160 Meter Shunt Fed Tower

I am trying something different for 160 meters this winter. The T-top wire vertical hung from the 150' tower the past few years is a great antenna but not without its negatives attributes. These include:

• Potential for interactions with yagis, of which I now have more
• Tower interactions affecting directivity and efficiency, up to 3 or 5 db in some directions and down to -2 db, respectively
  
• Low radiation resistance that reduces efficiency by, perhaps, -1 db
  

A few possible alternatives were reviewed in an article last month. My intention is to do the best I can within the constraints I have. My constraints are atypical in that I have two towers that are approximately λ/4 tall on 160 meters, and that makes many things possible that are unattainable for most hams.

After more analysis, research and modelling I decided to shunt feed the 140' tower. Shunt feeding a tower with yagis on top has long been a popular choice for hams with limited space and height since it exploits the modest height tower they have, very often with relatively efficient top loading provided by a tri-band yagi on top. 

In my case the electrical length of the tower is far greater than λ/4 and while that's impressive and potentially very effective it does introduce a few unique problems. These I knew in advance because they are covered in depth in ON4UN's Low-Band DXing book. 

This is a timely reflection on the book and John DeVoldere's remarkable legacy since he died on November 9. If you have an interest in the low bands you must have this book on your bookshelf. The antenna described in this article behaves almost exactly according to the chart and equations to be found in that book. It is the marriage of theory with the practical that makes the book a treasure.

A shunt fed tower is pretty simple: run a wire parallel to the tower, attach it to the tower where the resistance part of the impedance is 50 Ω and use one (gamma match) or two (omega match) capacitors to tune out the net inductive reactance at the base of the gamma rod (or wire). 

It is straight-forward to model the antenna in NEC2 (I use EZNEC), with a few cautions:

• Align the segments of the tower and gamma rod (wire)
• Use MININEC ground and place resistance loads in a one segment wire connected to ground to emulate the loss due to the tower ground and radial system
  
• Rather than include yagis in the model it is easier and equally effective to measure the electrical length of the tower and make the tower that height in the model
• The short wires connecting the tower and gamma rod should be one segment
• Put the gamma capacitor and source at the bottom of the gamma rod
• Connect the bottoms of the gamma rod and tower with a one segment wire, and do connect them in the built antenna
  

The guidelines on how to proceed are addressed in ON4UN's book. Recall that I measured the resonant frequency of the tower to be in the vicinity of 1200 kHz. This is an electrical length of about 62 meters, which is 135° or ⅜λ at the low end of top band. Although this is quite a bit more than 90° (λ/4) the pattern does not sprout any lobes at high elevation angles.

I was successful replicate the tabulated data from the book with the model I developed in EZNEC. The initial height estimate for tapping the tower with a wire spaced 1.5 meters is about 25 meters. Perhaps the largest uncertainty is the diameter of the tower which, as a lattice structure, is typically equivalent to a wire somewhat less than the tower face dimension and depends on the structural details. Gamma wire diameter and insulation are other variables with an effect.

For simplicity in getting to a first measurement I used the boom of the lower 5-element 20 meter yagi that is ~22 meters high. A clamp holds the wire in position along the boom and the wire is bonded to the tower metal by scraping away the paint around a hole in the adjacent tower girt. I did not rely on continuity between boom and tower because there's a layer of paint between them. I scraped the paint off the galvanizing at the attachment point.

It was no fun to lean out from the tower on a face with no climbing horizontals to clamp the wire out 1.5 meters (5') from the tower centre. I couldn't get it quite that far without additional acrobatics, an unwarranted effort since the tap would almost certainly have to be moved.

That was indeed the case. After cancelling the inductive reactance with a capacitor (low voltage capacitors are suitable for analyzer measurement) the resistance, at 122 Ω it was far wide of the mark. Part of that was due to the mess of clip leads but no where near 150% error's worth. Apart from the high impedance the antenna seemed to work okay. I am using the same set of 8 × 30 meter radials from my earlier antenna.

It was a windy during the first trials and the periodic billowing of the long gamma wire during the analyzer's scan made for peculiarly wavy SWR curves. It is worthwhile to place insulating arms at several points on the tower to hold the wire in place to avoid impedance swings and wire fatigue. I have not yet done so since with the antenna incomplete the positions were subject to change.

I returned to the computer to re-calibrate the model based on what I learned. It turned out that the error was not as bad as I feared. I estimated that the 50 Ω tap could be found 3 to 5 meters lower. I split the difference and used the available tower girt about 3.5 meters lower (~60' above ground).

I built an extendible arm to suspend the gamma wire from the tap point. A length of angle steel and an ABS pipe are joined with hose clamps. The outer end of the ABS pipe is notched to hold the wire in place. The bolt that connects the arm to the tower doubles as the electrical bond to the tower. The end of the wire is a tinned loop that fits on the bolt and minimizes galvanic corrosion when sandwiched within the galvanized hardware.

Minimal acrobatics are required to adjust the telescoping arm. To change the distance I disconnect the wire on the ground, loosen the hose clamps and slide the pipe to its new position. A retractable steel tape measure can be extended out to the end of the pipe to set the wire separation.

The separation between the gamma wire and tower was not constant. The wire was farther at the bottom than at the top. Although this is not ordinarily discussed in the literature there is no reason for the two to be exactly parallel. All that does is improve predictability. The effect of the angled wire is that of a parallel wire with a separation between those of the top and bottom.

The reasons for the non-parallel wire were to avoid more tower acrobatics and because the radial hub is not easily moved. I only moved the radial hub, along with all 8 radials, when the antenna was approaching completion. The general layout of the antenna base is shown in the photo below.

Pictured are the final positions of the components, after all the experiments and final adjustment. A wood stake has a wood platform for the plastic container with the gamma capacitor and an insulator for the gamma wire.  This keeps the capacitor and feed point off the ground and safe from snow, puddles and even small animals.

This is a high voltage point so beware of the presence of incidental conductors. Insulated wire is recommended for the vertical gamma rod (above the capacitor). A wire snakes along the ground between the tower lightning ground rod and the radial hub (ABS pipe next to the stake) to prevent the high loss of a return path via the soil. A short length of RG213 runs from the main Heliax transmission line to the feed point adjacent to the capacitor. The Heliax is buried in a trench along with all the other transmission lines and control cables.

It is important to check for continuity between the gamma wire and the tower ground. Despite the many splice bolts between tower sections it is possible for paint to insulate tower sections. A friend in the business once explained that the metal ridges I found on several of my used commercial tower sections are evidence of the tower having been the radiator of an AM broadcast station. After installation all the legs at each section splice are spot welded to ensure continuity and no resistance loss due to high RF currents.

Returning to the tuning discussion I have to say that we got lucky with the second trial using the lower tower tap. After tuning out the inductive reactance with the capacitor the impedance was almost exactly 50 Ω. This is what I call hitting the bullseye! Unfortunately after tidying the site -- changing connecting wire lengths and location of the radial hub -- the impedance dropped to 38 Ω. Further adjustment of the gamma wire got us back to 50 Ω.

I drew a diagram of the adjustment process for the gamma wire. Coarse tuning to get to 50 Ω is done by changing the height of the tower tap. Higher is higher impedance and lower is lower impedance. Once you're close -- 35 Ω to 65 Ω range -- you can fine tune the impedance by changing the separation of the wire and tower. Closer is lower and farther is higher impedance. There is an effect on bandwidth in different combinations of the two, however it is small enough that I solely focussed on achieving a match.

Since the radials are difficult to move you can either run a wire from the bottom to the radial hub or fix the gamma wire position at the bottom and extend or retract the tower arm. I performed trials using both methods. Do whatever works best in your situation. The extra wire on the ground for the former method changes the tuning and that can be tuned out with the gamma capacitor.

I centred the match at 1840 kHz. This is 10 kHz higher than for general DXing, for which most top band operators aim for 1830 kHz. Because contests are a primary interest of mine and the activity in top band contests can spread quite a lot higher in the band I centred the antenna higher. Alternatively the antenna centre can be moved higher just for contests, easily accomplished in a few minutes by adjusting the gamma capacitor.

The 2:1 SWR bandwidth is a little more than 75 kHz, as predicted in ON4UN's book and in the software model. This is narrow though typical of an electrically long shunt fed tower. Better than 100 kHz bandwidth can be had with a wire cage for the gamma wire. I'll consider doing that next year if I decide to keep this antenna.

With the gamma capacitor value and range determined once the 50 Ω impedance was found for the final antenna layout I proceeded to select a gamma capacitor from my ample junk box. The gamma capacitor has a high voltage across it due to the large inductive reactance it must cancel. By large I mean thousands of volts. The greater the inductive reactance the smaller the capacitor value required to cancel it and the higher the voltage it must withstand. For the 145 pf capacitor measured and modelled it is more than 2500 volts for 1000 watts.

A variable capacitor with the required range and voltage rating is large. It is better and cheaper to use a small value variable capacitor in parallel with a high voltage fixed capacitor. The smaller range of the variable capacitor is also easier to tune and this is important because the capacitor value is critical: a tiny change has a large impact on the feed point impedance.

My first attempt was a split stator capacitor with 50 pf per section in parallel with a 100 pf ceramic doorknob capacitor. The approximate range is 100 to 200 pf, plus stray capacitance in the wires connecting it to the gamma wire and feed point. 

The plate spacing is not enough to withstand 3000 volts, which is the minimum recommended in this application. Air requires at least 0.12" to survive 3000 volts, and that is for dry air with no margin for humidity and particulate contaminants. To determine the actual flash over voltage I followed the advice from an old joke:

To determine the load rating of a bridge drive heavier and heavier trucks over it until the bridge collapses. Rebuild the bridge as before and put up a sign with the load limit as the weight of the last truck that made it safely across.

I gradually increased power until the capacitor arced. This occurred around 650 watts. I had no difficulty operating at 600 watts. The load data in EZNEC tells the story.

Load #3 is the capacitor. At 600 watts it sees a little over 2000 volts. It is reasonable to assume from this (and a measurement of the plate spacing) that the capacitor is rated for 2000 volts. The box the capacitor comes in does not specify the voltage rating.

I rebuilt the capacitor by adding a 30 pf ceramic doorknob and wiring the split stators in series. The range of the assembly is now 130 to 155 pf. Recall that series capacitors of equal value result in a capacitance half that of each, which is 25 pf for the two 50 pf section. These act as a voltage divider with a combined rating of 4000 volts. Should you use different value capacitors in series the voltage division will be unequal and you may not eliminate arcing.

I added less than 10 pf to the range with a few inches of scrap RG213, which is 2.1 pf/inch and has a voltage rating of at least 3000 volts. The braid is trimmed near the edges to prevent arcing through the air.

For weather protection I put the capacitor in a margarine container. There are holes for the external connections and bottom holes to weep water and moisture. A scrap piece of wood on top reduces UV damage and, with a stone, keeps it from blowing away in a high wind!

Returning to the load data you'll see the 3 resistance loads for the radial system and the lightning grounds of the shunt fed tower and the similar tower 60 meters away. Ground loss has been reduced compared to my previous wire vertical and that of a wire vertical adjacent to one tower or between the two towers. 

It is difficult to know whether the efficiency increase is true in practice. For now all I can say is that the antenna is working very well and is certainly at least the equal of my previous winter 160 meter antenna. Cumulative experience over the coming winter's DXing and contests will provide additional performance data.

I'll close with noting a couple of issues, one that I ran into and one that may arise. After tuning the antenna and testing it in the shack I found that the resonant frequency shifted from 1840 to 1855 kHz. That may not seem a lot but for a narrow bandwidth antenna on 160 meters it is an almost 1% change that pushed the SWR over 2 at the bottom of the band. Scale this to 20 meters and it's equivalent to shifting resonance from 14.100 MHz to 14.220 MHz. Most hams would notice that!

The cause was the connection to the main transmission line. It is buried 20 cm below grade and behaves as an additional radial, one that is longer than the other 8. There is no common mode choke on the transmission line to defeat this behaviour. The gamma match provides a modest amount of common mode attenuation and the ground around the Heliax provides a degree of common mode choking because, by being buried, the common mode field loses strength along the way. However this choking effect is not enough to prevent it acting as a radial. With a small radial count there can be a current imbalance due to its different length and depth but imbalance is almost certain in any case with just 8 radials.

To compensate for the shift I tuned the antenna to resonance at 1825 kHz. With the transmission line attached the resonance shifted 15 kHz to 1840 kHz, which is where I want it.

The other concern, one common with shunt fed towers, is RF into electronics on the tower and heating of common mode chokes on yagis. At a minimum all control lines requires bypass capacitor or RF chokes, and even then there may be effects. I will be watching for problems with my home brew stack switches when they are installed in the next week or two. Since I use air core coax chokes there are no ferrite cores to overheat and damage. At least that's the theory. Again, I will have to see what happens.

Most often these problems do not manifest themselves. When they do occur they can be difficult to cure. Opting for a wire vertical adjacent to the tower rather than shunt feeding the tower itself can help but not as much as you might think since the coupled energy is almost as great as in the wire itself. More separation is needed to lower the risk.

There is no easy way to predict which installations will suffer from these problems. I will try it and see and take action if necessary. One reason I chose the tower with the 15 and 20 meter stacks is that it less likely I'll be on those bands at the same time as 160 meters than 40 or 80 meters.