Tuning the 80m Coil

Now it’s time to adjust the 80 meter coil. The current setting for all three bands are given below

L / Short
fc [MHz]
80m 36cm / 0 4.02 1.40 100 kHz
40m  26cm / 7.5 7.04 1.05 274 kHz
30m  26cm / 2.5 9.98 1.20 750 kHz

By decreasing the length of the 80 meter coil I could bring fc down to 3.67 MHz. Unfortunately, this messed up completely the 40 meter settings moving fc down to 6.9 MHz – quite unacceptable for me.

 SWR on 80m and 40m using L80=25cm
 HF2V SWR on 80  HF2V SWR on 40

Going back to L80=31cm makes 40 and 30 acceptable, however 80 meter is completely useless:

 SWR on all bands using L80=31cm
 HF2V SWR on 80 HF2V SWR on 40  HF2V SWR on 30

On the other hand, if I stick with L80=36cm, I could try to recover the 40 and 30 meter bands by tuning the 30m coil. This could indeed work out, since L30 did not have any influence on the 80 meter band. In any case, I have to do something about this because as it is now, I have an antenna, which works so-so on all 3 bands, but none of the bands is optimised.

FYI, an index of all SWR graphs can be found here.

Author: Alexandru Csete

Embedded software engineer in the satcom industry during the day. Radio amateur and SDR hacker during the night.