Tuesday, April 4, 2017

Building and Tuning the Remodelled A50-6

Sporadic E season is rapidly approaching. We can expect the regular appearance of propagation on 6 meters beginning in early May. Unlike the previous two seasons I plan to be active with a far better antenna. Hence my remodelling of my ancient Cushcraft A50-6 to conform with the optimized design by W1JR in the more recent A50-6S.

With resumption of serious tower work still several weeks in future this is a good time to prepare antennas for the season. I can't even get to the Beverage termination to continue experimentation with it since the fields and bush are a boggy mess. Eventually the ground will thaw and dry, but that takes a while in this climate and our soil conditions.

For 6 meters antenna preparation is a relatively easy task since I have everything I need. Most of my other antennas require more extensive modelling, tooling and parts acquisition before construction can commence. So it was time to work on the A50-6 and get it ready to go.

The modelling went quickly, requiring no more than minor adjustments to my earlier work. The final design largely conforms to the A50-6S dimension. I lengthened the reflector, driven element and first 3 director by ~¾", and the fourth director a little more. Half element lengths:
  • Reflector: 59-¾"
  • Driven element: 54-¾"
  • Director 1: 54-¼"
  • Director 2: 53-¼"
  • Director 3: 52-¾"
  • Director 4: 49"
The first change moved the best gain performance to the bottom of the band where all my operating takes place. The second change boosts the gain ~0.3 db (by tightening the resonance range of the array) while having little effect on F/B and not degrading SWR below 51 MHz.

Element spacing is identical to the A50-6S. All dimensions were measured to within ⅛".

The original A50-6 design used equal spacing between elements. This can work well -- as shown by W2PV -- but not in this antenna! The W1JR optimized designed does much better although it does introduce mechanical considerations due to the clustering of elements at the rear of the yagi.


Before I could assemble the boom I had to straighten one tube that had a slight bend. This goes back to when I used the boom as a wire antenna mast a few years ago. The 1-½"tube with a more pronounced bend was repairable with the pipes I had on hand. A different technique was needed for the 1-⅝" tube. Looking around my large stock of material I found a 1" solid steel rod and a large tree that suited the task.

With straight edges and careful rotation of the tube in my workshop I precisely located the peak of the bend and the section of the tube that was curved. The peak was not at the centre of that section. I marked these points on the tube and went outside to visit the tree I'd selected.

The peak point was centred within the V where the trunk branched, conveniently at shoulder height. The steel rod was measured and inserted so the end was similarly centered. A small length of pipe collared the pipe where it exited the tube in order to minimize the risk of kinking the tube.

With everything in position I proceeded to push the rod until the tube yielded. It worked quite well. I then repeated the procedure with the bend point to one side then the other of the peak to smooth out as much of the bent area as I could. Although the result wasn't perfect you can only tell by sighting along the full length of the assembled boom.

A further problem with the boom is that the centre of gravity is off-centre. The A50-6S compensates for this by using a longer length of 1-⅝" tube on the long side of the boom. The A50-6 tapers to 1-½" for the last few feet. At some point I may want to replace the boom with heavier wall tubing to increase its wind and ice survivability.

Pointing straight up

As many hams know and as I described in a previous article a convenient technique for tuning yagis is to point them straight up. The reflector can be quite close to the ground, even 0.25λ can work well. The reason is that cancellation of radiation to the rear of the antenna reduced ground interaction. Keep in mind that all we're doing is tuning for best match -- not gain -- and that F/B has to be moderately good over the desired bandwidth.

I used EZNEC to model the antenna in this position to see the effect of ground and to determine how low I could safely go. I settled on 5' (1.5 meters, or 0.25λ) for reasons of available props, modelled performance, safe handling and accessibility of the driven element. The prop is a length of schedule 40 water pipe with a 1.6" I.D. which fits reasonably snugly over the 1-½" boom (plastic end cap removed).

I didn't use the Trylon tower as the support since the tower is very wide and could interact with the antenna more than I'd like. Instead I used a log frame in my backyard. I did my best to orient the yagi so that its elements were nearly orthogonal to the 80 meter inverted vee overhead and off to one side. It seemed to work fine even though I ought to have temporarily moved the inverted vee.

Notice how the coax is routed along the boom and straight down so that it doesn't interact with the elements. A temporarily common mode choke consisting of two cylindrical ferrites reduces the effect of RF conducted onto the coax outer surface.

Tuning it up

The antenna uses a gamma match for transforming the low impedance of the antenna to 50 Ω. In EZNEC I used an L-network since it is easiest to adjust in the model. These and similar networks have almost identical matched SWR curves so they can be considered equivalent. The EZNEC model optimized SWR curve for 50 to 51 MHz is quite good. I initially setup the gamma match per the A50-6S manual. That turned out to be a good starting point despite my design changes.

The objective is to see how well I can adjust the gamma match to get the same result. This assumes the SDC (stepped diamter correction) is done properly so that the real antenna and the model have the same resonance, and therefore the same performance metrics of gain and F/B. Since I've done this before and had good results I am confident that I've got a close match to the modelled performance.

It took about 30 minutes of fiddling with the gamma match until I got what I wanted. Each adjustment required moving a step ladder into place, making the adjustment (usually in ¼" increments) then moving the ladder away.

Look at the measured SWR curve and see what you think. Perfection isn't necessary since the environment for tuning is different form that atop the tower and stacked above an HF yagi. Further tweaking can be done then, though not easily since the driven element is far from the mast.

Performance note

Tuning an antenna for a match is at best an indirect indication of good performance; at worst it is totally misleading. Optimum gain and F/B per the model can quite easily be frequency shifted in the tuned yagi.

It is the shape of the SWR curve that is of more interest since it indicates that the rate of impedance change conforms well to the model. As an example, yagis typically show a sharp decline in radiation resistance near maximum gain. The rapid impedance change will appear in the SWR curve since the matching network cannot deal with these sharp swings and the SWR will soar. Too flat a curve is similarly an indication that something is amiss.

That the measured SWR curve is so close to the model gives me some comfort that the antenna performance will be as modelled is however no guarantee. That requires field strength testing.

Packing up

Declaring success I tightened all the adjustment fasteners and took down the antenna. It has been placed out of the way and off the ground until I am ready to install it on the Trylon tower. I have high expectations for this year's sporadic E season.

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