Adding a yxm/mrf pre-amp will overcome the loss of the input attenuator
(apx 10dB) and will have a few extra dB to help things along. However, I have
had a couple of mails from people who expected the pre-amps to work when they
were added to simple small verticals or wires.
The pre-amps are 50 Ohms (ish) in and out while the impedance of a
typical 7MHz vertical is a very small fraction of 1 Ohm. The loss between them
is huge. You need a matching network.
I remember in the early days of 136k here in the UK working one
enterprising operator who had built a trasnsmitter and had matched his HF
vertical by adding a reel of enamelled copper wire in series. Tuning was
achieved by sliding a ferrite rod into the center of the reel and adjusting
for best signal. - Wonderful!
A small antenna that works well is the e-field probe as promoted by
AMRAD a few years ago. This is a short antenna with a very high input
impedance FET amplifier.
However, the e-field probe is an exception. If
you want to get any reasonable signal, you need to match the
antenna................then on most amateur sets, add a
pre-amp.
RE: Portable LF ATU article by G3LDO RadCom December
2003
In another posting, which I unfortunately deleted by mistake, it was
suggested use of the above for matching to "short" antennas.
I'd be very grateful to hear from anyone who's build this ATU and got it
working satisfactorily.
I can't get the LF Loading Circuit section of mine to tune-up at all,
it's completely flat with any values of selected C.
I suspect it's all down to my fabrication of the Loading Coil, ferrite
used etc. so it'd be very helpful if I knew at least the value of
inductance I should be aiming at for this particular cct. then I can mess
about with number of turns, ferrites and so on...
Mni tnx in advance.
..Dave
