[Amps] High SWR on 15 mtrs for SB220 - Follow-up

[Bill Spickler]

Here is follow-up to my question from about a month ago.  My problem was a high SWR into my SB220 on 15 mtrs only.  All other bands worked okay with SWR around 1.3:1.  On 15 mtrs the SWR was 3-4:1.  I had replaced the caps in the input circuit as I doubted the coil could be bad.  That didn't help.

I got a number of good suggestions from the list and most recommended re-tuning the input coil on 15.  I also know where to find new coils if needed.  Thanks to everyone for your suggestions.

So.... here's what I did.  I purchased some nice ceramic tuning wands and re-tuned the input circuit on 15 with the amp hot.  I was nervouse but kept one hand in the back pocket and survived the ordeal.  I was able to null the input SWR at about 3:1 with the tuning slug all the way in the coil.  Next I tried changing frequency and found a null with SWR of 1.8:1 at 21.700 MHz.  So I reasoned I needed more C in the input circuit.  I tacked an additional 22 pf and rechecked the SWR at 21.100 MHz --- no improvement.

I have been able to tune the SWR down to 1.2:1 with the built-in tuner on the TS940 so I finally decided that would be the best solution.  One last thought was to try a different length of coax between the 940 and the amp.  I was using a 44.5" piece of RG8U.  I added another short coax of the same length (kept the original in series as a test to see if it was bad) and guess what..... SWR 1.3:1, no tuner.  I then tried another jumper, RG8X 60".... SWR 1.2:1.

The experience has been educational.  One lesson is don't change your station setup once it's working well, and remember the fundamentals.  I'm not sure what's magical about 44.5" of RG8U and maybe someone will calucalte what wavelength this is, but it is working now so I'm not complaining.  Thanks again and keep those filaments glowing.

73s

Bill, NU6I