Laurence KL1X/5
Date: Tue, 4 Jan 2005
Tom see
http://www.g0mrf.freeserve.co.uk/preamp.htm
I fitted two of this is a small box and literally connected them in series
They are nominal 50 ohm in/out devices and only cover the 137Khz band - sounds like a Q mulitplier as you tune across the band with the pre amp on...
Ive not run figures on them in series but the gain is sufficient for the 817 with a bit to spare on a medium sized loop
Ive just modified the outboard preamps for the FT817 and now have to "seried" 2 x g3yxm/mrf bandpass amps and thats woken the 817 up! More than sufficient gain and I may end up putting a T 50 ohm attenuator between the two small boards to limit it a bit...

Steve Dove W3EEE
Date: Tue, 4 Jan 2005
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:
http://w7iuv.com/preamp.jpg
only using FT50-J cores instead. It is broadly similar to a preamp by our very own Mr. Rusgrove, I believe the details of which are on John, W1TAG's site.

David G0MRF
Date: Tue, 5 Jan 2005
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.

David G0MRF
Date: Tue, 8 Jan 2005
As supplied by Toko, the variable Inductor measures 235uH. They require a small adjustment of the ferrite core to bring them onto 136kHz. You can probably calculate the final value from the resonating capacitor values used in the pre-amp.

Mitch VE3OT
Date: Tue, 9 Jan 2005
Try
http://technology.fanshawec.on.ca/eltn124/loops.htm
and a rough circuit of the Burhans article and circuit has just been dropped off at
http://www3.sympatico.ca/mitch.powell/burhanx.jpg

Alberto I2PHD
Date: Tue, 10 Jan 2005
> I'm sure some people have described mods where this transformer is changed to improve LF performance, but I can't think who at the moment.

http://www.mods.dk/view.php?ArticleId=2711