Dear all,
I want to share an experience hat might be interesting for others too:
After moving from 501-504kHz to 472-479kHz I noticed that
even my lossy antenna needs retuning within the band.
As it is not very convenient to do that at the loading coil
(abt 30m from the house) I decided to install a small DC
motor for remote tuning the loading coil. Tuning can be done both
at the TX (in the shack) and at the loading coil. Between both
is about 40m of coax cable.
A first test done at the loading coil showed that it worked
fine and I tuned the antenna to maximum current (550 mA for 20W TX power) on
475 kHz (+/- band mid).
When I went into the shack and from there I tuned
the antenna to maximum current at the lower band end (472 kHz) I noticed 2 things:
1. The variation of the loading coil was much more than expected.
2. The maximum current (at the TX output) was 760 mA.
Very strange. So I went back out to the loading coil to measure
the antenna current (leaving the remote tuning in the position for
maximum current at the TX). The current had dropped to 410 mA and I had to do some serious retuning
to get it back to maximum (550 mA).
Back in the shack I measured the RF current at the TX: also 550 mA (as it should
be).
So despite the fact that the coax cable is merely
10% of the wavelength it seriously affects remote tuning.
I did some calculations and even with a short coax cable
(10 m) this effect should be noticable.
By the way: remote tuning can be done correctly by tuning for best SWR instead
of highest RF current at the TX output.
73, Rik ON7YD - OR7T
