Hi Joe,
Laurence,
I have an FT857D (a wattier 817) in my car, and its deafness
is of heroic
proportions. If I were asked to come up with something
that worked so well at
HF but so dismally at LF, it'd be a struggle.
It can't hear an NDB unless it's
within about a hundred yards of it;
absolutely amazing.
So similarly, I use an Altoids tin containing a
500kHz low-pass filter and two
series'd preamps to render it useful.
Each preamp's configuration is based on:
Hello and happy new year to all.
There have been a number of postings about the poor response of amateur
radio receivers on 136kHz.
The main reason for this lack of performance is usually the internal
attenuator that is switched in below 1.8MHz. On ICOM radios, the broadband
transformers also run out of performance below 500kHz. This general problem
seems to be more severe with radios that have already had their
performances streatched to cover the 6m band. e.g. The FT1000 and TS850
work well ( 28MHz max) but the IC756 746 and the FT817 are about 20dB
'deaf'.
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.
73
David G0MRF
