Repairing a Steppir Biggir Vertical – part 3 – Chasing My Tail
This is part 3. You might want to start with part 1 of this adventure.
I have been repairing and designing electronics for decades. I have been doing this for so long that I know others that have been in the business this long have also experienced situations where our judgement was wrong, so we do not judge others.
After rebuilding and testing the Steppir 80m Coil assembly, I installed it on the base of the vertical. The Coil Assembly mounts to the Element Housing Unit (EHU.)
When I put up the Biggir early 2019, I did NOT install the “High Wind Kit” (HWK). As stated earlier, the HWK was made of aluminum and Steppir clearly states that you can not use the HWK on the Steppir Biggir vertical if you have the 80m coil AND want to run an amplifier.
Well, I had bought the newly available fiberglass plate that is used instead of the aluminum plate, so now there is no problem running an amplifier.
For the two weeks or so that I the 80m coil kit was removed from the vertical, I was happily operating 40 – 10m, sometimes using the amplifier. So after reinstalling the 80m coil I was happy to be able to return to 80 meters.
Nope! I watched the SWR jump from 1.4:1 to ‘full scale’ and immediately shut off the amplifier. Hhhmmmm…. at 100 watts everything was ….. oh oh… it was fine for a few seconds, now SWR is ‘full scale’ again. Turn the rig down to 20 watt and SWR is 1.4:1 again. Turn power up to 30 watt and suddenly I get “full scale” reflected power.
Investigating further, I discovered that everything was fine on 40m through 10m. Even with the amplifier on, everything was fine on 40m through 10m.
Clearly, there is something wrong with the 80m coil kit. So I removed it, dismantled it and inspected it. Everything looked fine. Suspecting that the 4:1 transformer had gone bad, I rebuilt it. No improvement 🙁
Repeatedly, I removed the 80m coil, cleaned stuff, checked stuff, and reinstalled and I got the same exact symptoms. I unwound the coil…. cleaned the coil form again.. tested the coil form in the microwave oven, reassembled, and no improvement. I was chasing my tail and getting nowhere.
This is baffling. Everything seems fantastic on 40m through 10m running 1.5 kW, but I can not run more than 30 watt on 80…. how can it be anything other than something wrong in the 80m coil kit?
Ahhhhh…. remember the case of the low-power-only antenna ? So now it was test, test, test, test…. what was wrong with the 80m coil assembly?
THINK ! THINK ! THINK ! What can it be?
OK…. the feedpoint of the antenna on 40-10m is in the ballpark of 50 ohm. 1.5 kW into 50 ohm results in 5.5 amp rms and 273 volt rms. What happens on 80m?
So 5.5 amp, 273 volt is applied to the input of the 4:1 transformer (impedance ratio) when we are on 80m, so out comes 11 amp, 137 volt… where does this go? Into the coil. The coil taps are approximately
- tap 1 (switch position 2) 5 uH
- 15 uH
- 16 uH
- 17 uH
- tap 5 (switch position 6) 18 uH
At 3.5 MHz tap 5 will give us approximately 400 ohm reactance.
Let’s calculate what happens at 50 W (because the numbers are so easy.) At 50 ohm in our coax, our 50 W results in 1 amp at 50 volt. That 1 amp goes into the transformer and comes out as 2 amp, 25 volt. 2 amp through that 400 ohm reactance creates 800 volt. Add that 800 volt to the 25 volt at the output of the transformer, we have 825 volt leaving the 80m coil kit and going to the Element Housing Unit (EHU.)
Aha ! 1,500 W on 40 through 10 meter is ‘only’ 273 volt, but 50 W on 80 meters gets us 825 volt.
More interesting is when we run 1,500 W on 80m. We have 137 volt, 11 amp going into the loading coil. 11 A x 400 ohm reactance is 4400 volt. Plus the 137 volt at the input end of the coil, there is 4537 volt going to the E.H.U.
So now I understand how the problem might not be in the 80m coil assembly.
Let’s disassemble the EHU in the part 4 of “Fixing a Steppir Biggir Vertical”