Message-ID: <001b01c4f0bd$9a6f7f20$eb1686d4@erica>
From: "g3ldo" <g3ldo@ukonline.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <82.1ecc794c.2f0d0993@aol.com>
Date: Sun, 2 Jan 2005 11:23:27 -0000
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Subject: LF: Re: Pre-amps on LF
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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