Dave G0MRF said:
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 built a pre-amp/LFtuner to add to the front end of a IC-706, which I used
when on vacation in Marrakech, October 2003. The unit used two G0MRF preamps
in series and must have worked reasonable well because it was used in a
crossband 136kHz/7MHz QSO with G3AQC. The antenna was a 20m long doublet,
fed with 6m of 400ohm ladder line, resonated using the tuner against the AC
supply ground.
The pre-amp/LF tuner was described in RadCom, Antennas column, December
2003.
I have a PDF file of the article for anyone who would like a copy.
Peter, G3LDO
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