Thursday, September 4, 2014

80 Meters Loaded Half-sloper

A half-sloper for 80 meters was listed in my antenna plans for the new tower. I have had good luck with half slopers for 80 in the past, so perhaps one cut down to fit a 14 meters tall tower can also do well. I don't know the provenance of this antenna but it was already popular in the 1980s with hams who needed decent DX performance that works in a small lot with the typical modest-height tower and tri-bander.

When I put up a half-sloper in 1985 it was on a whim. It was easy to build and I had no other good ideas a good 80 meters DX antenna. Since then I have been intrigued by this class of antenna since, to my surprise at the time, it worked very well indeed. All it needed was coax to the top of my 19 meter tall tower and a wire sharply angled downward to...somewhere. The "somewhere" was a challenge since I discovered that antenna impedance and resonant frequency were very sensitive to wire placement and length. I had to deal with interactions since it strongly coupled to the unshielded rotator cable running down the tower.

Once I had it working it worked great, netting close to 200 countries on 80 meters and it made for quick work in the pile-ups. Of course the Collins 30S1 helped.

When ELNEC arrived I tried to build a model of the antenna. This was difficult since MININEC ground didn't deal especially well for antennas close to ground. The need to add in my stacked yagis to the model added more complexity. With so many segments on a 20 MHz 386 PC (early 90s state-of-the-art) some runs took 30 minutes.

In the end I managed to squeeze results out of ELNEC that resembled what I saw and measured on the actual antenna. Later, after upgrading to NEC2-based EZNEC, I discovered that my model was nonsense. Shortly afterward I left the hobby and did not pursue the matter further. Twenty years later it is time to give this antenna another look.

Objectives

Even when it works well this antenna does not truly measure up to the best single-element 80 meters antennas currently in use. It is a compromise antenna, so our objectives must be realistic. Here are mine:
  • Vertically polarized, with the bulk of the antenna current high up rather than close to the ground. The idea is to get good DX performance with modest near-field ground loss and no radials.
  • Fit comfortably on the tower and within the property, without encumbering other yard uses and without deleterious interactions with the tri-band yagi and other wire antennas.
  • Develop a good antenna model so that the antenna can be better understood, optimized, experimented with, and effectively compared to alternatives.
Model

The interaction model I recently developed is also a pre-requisite for modelling this antenna. The tower and yagi, and even cables, are an inevitable part of the half-sloper antenna. That is why the half-sloper is the first wire antenna I added to the interaction model.

Although this is not a full size half-sloper due to the height of the tower my hope is that the modelled characteristics are indicative of a non-loaded version. If I have time I will play with various tower heights. But for now my focus is on what I can use, and that means sticking with my tower and environment.

Here are some of the things I hoped to learn:
  • Contribution of the yagi to antenna tuning and performance
  • Effect of inductively loaded wire
  • Varying the angle between wire and tower
  • Directionality and polarization
  • Alternative source placement
To begin the modelling process I inserted a 17.5 μH coil 60% of the way down the wire. The inductance is not critical, so I chose that of a suitable coil I had on hand. The wire is 12 AWG insulated wire 18.5 meters long making an angle of about 45° angle with the tower.  The wire end is 1 meter above ground, which is not ideal (safety hazard) but acceptable at this stage in the design. The SWR is above 2 due to a feed point impedance of 22 Ω at resonance.

Variations on the above modelling parameters will be explored in the article, and possibly in future articles.

Note: This antenna is a good example of how to drive NEC2 crazy. With the source near a multi-wire junction (tower, wire and mast) and with very different diameters strange things can occur. W7EL warns about this in the EZNEC manual and provides suggestions on evaluating and correcting calculation errors that result in erroneous antenna gain (loss). In this case a slight shift in source placement made a difference of 4 db! I have corrected for this in the model.
Yagi contribution and interaction

Since the yagi is part of the half sloper that is not considered an interaction. The concern is that the half-sloper wire will interfere with yagi performance on 20, 15 or 10 meters. So let's look at that first.

The result isn't surprising to me since I've noted in the past that even wires that cross the aperture of a yagi at a sharp angle have little induced current. That is true here as well. The worst case I found was on 10 meters with the yagi elements pointing in the same direction as the sloping wire. As the plot at right graphically demonstrates there is little current induced on the wire even in this case. So far so good.

However there is current there even though its magnitude is small. That can still be a problem since the high F/B of the yagi is predicated on the precise phase and magnitude relationship among the yagi elements, which can be disturbed by induced currents elsewhere. For example a 20 db F/B ratio is a 99% power reduction. Gain is also be affected, though to a much lesser degree.

As it turns out the pattern is affected only slightly. The F/B is not so much degraded as having the frequency of peak F/B shifted a small amount. Some pattern asymmetry is introduced (seen more on 20 and 15 in this particular instance), but it is small enough that I judge it to be entirely inconsequential. The gain reduction on 10 meters is < 0.1 db, and is lower yet on the other two bands.

The yagi contribution to the behaviour of the half sloper is different from what I expected. On reflection I see that my expectation was unreasonable. Let's look at that now.

There are currents on the yagi elements and boom as we expected to find. After all, one side of the transmission line is connected to them and the tower. The current plot is taken at 3.550 MHz, the frequency where I tuned the antenna to resonance. Notice that the current on the yagi is significant but not large.

In retrospect this makes sense. The effect of capacity hat placement increases from a small value near the centre of a dipole (or dipole variant) to a far larger value as it approaches the element end. That is, its effect increases where the impedance and voltage are high. A coil is the opposite, with its effect greatest where current is highest.

I tested this in the model by doing what is quite difficult in a real antenna system: I disconnected the yagi from the mast, leaving it free-floating in space above the tower. When I did this the resonant frequency of the half sloper rose from 3.550 MHz to 3.650 MHz. This isn't much. The feed point impedance also rose a small amount, from 22 Ω to 34 Ω, for this particular arrangement of wire and coil (more on this later).

The difference should be larger for a larger yagi (e.g. TH6), a longer mast or a stack of yagis. Even so, the effect of the yagi is less than I had believed.

Pattern

Once I had a functional model and began plotting the antenna patterns I got a few surprises. By that I mean my recollection of my 1980's half-sloper performance showed distinct differences compared to the present model. I believe that the new model steers closer to the truth.


I've focussed on the pattern at 10° elevation since DX is more of a challenge than short path for a small antenna on 80 meters. The antenna is clearly directional in the direction the wire points. However this is only true at low angles. At higher angles the azimuth pattern is less directional. At 10° the gain is -5.85 dbi, with a F/B of -4 db and F/S of -5 db.

Radiation off the ends at low angles is vertical, and is largely horizontal off the sides and in all directions at high radiation angles. This is an interesting mix in that it has radiation components that favour both DX and short path. It is however a compromise antenna, doing neither especially well.

I plan to point the antenna to Europe since it the most productive DX path for this locale and is short enough of a path that QRP has a chance. The forward gain of about -6 dbi may seem poor but for a simple antenna of this type on 80 it is actually quite good in comparison to alternatives. A large ground plane would lower the overall system loss but would contribute little to the far field gain at low radiation angles.

From my past experience I would have expected the DX performance to be better and the short path performance to be worse. That's the problem of only having one antenna for a band: there is little opportunity to quantitatively assess performance. This is where modelling can help if you are unable or unwilling to spend time and money experimenting with real antennas. In fact, it can help you direct your efforts in more fruitful directions.

Inductor placement

The placement of the inductor matters. The closer to the "centre" of the antenna it is placed the greater it will lower the resonant frequency. While that may seem attractive there is a cost: inductor loss. Current is highest at the centre and therefore the loss due to the ESR (equivalent series resistance) of the coil increases. But place it too far towards the far end of the wire and its effect is greatly reduced.

At the initial coil position 60% out along the wire the loss is about -0.1 db, assuming an ESR of 0.5 Ω for my junk box 17.5 μH coil. Loss increases to -0.2 db with the coil 40% out along the wire. I consider this negligible. Just be careful of losses if you are tempted to move the coil further inward or use a higher inductance in an effort to shorten the length of wire needed.

The other impacts are feed point impedance and SWR bandwidth. As you move the coil inward the impedance drops and the SWR bandwidth decreases. Again, the range of 40% to 60% out along the wire is a reasonable placement to balance the pros and cons.

If the wire can be lengthened it may be possible to eliminate the coil entirely even with a relatively short 14.1 meters tall tower. The wire will have to go out form the tower more horizontally. This requires more yard space and increases yagi interaction with the half sloper wire on the high bands. Another alternative is linear (capacitive end loading) of the wire element instead of a coil.

Matching

With a loaded element and a 45° angle between wire and tower this antenna is not a good match to 50 Ω coax. Its impedance at resonance is 22 Ω and, when matched, the SWR 2:1 bandwidth is 90 kHz. The feed point is near the electrical center of the antenna, in a 1 meter long wire extending horizontally from the top of the tower to the top end of the sloping, inductively-loaded wire.

I can live with the narrow bandwidth since with QRP my interest is primarily CW. A good match from 3.5 to 3.6 MHz meets my needs for both DX and contests. SSB QRP on 80 meters is not to my taste.

There are several ways to match the antenna at the feed point, and not with an in-shack tuner and the transmission line losses that can entail.
  • Quarter-wave transformer: If the model proves accurate in a real system (unlikely for this peculiar, highly-interacting antenna) the transformer characteristic impedance must be 32 Ω. You can get close to this with parallel runs of RG-59 or RG-11 (or RG-6).
  • Move the feed point: Impedance rises as the feed point moves towards the end of a doublet (of which this antenna is an example). Since the feed cannot easily be inserted into the tower leg of the antenna it would have to be moved further out on the wire element. Since the impedance changes slowly near the centre the feed point has to be moved quite far out to get a 50 Ω match. A current choke (or balun) would provide some protection from imbalance.
  • Gamma or omega match: One way to accomplish this is to run the coax out a suitable distance and parallel to the tower and use the outer conductor of the coax as the arm to short to the tower at a lower height. Variable capacitors can be inserted to assist with tuning.
The downside is that despite the model this is an unpredictable antenna with regards to environmental interactions. Therefore there is no certainty which of these techniques might work best, if at all. On the other hand you may get a good match by doing nothing other than plugging in the coax. I've found that this will sometimes work. I suggest putting up the antenna and measuring the feed point impedance, and only then decide how to proceed with the match.

Wire angle and direction

In the section above on inductor placement I mentioned that the wire can be tilted upward. In the model I tilted the wire upward by 30°, so that the angle between tower and wire is 75°. This is getting close to horizontal. If the wire were made fully horizontal you would have a loaded inverted-L. But let's stick with the intermediate step and see what happens.

There is now a deleterious interaction with the tri-bander when its elements point in the same direction as the wire. However, in this particular case with the inductor-loaded wire the only significant affect is on 15 meters due to the induced current on the half sloper wire. The azimuth pattern widens and gain is reduced by -1 db or more. Impact on the 20 and 10 meters is small enough to be ignored. Because the yagi feed system is imperfect in the interaction model I did not attempt to measure the impact on SWR. However there must be a change.

On 80 meters there are also interesting changes. First, the SWR improves to better than 1.5 at resonance. This is expected since the tower and wire are approximately equivalent to an inverted vee (actually, so is an inverted-L), and the feed point impedance of such an antenna increases with increasing interior angle. The resonant frequency also declines, in this case by 50 kHz.

Before you become tempted to do this there is more to be dealt with than tri-bander interaction. The pattern is also affected. While system loss improved from -6.2 db to -3.8 db that recovered power primarily goes to high-angle radiation. That isn't good for DX. At 10° elevation the gain degraded by -2 db. This should not be surprising since by raising the wire it becomes more horizontal.

With a more acute angle the antenna match becomes a challenge but it is a better DX antenna and yagi interactions are negligible. The more the antenna resembles an inverted-L the better the match, and the worse the yagi interactions and DX performance.

Comparison to an inverted-vee

The simplest alternative to this antenna is a loaded inverted-vee with leg configuration similar to that of the half-sloper wire element. Mutual coupling with the yagi is small and, if symmetrically positioned, negligible with the tower.

Without going into model details here is a summary of my findings:
  • Radiation at 10° elevation is -1 dbi worse in comparison to the loaded half sloper, but is more omnidirectional. Low-angle gain is highest in the end-fire direction, the same as the half sloper.
  • Impedance at resonance is 40 Ω, which is a good match to 50 Ω coax.  SWR bandwidth is about the same as the half-sloper.
This is a favourable comparison. You get some benefits at the cost of another leg and coil. These benefits include: match; omnidirectionality; more predictable match; and better match.

Of course there are far better antennas for 80 meters than either a half sloper or inverted-vee. But these are not compatible with a suburban lot and limited height. For example, I recently visited a super-station with a full-sized, steerable 4-square vertical array in a wide-open field and lots (lots!) of radials. Very impressive, but impossible for me.

Conclusions

It should be clear from this analysis that there is nothing particularly special about the half-sloper antenna. It is similar to an inverted-vee or inverted-L antenna, but is oddly structured, with an acute interior angle and unpredictable relationship to ground. That is, it really doesn't belong in a distinct antenna category.

Yet the antenna has advantages:
  • Easily turns the tower into "half" a low-band antenna, eliminating wire(s) and yagi interactions.
  • Decent DX and short-path performance, allowing a small station to do reasonably well on 80 meters.
  • Modest system losses without the need for an artificial ground plane.
Among the disadvantages:
  • Unpredictable match means that time may need to be spent adjusting wire placement for best performance.It could be aggravating to find that the perfect spot for the wire is where the kids will run right into it.
  • A matching network (e.g. gamma or omega match) may be desirable to avoid the need for a tuner and transmission line (coax) loss. 
  • Narrow SWR bandwith means this one antenna can only cover one 80 meters band segment. That implies the need for a tuner (remote or in the shack) or more than one wire if you want to operate the full band. In my case, if I ever feel to need to operate an SSB contest on 80 I can simply place a junction near the end of the wire and open it to move resonance up to 3.8 MHz.
  • The antenna has directivity so you'll have to choose which direction matters most to your operating objectives or, as already said, you'll have to add one or more wires.
My options are limited by space and antenna interactions. I will most likely build this antenna. When I do and I have some experience with it I'll report back. If I don't like it I can opt to convert it to an inverted-vee. But that could interfere with my plans for a vertically-polarized wire antenna for 40. There are further alternatives I have yet to explore.

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