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01/21/12 |
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This section is to highlight our antennas - we'll add more as we have time to evaluation them and post the analysis here 110619 - From Gary - VE7AS Rebuilding an Old Classic When Bill took possession of a gift of an old Wilson 5 element 20 metre mono-band Yagi, I was given the assignment of getting it back on the air. I was delighted when the antenna came with its original manual, a bonus for sure. It also came with a lot of handwritten notes, mods and upgrades with someone's ideas on making the original design better – which I ignored the second time... This antenna, appeared to have had at least 2 owners before Bill took possession, each adding their own concept of how the antenna should be put together. So the challenge was to decide what version to use for assembly. To digress for a moment, the Wilson M520 Yagi was designed by W7GVA for Wilson Electronics in Henderson, Nevada in or around 1970. The Yagi Bill has, appears to have been initially purchased January 29th, 1972 by WA7BAY. At some point, the antenna was bought by Heinz VA7AQ and was in operation of over a decade, then disassembled and sat neglected for years...as old antennas often do. When it came time to unwrap the bundles of tubing and do an inventory, there were pieces that needed replacing – some because the tubing had cracked over time, and some because they were simply missing. About $50 worth of new tubing and hardware and we were on the road to putting some signals through the old Yagi once again. Spec time... It was amazing to see the original 1972 price tag was only $159.95 USD! Today, you would have to spend around $750 for the very same antenna....amazing... Gain was stated at 12db, very nice... that means if you pumped 1000 watts into it, you could expect 10,000 ERP for the first 10db and if it was 13db the extra 3db would double that to 20,000 watts effective radiated power, so 1 db less would still give you somewhere between 18K-19K ERP! Pretty respectable. B/F ratio, 20db – or 1000 times difference between signals received from the front and the back of the antenna. Boom 40 feet, width 36 feet. 6 square feet of surface area, 145 pounds of wind load up to 100 mph before the self-destruct sequence. Weight – 85 pounds. All-in-all, a very impressive looking antenna when it's all put together. Now I won't go through the gritty details of the first attempt to assemble the antenna, by suffice it to say, I learned this; 1. Use a tape measure with only ONE scale on ONE side; 2. Use friends who know how to read a tape measure; can see the numbers (make sure they wear their glasses) and can divide that number by two (to find centre); 3. Don't rely on several people taking turns “helping' you assemble the elements, because it is a documented fact, men can't multi-task well and; 4. Don' rely on those same friends to actually tighten all the clamps on all the elements for the same reason mentioned in item #3.
The end result is :
Above is a 14.0 to 14.350Mhz. VNA scan. The blue line represents the Return Loss, which should be greater than -30db down, it's only -8db – dismal. The red line is the SWR or VSWR and it's only below 3:1 between 14.080 to 14.140 – not good. The Impedance line [green] shows that it's below 50 ohms...not what I was expecting...so something's wrong... See items 1 to 4 above for the reason... So, a week or two later, I took it all apart and started from scratch. Used the original measurements, a better tape measure and worked alone. I added new tip elements, re-measured everything and decided to assemble the Yagi one element at a time and adjust the gamma match as I went, starting by taking the gamma measurements back to the manual's original lengths. Best decision I made...as it turned out. No success getting it to work with just the driven element on the boom. Discovering that a gamma matched “dipole” doesn't work well on its own – unlike a trapped dipole. So I added the reflector and made another measurement. As you can see, the 2:1 bandwidth expanded to 14.050 to 14.350, or basically 90% of the 20m band. Resonance was around 14.190 Mhz. And the Z [impedance] very close to 50 ohms, but more importantly almost flat across the band. So with just adding the reflector to the driven, we have a 3db gain 2el Yagi... Useable as is! Why? Because the gamma match needs the interaction of the parasitic elements to work it's phasing magic. Now time to add the first director...after re-adjusting the spacing, here is what I got on the VNA:
Getting better! 1.5:1 SWR or better across the entire band.... Impedance improvement too. Amazing how the interaction affects things. We now had a 3el Yagi with about 6db gain. Rather than adding the last two directors, one at a time and spend another hour playing with the gamma match, I added the last two directors, checked the spacing and then pointed the fully assembled Yagi north [the reflector near the C-cans]. I forgot to save a screen shot, but it was slightly higher at the top end of the band, more resonant toward 14.300 than the desired 14.200.... I should have left it, but I decided to tweak the gamma just one more time.... Note to self: If it's pretty darn close to a low SWR [below 1:5.1] and it's “almost” flat across the band...then leave it the heck alone! Oh, but no...I had to tweak it..and do you think I could make it better or even get back to the above measurements? No... sigh. L After admitting that I could not make it perfect, and rotating it 90 degrees – the antenna now pointing ESE or the boom basically parallel to the C-cans, here is what we ended up with: [screen print shot] 
Ok, we see the resonance has dropped considerably – down to 14.098 with a VSWR of a respectable 1.05:1[red line]. The return loss [blue line] is well below -30db which is what is expected with a low VSWR. The green impedance line is good too, staying close to 52 ohms. NOTE: The antenna is only 10 feet off the ground, too close to the C-cans and the propane tank so this graph will [no doubt] all change once the antenna is on top of one of the 50 foot telephone poles. We'll all just have to wait for the last VNA measurement...if only I we had the same success with the AP8 vertical. Stay tuned! 110517 - From Gary - VE7AS - Here are
two MiniVNA shots of your 6m antenna. One of them shows the entire 6m
band and where your yagi is in the whole 6m band. The bandwidth picture
shows the 2:1 bandwidth -- basically 50 to 51.100 without a tuner. The
red line is the SWR and the green line is the antenna's impedance, which
you want to see around 50 ohms give or take 10 ohms... So the antenna is
good for the CW & SSB section of the band, which is what we'd expect it
to do... You can expect 8.4 to 10 dbd forward gain from this yagi.
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