Sunday, June 7, 2015

Solstice Antenna for 6 Meters

From 1985 to 1992 I was very active on 6 meters. I enjoyed the propagation surprises it offered, including some fascinating DX. I worked hundreds of grid squares, worked every province and territory, all US states, all but one continent and about 65 countries. I'd have worked many more countries had there been more countries that permitted the band to their hams back then. In short, 6 meters was one of my favourites.

Since returning to the air in late 2012 after 20 years absence I occasionally pondered putting up an antenna for 6. I have made a few contacts using my KX3 and antennas for the HF bands though not without difficulty. After putting up the new tower in 2014 there was no point since the sporadic-E season was long past its peak. So 2015 is the year to get it done.

With the CQ WPX CW contest out of the way and no other HF contests I want to enter the rest of the summer I have an opportunity. These seemingly unrelated endeavours are connected by the availability of transmission lines on the tower: a yagi for 6 will need to use the coax currently feeding the 80 meters half sloper. Of course that means this is a temporary configuration that I will undo once the mid-summer (solstice) sporadic-E season is over.

Thus my objective of a "solstice antenna". It has to be small yet effective. Some trade-offs are allowed since it will be in place for only 2 months. These are the challenges I must deal with:
  • Use only parts on hand. This, as we'll see, is the easy part.
  • It must go up fast and easy. Sporadic-E season is right now so time is of the essence.
  • Design or copy the design of a small yagi.
  • Tune and optimize performance.
  • Interactions with the Explorer 14 antenna sharing the same mast.
I'll walk through the steps to get myself back on "magic band".

Parts on hand

As I said above, this is the easy part. It's easy since I still have the yagi I used so many years ago: Cushcraft A50-6. That's a big antenna (1λ boom length) and unsuitable for the present circumstances. But with all those parts it is quite easy to build a smaller yagi.

In fact all of the parasites are mechanically identical, only requiring adjustment of the element tips for tuning. The centre section is 4' ( m) of 3/4" (19 mm) tubing and the tips are 5/16" (16 mm) tubing. The boom can be built by selecting tubes of the total 20' (6 m) original boom length. Alternatively another tube, aluminum or other non-conducting material can be substituted. There are numerous options to choose from my junk box.


I will not dawdle with this antenna. I aimed for fast turnaround from concept to on-air. This objective not only explains why I would only use parts on hand but also my focus on tried and true designs. For example, while a Moxon rectangle is light, small, easy to feed and simple it would require fabrication and some experimentation with design and tuning.

For this reason I chose to stick with a yagi, one based on an existing Cushcraft design. If that latter point seems odd keep in mind that the tubing schedule affects element resonance and I have Cushcraft elements. So all I needed was a Cushcraft manual for a yagi smaller than the A50-6.

Primarily for reason of interaction with the Explorer 14 (see further below) I opted for a 2-element yagi rather than a 3-element yagi. Other reasons are to keep wind load to a minimum (1 ft² in this case) and a 1 meter long boom is easy to lift straight up the tower between the guy wires. The 2 meter boom of a 3-element yagi would require more work to get past the guys. The sacrifice of up to 2 db gain is acceptable for this temporary antenna. Bandwidth and F/B are unimportant and were ignored in the design.

Design process

With the decision made on the antenna type I looked online for an A50-2 manual and...there is no such product. The smallest Cushcraft yagi is the A50-3. Unfortunately you cannot make a 2-element yagi from that by simply removing one of the parasitic elements. Antenna resonance will drastically shift. This called for some computer modelling.

In a typical 3-element yagi the director and reflector elements are resonant at equal distances from the centre operating frequency. In the case of the A50-3 these are approximately ±4%. Since in a 2-element yagi with a reflector the reflector should be resonant at the centre operating frequency, or +4% from where it is in the A50-3. However that is an approximation.
Dimensions for the Cushcraft A50-3

What I did was take an optimized 3-element yagi model of similar dimensions and remove the director. I then adjusted the reflector length to re-centre the antenna at the desired frequency. I found that for a gain-optimized spacing of 0.17λ (1.0 meter at 50.1 MHz) the resonant frequency needed to be shifted upward by 3%. Since I don't know that the A50-3 dimensions for the low end of 6 meters are what I want it is risky to shorten the reflector by 3% and expect the tuning to be correct. The element must be precisely modelled.

Unfortunately NEC2 does not correctly calculate reactance on wires made from telescoping tubing. It is necessary to use EZNEC's Leeson stepped diameter correction feature. I did this, following the necessary design steps to ensure the correction can be applied and would be reliable. The metal boom was added to the design since that can have a small effect -- the boom increases the effective element diameter where they cross.

I modelled the A50-3 reflector as a standalone element, confirmed the frequency of resonance was where it ought to be (~49.8 MHz), then shortened the outer tube sections by 3%. I added the driven element, using the A50-3 dimensions, 1 meter in front of the reflector. The boom was extended up to, but not touching, the driven element. The boom cannot connect to the driven element in a model with the split source required for the Leeson correction.

To my surprise the gain and F/B performance centred right where I wanted them. Free space gain is 6.9 dbi. The antenna impedance was also as expected from theory, ranging from 33.8+11j Ω at 50.0 MHz to 37.0+18j Ω at 50.3 MHz. The reactance is likely inaccurate, due to the boom. Even so I expected the impedance would be within the adjustment range of the gamma match -- I was almost right, as we'll see. Impedance declines as the boom length is shortened, and might be required if a match cannot be obtained (the A50-6 has a lower impedance and this gamma match is from that antenna).

The elevation plot of the 6 meter yagi at its intended height of 14.1 meters is on the right. There are many minor lobes at higher angles due to the wide beamwidth and being 2.4λ high. For the same reason the modelled ground loss models as -1 db.

Tuning it up

The antenna does not need to be high off the ground for tuning. Even 2 meters (0.35λ) is enough to ensure that the tuning will be the same when raised onto the tower. Adjusting the antenna at this height is easy. It is only necessary to get your body out of way to take measurement (your body couples to the antenna) and to keep some free air around the antenna so that the environment does not interact.

The test setup is shown at right. Antenna work on the deck on a sunny warm day is a pleasant activity. The KX3 and an SWR bridge are on a table. Notice that the reflector is nearest the house so that open air is in front of the antenna. This minimizes interactions with the house and deck. A step ladder is used to make adjustments to the elements and gamma match. It must be moved out of the way each time to avoid interactions. A fibreglass ladder would be a better choice.

I adjusted the reflector tip length to 83 cm per the model as adapted from the A50-3, as discussed above. For now I have to trust that this will place maximum gain at 50.1 MHz since I have no easy way to confirm it. If I'm off even 1% (500 kHz) the gain remains near maximum. One way to test that reflector tuning is not wildly off is if the gamma match can't come close to achieving a good match at 50.1 MHz.

I couldn't get the SWR lower than 1.2 at 50.1 MHz with the gamma match alone (maximum capacitance at the best impedance tap point). I shortened the driven element 5 mm per side to get the job done. SWR is no more than 1.1 at 50.15 MHz. Since the required bandwidth is narrow (0.5%) the SWR is near perfect from 50 to 50.2 MHz.

I did have a bad interconnect cable that created an impedance bump between the rig and SWR bridge so the rig's SWR bridge read a higher value until I fixed that problem. I recommend testing all coax or you could end up wasting a lot of time with fruitless tuning. Sometimes coax that tests good at HF shows an anomaly at VHF.

As a last test I raised the antenna by one more 4' fibreglass mast section. As expected the impedance was unchanged. If 6 meters had been open I should have been able to work someone. The band was dead so I dismantled the test setup.

Interactions with the tri-bander

This antenna will have to be positioned close to the Explorer 14. When mounted directly atop the mast bearing it is only 80 cm below the tri-bander. Interactions are inevitable. The expected risk was more on 6 meters performance since it's the smaller antenna. However I did have some concerns about the Explorer 14 para-sleeve, which is short and could compromise 10 meter performance.

I used EZNEC to place a model tri-bander and the new 2-element 6 meter yagi together as they will be on the tower. Although I know that the model cannot be accurate I want to at least identify areas of concern. The model should be sufficient for that even though the Explorer 14 is not the same as the model tri-bander.

The current plot above is with the 6 meter yagi fed at 50.1 MHz. This is the worst case condition which I will further analyze. I was gratified that the effects of the 6 meter yagi on 20, 15 and 10 gain and impedance were negligible. Even the F/B, which is the most sensitive performance parameter, was little affected, even on 10 where interactions are maximum.

The presence of the tri-bander has a more profound effect on the 6 meter pattern. Somewhat surprisingly the effect on impedance was very small: only 2 or 3 Ω in both real and imaginary components. When the antenna goes up this can be easily confirmed, more easily than gain can be.

While small in magnitude the current induced on each of the tri-bander's elements has a significant impact on all aspects of the pattern. To see this more clearly I moved the model to free space then overlaid the patterns with the tri-bander absent and present. It is perhaps unsurprising that the tri-bander elements acted as weak reflectors, tilting the elevation pattern downward. More interesting is that the side nulls common to any dipole or yagi with dipole elements have been filled.

When returned to real ground and the stack at the design height the 12.57 dbi gain shown earlier was reduced by -1.5 db. This is not desirable though I am prepared to live with it as a temporary way to get on 6 meters this summer.

Since I did not include a boom in the tri-bander model I chose not to model interactions with the 6 meter yagi rotated 90°. I may try this arrangement after the antenna if the interactions prove to be a significant problem.

Up in the air

Not a pleasant perspective for those with vertigo
From start to finish this project spanned 7 days. Design and testing were the largest tasks. Actual time spent was minimal. The biggest gap in that week was between completing tuning and hauling it up the tower. The weather was fine Sunday morning and I had a couple of free hours so I went ahead and installed it.

The first thing was to orient the antenna so that it would not tangle the guys during lifting. Without help on the ground to pull the antenna outward at critical points this can be a problem. I had just such assistance the previous day when I put up an antenna for another ham that had to be steered around various obstacles.

I rigged the rope so the element ends were pointing upward. Of course it all went awry when I got to the top of the tower and pulled. With such a short boom I was still able to maneuver it by jostling and swinging the rope. This would have been much more difficult if I had built the antenna with 3 elements. So far so good.

The next problem was the stacking distance. I neglected to account for the mast bearing height and tri-bander distance below the mast apex in my interactions model. Even with the small boom-to-mast clamp the vertical distance between the yagis is 60 cm (2'). Not surprisingly this increased interactions, easily seen by the increased SWR.

The coax for the drip loop is a bit short. I chose it anyway to avoid having to make a new one. To compensate I slid the boom so that the driven element is closer to the mast. I couldn't get too close or the gamma match would touch the tower! Asymmetry in such a small antenna will not unduly stress the rotator.

After sealing all the coax connections and securing the cables I came down the tower to test out the antenna.

Testing it out

The 40 meter (130') run of RG-213 is less than ideal for VHF. The loss for this run calculates to -1.9 db. Although the cable is good it is old so let's say that the loss is -3 db. With a shack-measured SWR of 1.3 the overall transmission line loss is still approximately -3 db. The 8 watts from my KX3 is only 4 watts at the antenna. Receive sensitivity is reduced by the same amount.

There was a marginal sporadic-E opening at the time I was testing the antenna. I did not attempt a QSO since signals were weak and fleeting, unworkable with my low power. Instead I compared received signal strengths versus my other, non-resonant antennas. On that basis the antenna was working, though perhaps not as well as I hoped. A better assessment of gain is difficult form this test since the Explorer 14 on 6 meters is so large that its pattern would be quite complex, and likely has gain in some directions.

To be continued

Later Sunday there was another opening. This time I made one QSO. That at least showed it is doing something right. For what it's worth I was complimented on my QRP signal.

Since this is a temporary antenna I declared that, for good or ill, this project is a success. I'll be back with an update once I get to work a better opening, including checking its F/B. I am even considering playing in next weekend's ARRL VHF contest. If 6 meters opens.

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