Radio amateurs typically put in a 100 pF high voltage capacitor across the primary of an end fed half wave transformer. I had the opportunity to test the range between 60 pF and 180 pF using a polyvaricon type of variable capacitor.
The results are as follows:
Fully CCW is 180 pF fully CW is 60 pF
On 40m the 2:1 SWR bandwidth is slightly improved from 370 kHz to 390 kHz
The SWR dip is also slightly better at 180 pF as compared to 60 pF which went from 1.45 to 1.38
On 20m the 2:1 SWR bandwidth was improved from 615 kHz to 645 kHz
The SWR dip was also improved from 1.3:1 to 1.09:1
On 15m the 2:1 SWR bandwidth was improved more dramatically from 0 kHz (the 15m band was above 2:1) 1155 kHz
The SWR dip was also much improved from >2:1 to 1.02:1
The 10m band was hovering just above 2:1 SWR
My standard QRP transformer on a recent build with the 100 pF capacitor measures as follows:
40m 580 kHz 2:1 BW with a dip at 1.8:1 7.00 MHz
20m 760 kHz 2:1 BW with a dip 1.03:1 14.14 MHz
15m 960 kHz 2:1 BW with a dip 1.2:1 21.4 MHz
10m 970 kHz 2:1 BW with a dip 1.5:1 28.8 MHz
Moreover, checking the SWR with a radio configured for field operation with the ATU bypassed with an 8 foot RG174 feedline. The following are the results using the SWR bridge in my KX2
The 40m band is usable from 7 MHz to 7.25 Mhz with the lowest dip at the CW end at 1.5:1
The entire 20m band is < 2:1 with SWR being 1.5:1 at 14 MHz dropping to 1.2:1 at 14.35 MHz
The entire 15m band is < 2:1 with SWR being 1.7:1 at 21 MHz dropping to 1.0:1 at 21.45 MHz
The 10m band is usable between 28.68 MHz and 29.6 MHz with a dip at 1.7:1 at 28.8 MHz
So indeed the "goldilocks" compromise for the transformer is with a standard fixed high voltage (1 kV) 100 pF capacitor. Fine tuning it with a variable capacitor is probably not worth the effort. Furthermore, there is a penalty in transformer loss in the 10m band
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