the rain did, and indeed I knew before I raised the antenna. It has to do with wire termination.
Any good antenna book recommends high-quality terminations for wire antennas. A ceramic insulator is best. The ends of antennas are high-impedance (low current, high voltage) points, and are prone to coupling with other conductors in the vicinity. The problem is worse with wires than tubes. With high power, a pointy wire and low humidity you can even get corona discharges to the air itself.
I did not use high-quality terminations for the parallel inverted vee antenna. I would need 8 of them which I did not have and, importantly, would add undesirable weight (and sag). Instead I created end loops out of the wires themselves by tying the bare end back on to the wire. These loops partly compensate for the wire's narrow diameter (lower the terminal impedance). However the problem lay elsewhere: in the nylon ropes.
Ropes contain a lot of air space between the strands. When it rains water fills those spaces. Synthetics such as the nylon rope I used shed water more readily than natural fibres. But that only comes into play after the rain stops, with the nylon drying faster than natural fibre rope. While fresh water is not a great conductor, electrolytes are usually present from collected debris. The high voltage at the ends of the wires amplifies the coupling.
The result in my case is that in the rain the antenna's resonant frequency drops quite a lot on all bands. It averages about 200 kHz. Since my antennas are tuned to favour CW band segments this raised the SWR on much of the phone segments, sometimes well above 3. Since I primarily operate CW this didn't slow me down, at least not after the initial surprise. Some loss due to coupling to the wet rope is also to be expected.
The pattern of a dipole or inverted vee is not appreciably affected by rain, just the resonance and impedance. If you are using a wire array such as a yagi the pattern will be highly distorted. Not only is the driven element affected, so are the parasitic elements. Drop the resonant frequency of a director or reflector by 200 kHz and your front-to-back may entirely vanish. Gain will also drop, though by a smaller factor.
As an example of this effect I've included here the azimuth patterns of a 3-element NBS yagi for 20 meters in free space. One at resonance on 14.150 MHz and one when resonance is lowered by 200 kHz. The latter is simulated by setting the test frequency to 14.350 MHz. While not shown here, the SWR would also rise sharply since the radiation resistance is 2x higher and there is substantial reactance.As a general rule the tolerances of multi-element arrays are always tighter than in any single element antenna. For this reason I always used ceramic insulators on the wire yagis I built in the past.
The second and last point about the multi-band inverted vee I want to mention is the arrangement of the wires and spreaders. Recall that there are 4 inverted vees in the shape of a virtual cylinder, cut for 30, 20, 17 and 15 meters.
The outermost spreader is placed just inside the termination of the 20 meters antenna. That spreader (and its mate on the other side of the antenna) support wires for 20 and 30 meters, and the ropes coming from the ends of the 17 and 15 meters antennas. To assure the stability of this spreader it is important that the wires for the two lowest bands be positioned opposite each other. Since the nylon ropes run nearly frictionless through the spreader notches (necessary for tension adjustment) the spreader would tip over if those wires were adjacent rather than opposite. The same reason dictates positioning the spreader inside the termination loop of the 20 meters antenna.
Friday, September 20, 2013
Multi-band Inverted Vee
Following on from my previous article I can now report that I have built, erected and tuned the planned multi-band inverted vee antenna on my house-bracketed support mast (Site B). There are parallel inverted vee antenna for 30, 20, 17 and 15 meters. To cut to the chase, yes, it works. In fact it works better than I expected.
The picture shows the antenna as viewed from the backyard, facing approximately northeast. As with the 1.5-band inverted vee experiment the wires are anchored to the north side of the house roof and to the top of the Site-C tower.
A close-framed picture like this is needed to provide a sense of how it's constructed. Now that you've got the finished product in front of you I will step back a few paces and describe how I got here.
In the previous article I compared models of multi-band dipoles (also inverted vee dipoles) using two configurations: radial and parallel wires. The predictability of tuning and performance was significantly better with parallel wires.
If I had only wanted to cover two bands I'd have been done. Because I wanted more bands there are mechanical and electrical challenges with scaling that simple model. Instead of parallel wires I chose a cylindrical model, with antenna legs on the perimeter of a virtual cylinder. There is nothing innovative about this design, which you may have already encountered elsewhere.
Rather than linear spreaders (wire spacers) I needed spreaders that centred the wires equidistant from a common (cylinder) axis. Since my design is for 4 bands I went with an "X" structure for the spreaders. This required fabrication in my workshop. It had to be simple and fast since my time is limited.
There are ample choices of plastic that are suitable for this application: good dielectric properties, strong, easy to work with and resistant to the weather and ultraviolet. For example, Lexan. Wood is not a good choice since it will deteriorate in the weather and can be heavy enough to weigh down the antenna, causing wire sag. For some reason almost all the plastic-supply outlets seem to be on the opposite side of town. To avoid a long drive I decided to first see what I could scrounge nearby.
First I surveyed the inside of the house for derelict plastic that would suit the bill. I struck out so I went browsing through a building materials store. I was going there anyway so this was not a special trip.
Hams are not the only tinkerers. You can almost always find a few middle-aged men casually strolling the aisles of these and similar stores, touching, weighing and even shaking all manner of raw materials. We all have a final picture in mind to which we are trying to fit the available materials. That is, we don't know what we're looking for, but when we find it we will know that it's exactly what we need.
That should explain the following picture. Hopefully you'll find the lettering to be legible.
This is printed on a 10-foot length of ½" PVC water pipe. It's rigid (strong), nonmetallic (dielectric) and sunlight resistant (UV protection). It only cost $3.39, which is guaranteed to be considerably cheaper than almost every alternative. All it needs is a little bit of labour to transform it into antenna gold.
My tools were a tape measure, felt-tipped marker, Workmate, drill, hacksaw and half-round bastard file.
I made the spreader so that no wire was closer than 10 cm to any other. This is not a strict criterion, just enough to ensure against over-coupling caused detuning, even when wind and wire sag lessen the separation.With 4 wires this requires spreader arm lengths of at least 15 cm. I made them 18 cm (7") since the wires penetrate into the arm about 1 cm on each end.
This was not a precision operation! I put the pipe lengthwise into the Workmate and made two cuts along the length of the pipe, dividing it into 4 quarter-round sections. Doing it this way results in wobbly lines and varying widths along each arm. The PVC is thick and strong enough to withstand a few weak points. I could have made them uniform and pretty, if I had taken twice as much time. Friction-fit notches at each end of every arm were made with a hacksaw and then reaming the cut with a drill bit slightly narrower than the wire width.
I used the file to clean the edges, and then to carve a mid-point recess in half the arms. Since the arms have the curvature of the pipe this allows one arm to comfortably nest within its partner. These were then drilled in the centre and screwed together. Since they could still move when forced -- the plastic is very... plastic -- I glued them with cyanoacrylate (super glue). Now they hold their shape.
I made 6 spreaders. There are 3 for each leg: one adjacent to the feed point (see picture below), one just inside the end of the 20 meters leg, and the third one positioned midway between them.
I next proceeded to cut the antenna wires. The 30 meters antenna was the easiest. Since the model showed that the higher-frequency wires would not affect its tuning I simply modeled it by height and interior angle and then cut it about 10 cm longer (5 cm on each side). I made sure to model the insulation covering the wire since it has a significant impact on the wire length. You should typically expect that the wire must be cut about 2% shorter than bare copper. I used #12 stranded wire for 30 meters since it was not only the longest of the four wires but would also be used to set the tension for the other three.
I reused the 20 meters antenna from my 1.5-vee experiment. This was easiest since it was already soldered to the center insulator. However, I knew that it would likely need to be lengthened since, unlike the previous antenna, 20 meters would not be the lowest band. Interaction with the 30 meters antenna would require lengthening the 20 meters antenna.
The single 17 meters leg was similarly reused. The second 17 meters leg and the 15 meters antennas were newly cut. The 20, 17 and 15 meters antennas were made from #14 insulated stranded copper wire.
I modelled each antenna as I did for 30 meters. I then mentally added additional length to each, and added several more centimeters for the end loop and wrapping of the wire on the centre insulator wire loop. I hoped this would give me enough wire to allow me to cut and trim each antenna to length. Doing a full model of this antenna would have taken time I do not have right now. I might do so later, long after the fact.
The end of each 30 meters leg is wrapped onto a large galvanized flat washer. Nylon ropes tie the wire-end loops of the other antennas to the same flat washer. Like the wires themselves, the end ropes for the shorter 15 and 17 meters wires attach to the remaining spreaders on the way to this 'terminal' washer.
After soldering the wires to the center insulator the spreaders are placed at their intended positions, with the wires pressed into the spreader notches. The centre insulator is then anchored to some handy fixture in the yard (I used the tower). The tie-down nylon ropes are then pulled to draw tension on the antenna half and tied so that the entire assembly is suspended in midair. The tension for 20, 17 and 15 meters legs is then adjusted so that the wires do not appreciably sag.
Thin nylon rope (I used ⅛") is very stretchy which allows some latitude in setting wire tensions. Just be sure there is no significant slack that wind and gravity can affect. After setting the tension adjust the spreaders so that they are orthogonal to the wires. One or several spreader arms may have moved out of position during this procedure due to the friction fit.
Repeat the above procedure for the other half of the antenna. If, unlike me, you properly machined the spreader notches the wires and rope will not spontaneously self-eject from the spreaders.
This contraption is not too fragile, which is good since it will inevitably need to be dragged along the ground and then bounce against, and tangle with, who know what as it's hoisted by pulley to the top of the mast. There is also the problem of twisting near the terminal washers since the mess of nylon ropes and ends of the 30 meters antenna will twist around each other. This is not really a problem even if it does look ugly. The spreaders keep this from happening to the main body of the antenna.
You can see this happening in the picture below. The flat washer is hidden within that group of knots. It isn't as bad as it looks!
When I was done the antenna worked but was not without faults.
The 20, 17 and 15 meters were still too short, even though I anticipated the problem and compensated for that on 17 and 15. The undershoot was 1.3% on 20 (13 cm), 2.5% on 17 (22 cm) and 1.5% on 15 (10 cm). Wire needed to be added, which I did by tying and soldering additional wire. I did this all one one side of the antenna. There is no need to make the antenna halves exactly equal, but do make sure you are using a coax choke or current balun at the feed point. I worked on the roof (north) side since it was easier to access than the end going to the tower.
When trying out the antenna after lengthening the wires I noticed that on 17 meters the SWR continuously swung back and forth over the range 1.2 and 2.0. It turned out that one leg of that antenna had insufficient tension, causing one section of it to swing close to the 20 meters wire in the breeze. That was adjusted during the next iteration. Unfortunately this was on the tower end so I had to climb the tower, twice, to remove and return the tie-down rope.
The Site-B mast is very flexible. It was challenging to judge the tension from each end of the antenna, particularly on the tower end, so that the mast was not pulled too much to one side. As you can see in the picture the antenna halves are not straight. To do so would require too much tension. It takes a surprising amount of tension to straighten even a light structure such as this antenna. It is about more than aesthetics since the interior angle is more acute due to sag. The performance impact is small, but there is an impact.
After this lengthy description you might not realize how quickly this antenna came together. On day 1 I bought the pipe and then spent no more than 2 hours that evening fabricating the spreaders. On day 2 the wires were cut, the antenna assembled and raised into the air. The on-air performance and SWR were checked that evening, and calculations were done to lengthen the antennas. The third and final day the antennas and rope tension were adjusted, and the antenna was raised into position.
At this point everything worked as planned. I worked stations on every band (to confirm that it did indeed transmit well) and the SWR was below 1.5 on the CW and low phone segments of all four bands. Out of curiosity I briefly tried it on 12, 10 and 6 meters. On 12 it received well but the SWR was very high. On 10 and 6 meters it not only heard well, the SWR was quite low. On 6 it is 1.4 between 50 and 50.2 MHz, and on 10 it is 1.6 at 28 MHz and dips to 1.1 at 28.4 MHz. This goes to prove that if you get enough wire in the air can be unexpected resonances showing up here and there.
I have yet to attempt any QSOs on 10 and 6. It will be interesting to see how it plays on these bonus bands. DX performance is expected to be erratic since dipoles longer than 1 wavelength have multiple smaller lobes, and nulls between those lobes. This is true for both azimuth and elevation, and their number increases as the antenna gets longer.
Once I have used the antenna long enough to assess its DX performance I'll report back. Perhaps within the next week. It should, and so far seems to, show the far field characteristics of an inverted vee on the 4 primary bands.
The apex is up 14 meters, the north end is 6 meters high and the south end is 8 meters high. Its orientation favours south Europe, Africa and Oceania. The current orientation of the TH1vn tri-band dipole fills the gaps in the inverted vee's azimuth pattern. It is currently up 9 meters, having been lowered from 10.5 meters so that I can work on it.
The picture shows the antenna as viewed from the backyard, facing approximately northeast. As with the 1.5-band inverted vee experiment the wires are anchored to the north side of the house roof and to the top of the Site-C tower.
A close-framed picture like this is needed to provide a sense of how it's constructed. Now that you've got the finished product in front of you I will step back a few paces and describe how I got here.
In the previous article I compared models of multi-band dipoles (also inverted vee dipoles) using two configurations: radial and parallel wires. The predictability of tuning and performance was significantly better with parallel wires.
If I had only wanted to cover two bands I'd have been done. Because I wanted more bands there are mechanical and electrical challenges with scaling that simple model. Instead of parallel wires I chose a cylindrical model, with antenna legs on the perimeter of a virtual cylinder. There is nothing innovative about this design, which you may have already encountered elsewhere.
Rather than linear spreaders (wire spacers) I needed spreaders that centred the wires equidistant from a common (cylinder) axis. Since my design is for 4 bands I went with an "X" structure for the spreaders. This required fabrication in my workshop. It had to be simple and fast since my time is limited.
There are ample choices of plastic that are suitable for this application: good dielectric properties, strong, easy to work with and resistant to the weather and ultraviolet. For example, Lexan. Wood is not a good choice since it will deteriorate in the weather and can be heavy enough to weigh down the antenna, causing wire sag. For some reason almost all the plastic-supply outlets seem to be on the opposite side of town. To avoid a long drive I decided to first see what I could scrounge nearby.
First I surveyed the inside of the house for derelict plastic that would suit the bill. I struck out so I went browsing through a building materials store. I was going there anyway so this was not a special trip.
Hams are not the only tinkerers. You can almost always find a few middle-aged men casually strolling the aisles of these and similar stores, touching, weighing and even shaking all manner of raw materials. We all have a final picture in mind to which we are trying to fit the available materials. That is, we don't know what we're looking for, but when we find it we will know that it's exactly what we need.
That should explain the following picture. Hopefully you'll find the lettering to be legible.
This is printed on a 10-foot length of ½" PVC water pipe. It's rigid (strong), nonmetallic (dielectric) and sunlight resistant (UV protection). It only cost $3.39, which is guaranteed to be considerably cheaper than almost every alternative. All it needs is a little bit of labour to transform it into antenna gold.
My tools were a tape measure, felt-tipped marker, Workmate, drill, hacksaw and half-round bastard file.
I made the spreader so that no wire was closer than 10 cm to any other. This is not a strict criterion, just enough to ensure against over-coupling caused detuning, even when wind and wire sag lessen the separation.With 4 wires this requires spreader arm lengths of at least 15 cm. I made them 18 cm (7") since the wires penetrate into the arm about 1 cm on each end.
This was not a precision operation! I put the pipe lengthwise into the Workmate and made two cuts along the length of the pipe, dividing it into 4 quarter-round sections. Doing it this way results in wobbly lines and varying widths along each arm. The PVC is thick and strong enough to withstand a few weak points. I could have made them uniform and pretty, if I had taken twice as much time. Friction-fit notches at each end of every arm were made with a hacksaw and then reaming the cut with a drill bit slightly narrower than the wire width.I used the file to clean the edges, and then to carve a mid-point recess in half the arms. Since the arms have the curvature of the pipe this allows one arm to comfortably nest within its partner. These were then drilled in the centre and screwed together. Since they could still move when forced -- the plastic is very... plastic -- I glued them with cyanoacrylate (super glue). Now they hold their shape.
I made 6 spreaders. There are 3 for each leg: one adjacent to the feed point (see picture below), one just inside the end of the 20 meters leg, and the third one positioned midway between them.
I next proceeded to cut the antenna wires. The 30 meters antenna was the easiest. Since the model showed that the higher-frequency wires would not affect its tuning I simply modeled it by height and interior angle and then cut it about 10 cm longer (5 cm on each side). I made sure to model the insulation covering the wire since it has a significant impact on the wire length. You should typically expect that the wire must be cut about 2% shorter than bare copper. I used #12 stranded wire for 30 meters since it was not only the longest of the four wires but would also be used to set the tension for the other three.
I reused the 20 meters antenna from my 1.5-vee experiment. This was easiest since it was already soldered to the center insulator. However, I knew that it would likely need to be lengthened since, unlike the previous antenna, 20 meters would not be the lowest band. Interaction with the 30 meters antenna would require lengthening the 20 meters antenna.
The single 17 meters leg was similarly reused. The second 17 meters leg and the 15 meters antennas were newly cut. The 20, 17 and 15 meters antennas were made from #14 insulated stranded copper wire.
For modelling geeks only: I did not directly measure the insulation thickness. I looked up the diameters of #14 and #12 stranded copper wire, measured the width of the insulated wire, and subtracted the first number from the second, then divided by 2. The results are approximately 0.4 and 0.5 mm, respectively.
 |
| Close up of the fully-constructed feed point, including the first set of wire spreaders |
The end of each 30 meters leg is wrapped onto a large galvanized flat washer. Nylon ropes tie the wire-end loops of the other antennas to the same flat washer. Like the wires themselves, the end ropes for the shorter 15 and 17 meters wires attach to the remaining spreaders on the way to this 'terminal' washer.
After soldering the wires to the center insulator the spreaders are placed at their intended positions, with the wires pressed into the spreader notches. The centre insulator is then anchored to some handy fixture in the yard (I used the tower). The tie-down nylon ropes are then pulled to draw tension on the antenna half and tied so that the entire assembly is suspended in midair. The tension for 20, 17 and 15 meters legs is then adjusted so that the wires do not appreciably sag.
Thin nylon rope (I used ⅛") is very stretchy which allows some latitude in setting wire tensions. Just be sure there is no significant slack that wind and gravity can affect. After setting the tension adjust the spreaders so that they are orthogonal to the wires. One or several spreader arms may have moved out of position during this procedure due to the friction fit.
Repeat the above procedure for the other half of the antenna. If, unlike me, you properly machined the spreader notches the wires and rope will not spontaneously self-eject from the spreaders.
This contraption is not too fragile, which is good since it will inevitably need to be dragged along the ground and then bounce against, and tangle with, who know what as it's hoisted by pulley to the top of the mast. There is also the problem of twisting near the terminal washers since the mess of nylon ropes and ends of the 30 meters antenna will twist around each other. This is not really a problem even if it does look ugly. The spreaders keep this from happening to the main body of the antenna.
You can see this happening in the picture below. The flat washer is hidden within that group of knots. It isn't as bad as it looks!
 |
| View from the roof of the antenna on the mast, looking towards the southeast |
The 20, 17 and 15 meters were still too short, even though I anticipated the problem and compensated for that on 17 and 15. The undershoot was 1.3% on 20 (13 cm), 2.5% on 17 (22 cm) and 1.5% on 15 (10 cm). Wire needed to be added, which I did by tying and soldering additional wire. I did this all one one side of the antenna. There is no need to make the antenna halves exactly equal, but do make sure you are using a coax choke or current balun at the feed point. I worked on the roof (north) side since it was easier to access than the end going to the tower.
When trying out the antenna after lengthening the wires I noticed that on 17 meters the SWR continuously swung back and forth over the range 1.2 and 2.0. It turned out that one leg of that antenna had insufficient tension, causing one section of it to swing close to the 20 meters wire in the breeze. That was adjusted during the next iteration. Unfortunately this was on the tower end so I had to climb the tower, twice, to remove and return the tie-down rope.
The Site-B mast is very flexible. It was challenging to judge the tension from each end of the antenna, particularly on the tower end, so that the mast was not pulled too much to one side. As you can see in the picture the antenna halves are not straight. To do so would require too much tension. It takes a surprising amount of tension to straighten even a light structure such as this antenna. It is about more than aesthetics since the interior angle is more acute due to sag. The performance impact is small, but there is an impact.
After this lengthy description you might not realize how quickly this antenna came together. On day 1 I bought the pipe and then spent no more than 2 hours that evening fabricating the spreaders. On day 2 the wires were cut, the antenna assembled and raised into the air. The on-air performance and SWR were checked that evening, and calculations were done to lengthen the antennas. The third and final day the antennas and rope tension were adjusted, and the antenna was raised into position.
At this point everything worked as planned. I worked stations on every band (to confirm that it did indeed transmit well) and the SWR was below 1.5 on the CW and low phone segments of all four bands. Out of curiosity I briefly tried it on 12, 10 and 6 meters. On 12 it received well but the SWR was very high. On 10 and 6 meters it not only heard well, the SWR was quite low. On 6 it is 1.4 between 50 and 50.2 MHz, and on 10 it is 1.6 at 28 MHz and dips to 1.1 at 28.4 MHz. This goes to prove that if you get enough wire in the air can be unexpected resonances showing up here and there.
I have yet to attempt any QSOs on 10 and 6. It will be interesting to see how it plays on these bonus bands. DX performance is expected to be erratic since dipoles longer than 1 wavelength have multiple smaller lobes, and nulls between those lobes. This is true for both azimuth and elevation, and their number increases as the antenna gets longer.
Once I have used the antenna long enough to assess its DX performance I'll report back. Perhaps within the next week. It should, and so far seems to, show the far field characteristics of an inverted vee on the 4 primary bands.
The apex is up 14 meters, the north end is 6 meters high and the south end is 8 meters high. Its orientation favours south Europe, Africa and Oceania. The current orientation of the TH1vn tri-band dipole fills the gaps in the inverted vee's azimuth pattern. It is currently up 9 meters, having been lowered from 10.5 meters so that I can work on it.
Sunday, September 15, 2013
Fall Antenna Work Starts
With the contractors gone I have to catch up to 2 weeks of lost time. That is, time not spent on antenna work. Although work on the antennas wasn't possible I was still able to do computer modelling with EZNEC. I have many ideas brewing in my head, some for implementation this year and some that must wait until next year at the earliest.
In 30 minutes Sunday morning I pulled down the TH1vn tri-band dipole and the 1.5-bands inverted vee. The inverted vees are down for at least a few days as I put my new plan into play. The TH1vn is now back on the tower but at a lower height where I can easily work on the feed system over the next week or two.
First I'll talk about the TH1vn. As mentioned in the original articles I wrote about this antenna, it is the driven element of my ancient TH6DXX yagi. When I put it up a few months ago I mentioned some odd behaviour on 15 meters. This eventually was discovered to be an intermittent connection...somewhere. I suspected one of the traps for 15 meters, a guess I based on the large effect on that band but lesser effects on 10 and 20.
Doing a search on the web turned up some scary stories of the difficulties of working on these traps, especially those for the driven element. Once I had them in my hands I discovered they came apart quite easily, and that they are straightforward to service. I found no obvious faults. I torqued all the screws, rebuilt the element clamps with stainless steel bolts (the old, rusty ones fell victim to my bolt cutters), and put it back on the tower. It now works as it should.
If the dipole stays problem-free for a few days I will again bring it down for my next experiment, which is to add 17 meters to it. I'll talk more about this in the future after I've made the modification. The modelling for the added band was straight-forward. However I need to consider a lightweight, low-cost and robust mechanical design. The antenna must last for at least the winter. Next year (2014) I may subject it to more modifications for additional performance.
Once the dipole is operational on 20, 17, 15 and 10, it will be time to put up a delta loop for 40 meters. If you've followed along you'll know that I decided on the delta loop as the best of several options for decent DX performance on that band.
Now on to my experiment with the multi-band inverted vee. I chose to do some research and model designs with EZNEC before tackling "cut-and-try" again. Like most hams I used to mostly rely on cut-and-try with dipoles and fan dipoles even though there were instances of deleterious interactions. Since the elements for each band would need to run in the same vertical plane -- the case where interactions are most intense -- I wanted to avoid a lot of physical labour, and frustration.
One thing I did know is that when the elements of a fan dipole (or inverted vee) are maximally separated the interactions are small enough that cut-and-try is a reasonable approach. Some adjustment is usually needed in any case, since even with modelling software there are inevitable interactions with ground, building wiring and so forth. When in the same vertical plane, tuning can be highly sensitive to small changes in wire position.
To this end I performed a couple of modelling experiments. First up is a 30 plus 20 meters fan dipole with a common feed point. In this configuration the 4 wires move out radially from the feed point, in accord with my chosen mechanical feed point design. The model uses dipoles in free space to aoid additional variables. Those can be added to model once a suitable design is selected.
I rotated the 20 meters dipole in 10° increments from 90° (dipoles are orthogonal) to 10°. In the orthogonal start position I "trimmed" the dipole so that the resonant frequencies were 10.125 and 14.100 MHz. The table shows what happens as the 20 meters dipole is rotated.
Although the affect on the lower frequency antenna is modest, on the higher band the interaction becomes severe as the angle between wires is lowered. This helps to explain the tuning sensitivity. Worse, since the 20 meters dipole would have to be far longer than a naive calculation would give, which make the "cut" part of cut-and-try difficult to put into practice!
There is a better way to construct a fan dipole, one where much of the sensitivity is eliminated. The big hint to me was the long-known technique of using ladder line as a two-band dipole. I do not mean a ladder-line feed system, but ladder-line for the antenna elements. The feed line is coax (with a common mode choke).
You choose a length of ladder line for the lower of the two bands (dipole formula). Then cut the line on one of the leads so that it is the proper length for a dipole on the higher of the two bands. Do this for each leg of the dipole. Tie the two sides of the ladder line together at the feed point, and connect those two to the coax feed line.
The model of the feed point looks something like the adjacent diagram. This is the second model I used for my parallel-wire fan dipole. I then modeled it in EZNEC.
As a starting point I used a 5 cm separation between elements and tuned the antenna for resonance as in the preceding case. Then the separation was increased in 5 cm steps up to 30 cm. To do this I left the horizontal lengths unchanged, only increasing the length of the vertical connecting wires (wires #4 and #5 in the diagram). As before I used free space. The 30 meters dipole is on top, connected to the feed line.
This is much better. Notice how the 30 meters dipole is unaffected by the separation, including any affect from the connecting wires #4 and #5. The impact on the 20 meters dipole is incremental, roughly in proportion to the length of the connecting wires. (Recall that the source is connected to the 30 meters dipole.)
For the modelled feed configuration and range a quick calculation shows that the equivalent length of each 20 meters dipole leg is 0.4x the length of wires #4 and #5 (the separation distance). For example, at a separation of 30 cm the equivalent lengthening of each of the 20 meters legs is 12 cm. I checked this by subtracting 12 cm from each leg of the 20 meters dipole and the antenna model resonated at 14.075. However, the impedance remained at 64 Ω. This is a curious benefit since we do not want the impedance to drop too low or the real antenna, when built, might otherwise have a higher SWR.
You should expect that the length ratio will be slightly different if wires #4 and #5 are a different distance apart. In my model the value is 8 cm (3").
The reason for the lower impedance on 20 meters is that current is induced on the other dipole. This also adds some gain to the antenna, a small 0.8 dBd. The reverse is not true, so the impedance on 30 meters is near to the nominal value of 73Ω in free space and the gain is close to 0 dBd.
This is the model I plan to build for the next iteration of Site-B inverted vees.
In 30 minutes Sunday morning I pulled down the TH1vn tri-band dipole and the 1.5-bands inverted vee. The inverted vees are down for at least a few days as I put my new plan into play. The TH1vn is now back on the tower but at a lower height where I can easily work on the feed system over the next week or two.
First I'll talk about the TH1vn. As mentioned in the original articles I wrote about this antenna, it is the driven element of my ancient TH6DXX yagi. When I put it up a few months ago I mentioned some odd behaviour on 15 meters. This eventually was discovered to be an intermittent connection...somewhere. I suspected one of the traps for 15 meters, a guess I based on the large effect on that band but lesser effects on 10 and 20.
Doing a search on the web turned up some scary stories of the difficulties of working on these traps, especially those for the driven element. Once I had them in my hands I discovered they came apart quite easily, and that they are straightforward to service. I found no obvious faults. I torqued all the screws, rebuilt the element clamps with stainless steel bolts (the old, rusty ones fell victim to my bolt cutters), and put it back on the tower. It now works as it should.
If the dipole stays problem-free for a few days I will again bring it down for my next experiment, which is to add 17 meters to it. I'll talk more about this in the future after I've made the modification. The modelling for the added band was straight-forward. However I need to consider a lightweight, low-cost and robust mechanical design. The antenna must last for at least the winter. Next year (2014) I may subject it to more modifications for additional performance.
Once the dipole is operational on 20, 17, 15 and 10, it will be time to put up a delta loop for 40 meters. If you've followed along you'll know that I decided on the delta loop as the best of several options for decent DX performance on that band.
Now on to my experiment with the multi-band inverted vee. I chose to do some research and model designs with EZNEC before tackling "cut-and-try" again. Like most hams I used to mostly rely on cut-and-try with dipoles and fan dipoles even though there were instances of deleterious interactions. Since the elements for each band would need to run in the same vertical plane -- the case where interactions are most intense -- I wanted to avoid a lot of physical labour, and frustration.
One thing I did know is that when the elements of a fan dipole (or inverted vee) are maximally separated the interactions are small enough that cut-and-try is a reasonable approach. Some adjustment is usually needed in any case, since even with modelling software there are inevitable interactions with ground, building wiring and so forth. When in the same vertical plane, tuning can be highly sensitive to small changes in wire position.
To this end I performed a couple of modelling experiments. First up is a 30 plus 20 meters fan dipole with a common feed point. In this configuration the 4 wires move out radially from the feed point, in accord with my chosen mechanical feed point design. The model uses dipoles in free space to aoid additional variables. Those can be added to model once a suitable design is selected.
I rotated the 20 meters dipole in 10° increments from 90° (dipoles are orthogonal) to 10°. In the orthogonal start position I "trimmed" the dipole so that the resonant frequencies were 10.125 and 14.100 MHz. The table shows what happens as the 20 meters dipole is rotated.
30 & 20
Meters Fan Dipole – Free Space – Radial fan-out
|
||||
Angle
|
30 Meters
|
20 Meters
|
||
Fr (MHz)
|
R (Ω)
|
Fr (MHz)
|
R (Ω)
|
|
90°
|
10.125
|
75
|
14.100
|
71
|
80°
|
10.100
|
70
|
14.130
|
73
|
70°
|
10.075
|
65
|
14.170
|
74
|
60°
|
10.050
|
62
|
14.220
|
73
|
50°
|
10.025
|
59
|
14.270
|
70
|
40°
|
10.000
|
56
|
14.360
|
64
|
30°
|
9.980
|
54
|
14.460
|
58
|
20°
|
9.970
|
51
|
14.610
|
50
|
10°
|
9.950
|
47
|
14.870
|
42
|
Although the affect on the lower frequency antenna is modest, on the higher band the interaction becomes severe as the angle between wires is lowered. This helps to explain the tuning sensitivity. Worse, since the 20 meters dipole would have to be far longer than a naive calculation would give, which make the "cut" part of cut-and-try difficult to put into practice!
There is a better way to construct a fan dipole, one where much of the sensitivity is eliminated. The big hint to me was the long-known technique of using ladder line as a two-band dipole. I do not mean a ladder-line feed system, but ladder-line for the antenna elements. The feed line is coax (with a common mode choke).
You choose a length of ladder line for the lower of the two bands (dipole formula). Then cut the line on one of the leads so that it is the proper length for a dipole on the higher of the two bands. Do this for each leg of the dipole. Tie the two sides of the ladder line together at the feed point, and connect those two to the coax feed line.
The model of the feed point looks something like the adjacent diagram. This is the second model I used for my parallel-wire fan dipole. I then modeled it in EZNEC.
As a starting point I used a 5 cm separation between elements and tuned the antenna for resonance as in the preceding case. Then the separation was increased in 5 cm steps up to 30 cm. To do this I left the horizontal lengths unchanged, only increasing the length of the vertical connecting wires (wires #4 and #5 in the diagram). As before I used free space. The 30 meters dipole is on top, connected to the feed line.
30 & 20
Meters Fan Dipole – Free Space – Parallel fan-out
|
||||
Separation
(cm) |
30 Meters
|
20 Meters
|
||
Fr (MHz)
|
R (Ω)
|
Fr (MHz)
|
R (Ω)
|
|
5
|
10.125
|
74
|
14.100
|
53
|
10
|
10.125
|
76
|
14.050
|
59
|
15
|
10.125
|
76
|
13.980
|
61
|
20
|
10.125
|
76
|
13.910
|
62
|
25
|
10.125
|
75
|
13.830
|
63
|
30
|
10.125
|
75
|
13.760
|
63
|
This is much better. Notice how the 30 meters dipole is unaffected by the separation, including any affect from the connecting wires #4 and #5. The impact on the 20 meters dipole is incremental, roughly in proportion to the length of the connecting wires. (Recall that the source is connected to the 30 meters dipole.)
For the modelled feed configuration and range a quick calculation shows that the equivalent length of each 20 meters dipole leg is 0.4x the length of wires #4 and #5 (the separation distance). For example, at a separation of 30 cm the equivalent lengthening of each of the 20 meters legs is 12 cm. I checked this by subtracting 12 cm from each leg of the 20 meters dipole and the antenna model resonated at 14.075. However, the impedance remained at 64 Ω. This is a curious benefit since we do not want the impedance to drop too low or the real antenna, when built, might otherwise have a higher SWR.
You should expect that the length ratio will be slightly different if wires #4 and #5 are a different distance apart. In my model the value is 8 cm (3").
The reason for the lower impedance on 20 meters is that current is induced on the other dipole. This also adds some gain to the antenna, a small 0.8 dBd. The reverse is not true, so the impedance on 30 meters is near to the nominal value of 73Ω in free space and the gain is close to 0 dBd.
This is the model I plan to build for the next iteration of Site-B inverted vees.
Sunday, September 8, 2013
1.5 Inverted Vees
In my previous article I described the antenna mast and feed/pulley system I installed at Site-B. As mentioned at the time I did get something in the way of antennas installed on that mast, but did not say what they were. At the time I was in something of a rush, preparing for the arrival of the crew that would be refinishing the exterior of my house.
Since then I have had only a little time to play with the antennas, and no easy way to do further work on them. After a bit of tuning and mechanical adjustments I took the adjacent picture from the roof and then left things alone for a bit.
The wires are all visible in the picture. That you can only see 3 of them is not a mistake. Some explanation is in order.
The wire going down to the left is half of an inverted vee for 20 meters. It is tied to the edge of the roof with a length of nylon rope. The other half of the vee is tied in the same fashion to the top of the tower at Site-C. That's the tower supporting the TH1vn tri-band dipole. The apex of the vee is ~14.1 meters above grade, with south (tower) end ~12 meters up and the north end ~10.2 meters up.
Total length of the antenna is 10.2 meters of insulated #14 stranded copper wire. Its resonant frequency is close to 14.0 MHz with an SWR of 1.2, and it stays below 2 across the band. The average height of the antenna current is approximately 13 meters above ground. This is 2.5 meters (or close to 25%) higher than the dipole. This matters since it is the current that determines the far-field pattern.
The third wire is 4.05 meters of the same wire. Its low point is also 12 meters. It is tied to tower as well, but lower down. Ignore this wire for now since it has negligible effect on 20 meters.
The orientation of the antenna is not ideal. I did what came easy also put the wires as high as possible. The antenna favours east and west directions, and so is -3 db towards Europe and west Asia, and worse towards central Asia. Nominal (modelled) gain in the major lobe is 7.8 dbi at a heading of 90° and elevation of 25°. The pattern is slightly asymmetrical, as expected from their positions. The null to the south is deeper than I'd like: -4 dbi. It is more omnidirectional than a dipole though not by much.
My purpose in putting up this antenna was to test whether a little more height and bending down the legs would outperform the TH1vn tri-band dipole at a height of 10.5 meters. In particular:
To be brief, the inverted vee works well but is not better than the dipole at the lower height. The EZNEC models appears to reflect my experience with the antennas. In one way this is unfortunate. On the other hand it does mean the modelling can be relied upon, including for future antenna designs at my location.
The highlights of the antenna comparison are as follows:
Now on to that third leg I mentioned earlier, the final half of this 1.5 inverted vee antenna. I had this odd notion that it might be possible to add a band with only one more wire. If the length is suitably chosen it should form an off-centre fed inverted vee in combination with one, or even both, of the legs of the 20 meters inverted vee.
The reasoning is that any combination of wire legs that has a high impedance on the target band would carry little in the way of antenna current. I decided to try this since it is easier than building a full fan dipole, and I was having some difficulty managing modelled interactions between the wires going to the tower. The closer the wires in a fan dipole come to each other the more they interact and affect the lengths of all antennas except the one for the lowest frequency band.
Since time was short I only modelled the SWR for the third wire, with the aim of making it resonant on 17 meters. I found that the closeness of the legs going to the tower had a large impact on the resonant frequency on 17, but almost no affect on 20. The spread of resonant frequencies went from 17.5 to 18.5 MHz over the range of the tower tie point for the 17 meters leg. You can see how close they are in the photo and model up above.
With that much sensitivity some experimentation is necessary. First, in a quest for height, I tied the rope off at 8 meters height, quite close to the tie point for the 20 meters leg -- the top of the tower is 8.8 meters above ground. That was too close. Then I tried going very low, at the 4.5 meters level. This swung too much in the other direction, much as the model showed. So I split the difference and had a 17 meters antennas that resonated at 18.050 MHz. I decided that was close enough since the band is so narrow. SWR is 1.2 at 18.068 MHz and 1.5 at 18.168 MHz.
I put it on the air and it worked out just fine. Not great, but ok. Stations went into the log. Then I got around to modelling the pattern. That was not fine.
The antennas showed a pretty typical pattern shape for an inverted vee. But something was amiss since the gain in the main lobe was only 3 dbi, several db lower than expected. I then modelled the currents and had a closer look at what was happening.
Imagine my surprise when the image at right appeared! The antenna current chose the adjacent 20 meters leg rather the far one, which I had incorrectly assumed would form the other half of the antenna. Moving the 17 meters legs up and down had little impact on the current in the far 20 meters leg.
This explains the low gain and (as I eventually noticed) the smaller than expected SWR bandwidth. The latter is unimportant on 17, while the former is a problem. I have not investigated further to understand what is happening. Regardless of the reason it seems my "innovation" is a sham. I may have to add bands in a more traditional fashion rather than taking shortcuts. Still, it is in a way quite interesting.
I'll end this article by describing the full extent of the EZNEC model. The model includes all the metal elements in the mast, going all the way to ground level, plus the coaxial cable. The coax is modelled as a wire (the insulated outer conductor of RG-213/U) that is not connected to any other at its ends. To keep things simple this assumes that the air-core coax chokes at the top and bottom are perfect on all bands of interest -- infinite impedance. House metal, in particular the aluminum eaves (my original antenna!) and soffits, are not modelled (at least not yet), nor is the Site-C tower and antennas.
The view of the complete model is shown at right. The perspective is that of an observer to the north-northwest and above the height of the mast. Hopefully it is recognizable.
Antenna currents are small on all these additional conductors on both 20 and 17 meters. I was concerned that the partly-vertical profile of the antenna legs could couple to the mast and coax. For the present I can ignore their small effect.
The wires are all visible in the picture. That you can only see 3 of them is not a mistake. Some explanation is in order.
The wire going down to the left is half of an inverted vee for 20 meters. It is tied to the edge of the roof with a length of nylon rope. The other half of the vee is tied in the same fashion to the top of the tower at Site-C. That's the tower supporting the TH1vn tri-band dipole. The apex of the vee is ~14.1 meters above grade, with south (tower) end ~12 meters up and the north end ~10.2 meters up.
Total length of the antenna is 10.2 meters of insulated #14 stranded copper wire. Its resonant frequency is close to 14.0 MHz with an SWR of 1.2, and it stays below 2 across the band. The average height of the antenna current is approximately 13 meters above ground. This is 2.5 meters (or close to 25%) higher than the dipole. This matters since it is the current that determines the far-field pattern.The third wire is 4.05 meters of the same wire. Its low point is also 12 meters. It is tied to tower as well, but lower down. Ignore this wire for now since it has negligible effect on 20 meters.
The orientation of the antenna is not ideal. I did what came easy also put the wires as high as possible. The antenna favours east and west directions, and so is -3 db towards Europe and west Asia, and worse towards central Asia. Nominal (modelled) gain in the major lobe is 7.8 dbi at a heading of 90° and elevation of 25°. The pattern is slightly asymmetrical, as expected from their positions. The null to the south is deeper than I'd like: -4 dbi. It is more omnidirectional than a dipole though not by much.
My purpose in putting up this antenna was to test whether a little more height and bending down the legs would outperform the TH1vn tri-band dipole at a height of 10.5 meters. In particular:
- Would the 25% increase in height have a noticable impact on low-angle DX performance. The TH1vn models with a maximum gain of 7.2 dbi at an elevation of 30°, and only negligibly lower at 25°. The antenna favours South Europe, at a compass direction of 70°.
- Would it be more omnidirectional. Wire antennas don't rotate, and I want to cover as many directions as possible, even at the expense of stateside QRM.
To be brief, the inverted vee works well but is not better than the dipole at the lower height. The EZNEC models appears to reflect my experience with the antennas. In one way this is unfortunate. On the other hand it does mean the modelling can be relied upon, including for future antenna designs at my location.
The highlights of the antenna comparison are as follows:
- On the longest paths (4J, FK8, VK, ZL, JA, BY, etc.) the antennas are roughly equivalent, though not in all cases. The vagaries of multi-path and other propagation effects are noticable even though both antennas are (mostly) the same polarization and in the same vicinity.
- The off-peak orientation of the vee does discriminate against much of Europe, as the model shows. Europe is typically 1 S-unit stronger on the dipole. This is a shame since that is the direction for the bulk of DX paths. However, for some reason northern Europe (TF, OH) slightly favours the inverted vee.
- On polar (north) paths the dipole wins. This includes UA0, BY, VU.
- There is some variation on southern paths, perhaps due to the different positions and depths of the side nulls. Since it can vary by 2 S-units it is a good idea to try both antennas and choose the best one for each station.
- Since the antenna is not up a multiple of ½-wavelenght there is lobe that points straight up. This was confirmed by the relative strength of many mid-distance W and VE stations. This is not desirable.
Now on to that third leg I mentioned earlier, the final half of this 1.5 inverted vee antenna. I had this odd notion that it might be possible to add a band with only one more wire. If the length is suitably chosen it should form an off-centre fed inverted vee in combination with one, or even both, of the legs of the 20 meters inverted vee.
The reasoning is that any combination of wire legs that has a high impedance on the target band would carry little in the way of antenna current. I decided to try this since it is easier than building a full fan dipole, and I was having some difficulty managing modelled interactions between the wires going to the tower. The closer the wires in a fan dipole come to each other the more they interact and affect the lengths of all antennas except the one for the lowest frequency band.
Since time was short I only modelled the SWR for the third wire, with the aim of making it resonant on 17 meters. I found that the closeness of the legs going to the tower had a large impact on the resonant frequency on 17, but almost no affect on 20. The spread of resonant frequencies went from 17.5 to 18.5 MHz over the range of the tower tie point for the 17 meters leg. You can see how close they are in the photo and model up above.
With that much sensitivity some experimentation is necessary. First, in a quest for height, I tied the rope off at 8 meters height, quite close to the tie point for the 20 meters leg -- the top of the tower is 8.8 meters above ground. That was too close. Then I tried going very low, at the 4.5 meters level. This swung too much in the other direction, much as the model showed. So I split the difference and had a 17 meters antennas that resonated at 18.050 MHz. I decided that was close enough since the band is so narrow. SWR is 1.2 at 18.068 MHz and 1.5 at 18.168 MHz.
I put it on the air and it worked out just fine. Not great, but ok. Stations went into the log. Then I got around to modelling the pattern. That was not fine.
The antennas showed a pretty typical pattern shape for an inverted vee. But something was amiss since the gain in the main lobe was only 3 dbi, several db lower than expected. I then modelled the currents and had a closer look at what was happening.
Imagine my surprise when the image at right appeared! The antenna current chose the adjacent 20 meters leg rather the far one, which I had incorrectly assumed would form the other half of the antenna. Moving the 17 meters legs up and down had little impact on the current in the far 20 meters leg.
This explains the low gain and (as I eventually noticed) the smaller than expected SWR bandwidth. The latter is unimportant on 17, while the former is a problem. I have not investigated further to understand what is happening. Regardless of the reason it seems my "innovation" is a sham. I may have to add bands in a more traditional fashion rather than taking shortcuts. Still, it is in a way quite interesting.
I'll end this article by describing the full extent of the EZNEC model. The model includes all the metal elements in the mast, going all the way to ground level, plus the coaxial cable. The coax is modelled as a wire (the insulated outer conductor of RG-213/U) that is not connected to any other at its ends. To keep things simple this assumes that the air-core coax chokes at the top and bottom are perfect on all bands of interest -- infinite impedance. House metal, in particular the aluminum eaves (my original antenna!) and soffits, are not modelled (at least not yet), nor is the Site-C tower and antennas.
The view of the complete model is shown at right. The perspective is that of an observer to the north-northwest and above the height of the mast. Hopefully it is recognizable.
Antenna currents are small on all these additional conductors on both 20 and 17 meters. I was concerned that the partly-vertical profile of the antenna legs could couple to the mast and coax. For the present I can ignore their small effect.
Monday, September 2, 2013
Site-B Antenna Mast
I temporarily installed an experimental 20 meters delta loop at Site-B. When it failed to meet my needs and expectations it was removed and dismantled.
Site-B remains a good one for wire antennas, so all is not lost. I proceeded to plan and construct a mast for wire antennas, one that would be affixed to the 6 meters high base steel pipe. Of course life got in the way, so this took some time to get underway. I have also been under a deadline since on September 3 a work crew arrives to erect scaffolding all around the house to start on needed maintenance to the house siding. That would delay progress until the last half of September. I wanted to avoid that delay.
In a flurry of activity this long weekend I manged to finish construction of the mast, install it and also test and tune an antenna. In fact I barely got the antennas, rope and tools put away before a thunderstorm struck.
The antenna I put up at present covers only 20 and 17 meters, which I'll describe in my next article. It deserves separate treatment. Besides which I have had little opportunity as yet to use it much. In this article I'll stick to the mast construction and erection, and a little about antenna construction.
Unlike the Site-C tower which is not easily visible from the street, this mast caught the immediate attention of many of my neighbours. There was interest and curiosity, but no complaints. Of course complainants rarely choose direct confrontation, so time will tell. I really do not expect any conflict to arise.
As with the delta loop, I installed the mast by securing myself with a safety line and harness, then standing at the edge of the roof and lifting the 8 meters plus contraption -- complete with mechanical add-ons and various ropes -- straight up into the air and plugging it into the steel pipe.
This is easier than it sounds! Unlike the delta loop which has a full fibreglass mast, this one is lighter and easier to manoeuver. In fact with just a little bit of arm and back strength it can be manually lifted from one end and swung up to its vertical orientation. One just has to move it with care once it's airborne since it does oscillate back and forth and could throw you off balance.
Starting from the bottom and working upward I'll describe the mast's features:
- The fibreglass sections nest nicely into the 1-½" Schedule 40 galvanized pipe. The duct tape I used as a shim to improve the fit was replaced -- it proved to be too sticky and would likely weather poorly -- with a thin sheet of aluminum, a big roll of which I had in my junk box. A hose clamp (visible as a bright spot in the photo) secures it so that the shim doesn't pop out with the mast when it is subsequently lifted out.
- The aluminum mast is the boom of a 1λ-long yagi for 6 meters. Up until 1992 it enabled me to accumulate many DXCC countries on the "magic band". The elements are gathering dust in the garage. Since it's a yagi boom it is widest at the centre, not the bottom. However this allows it to fit almost snugly inside the fibreglass mast. A mast clamp acts as a stop to prevent it sliding too far into the fibreglass section. The clamp also secures an aluminum sheet shim, and a length of #12 wire. The wire is there for "future consideration", which I'll talk about when I get around to using it.
- Near the top is a guy ring that fits neatly just where I want it on the mast. There are 3 rope guys to stabilize the mast. The mast is very flexible and I need their help to keep the mast vertical when the antennas are pulled towards their anchors. The guys ropes are light duty, which is all that's required in this application. One thing I did was add an extension angle steel to the pipe bracket so that the rightmost guy can be more effective. That guy attaches to an anchor lower down the pipe.
- At the very top is a pulley. This allows me to adjust and modify the wire antennas without pulling down the mast each time. The pulley is very small, sized to match the rope that pulls up the antenna. While it is a topic all its own I will just mention that you should never use a pulley that is oversized to the rope. If you've every had a rope get trapped between the wheel and pulley housing where it is far out of human reach you'll understand what I mean!
For the steel mast I finally installed a permanent base. As you can see from the picture it is a smaller version of the floating base that I designed and build for the Site-C tower. Since I strategically positioned the threaded side at the bottom (the other threaded end was previously cut off) I bought a galvanized floor flange as a suitable interface between pipe and base. The mast is off-centre since the vertical alignment must have been a bit wrong on the temporary base. The only negative impact is one of aesthetics: it doesn't look pretty although its performance is the same.The antenna that I put up is, as I said, the subject of the next article. I will describe the mechanical and electrical construction of the antenna's core now since it is closely related to the construction of the Site-B mast.
The attached picture show the key elements of the construction. It centres on the plastic insulator. I made the feed system independent of the antenna elements by installing several loops of solid #12 copper wire on each end of the insulator. Antenna elements will be secured to those, as are the leads from the coax.
The common-mode choke is wound from RG-58 for two reasons. Weight of RG-213 was a concern due to the stress on the pulley-drive system. I also wanted an effective choke that would work down to 40 meters, and that is difficult for an air-core choke made from RG-213. The 12-turn, 11 cm diameter coil of RG-58 should be effective on 40, and work well up through 15 meters. There is another choke lower down where the RG-213 leaves the mast which, like the wire clamped to the aluminum mast, is there for possible future applications.
There really is no need for a PL-259 on the feed side of the choke. Since the run of RG-58 I found in my junk box that was the right length happened to have connectors on both ends I left them there. I soldered leads from the connector to the insulator ends.
At top is the loop of rope that connects the assembly to the pulley rope. It isn't clear in the picture so I'll mention that the rope goes around the insulator, not the more fragile RG-58 choke.
The rope that hangs down is used to attach to the main run of RG-213 for strain relief. Because of the pulley system it is not possible to bind the feed line along the mast. As a result the weight of about 6 meters of RG-213 would otherwise stress the connectors and choke. This risks eventual failure of the cables or connectors.
One thing that proved to be a problem in the overall design is the difficulty of selecting the correct amount of pre-load tension to apply to the mast guys. The one that is most critical is the one opposite the pulley since it feels the full weight of the what the pulley is supporting, plus the tension applied to the antenna wires. The other two guys help to center the mast and only see dynamic loads in high winds.
I have had some difficulty judging the required pre-load. The key guy is one on the right (first picture above) that runs through the angle bracket extension. My design makes it is easy to adjust its tension using a step ladder on the ground. However the adjustment can only be done with the antenna lowered either part way or (better) all the way.
A bit of bend in the mast looks unsightly but does not really have much effect on its strength. But that strength does depend on the integrity of that one guy. This is something I may have to address before winter arrives.
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