472kHz - an analysis of EA5DOM with and without RX preamp

The night before last EA5DOM (1525km) was copied 45 times on 472kHz with the preamp in circuit, so the FT817 was at maximum sensitivity. My best report of him was -16dB S/N.

Last night he was copied again but without the preamp. Last night I copied him 35 times with a best sensitivity of -19dB S/N.

Although conditions may have been different, I think the 472kHz preamp is worth having. It allows signals too weak to copy to be successfully decoded. Incidentally, EA5DOM was copied as late as 0740z today at -26dB S/N without the preamp on 472kHz WSPR.

472kHz WSPR RX overnight with preamp

Overnight, I used the FT817 on MF RX with a preamp and the (short) earth-electrode antenna in the garden. Subjectively, the sensitivity is better and I am able to copy stations at better strength. I will do a straight comparison of sensitivity, with and without the preamp, using a signal generator later.

These are the unique station spots given to others overnight. Many stations were copied many times of course:

Unique 472kHz WSPR stations copied overnight

472kHz preamp

As mentioned some days ago, I received a 472kHz preamp from a very kind, anonymous, person recently. In the next few days I need to check FT817 sensitivity with and without the preamp. Assuming it makes a very useful difference, I'll try 472kHz WSPR receive to see what I can copy using the earth-electrode antenna at this time of year when the LF and MF bands are still quite noisy.

Loop preamp

After some thought I'm almost certain now to buy a Wellbrook RX loop for use on 136 and 472kHz receive. This loop will also be useful for a number of other applications such as 160m and 80m monitoring and possibly some VLF work.

The ALA1530LF looks perfect for the task being able to cover 20kHz to 30MHz with excellent IP2 and IP3 performance. One concern is that a receiver is being hit with signals from right across the spectrum: although the loop preamp itself will not fall over, the receiver after it might. So, I think a small, high dynamic range tuned preamp with a few dB of gain may be needed ahead of the receiver.  The additional gain may be needed as many HF rigs have poor LF and VLF sensitivity.

A simple preamp is easy to design for either 136 and 472kHz bands. Something along these lines (see below) should be suitable for 472kHz reception. I'm not actually sure that the ceramic filter will be needed: just the single hi-Q tuned circuit on the input may be enough to protect the subsequent RX from being blasted with everything.

472kHz (quiet) RX preamp

When the grandchildren and family have all gone home next week, my first project will be to improve the RX antenna system for 472-479kHz receive. As mentioned before, I am suffering badly from SMPSU noise pick-up inside the house, so plan to install a magnetic loop and preamp somewhere towards the bottom of the garden in an attempt to minimise pick-up from my own home and from neighbours. With a loop there is also the ability to sharply tune the antenna and to null interference from the worse directions. The circuit I propose to use is a version of my 9kHz tuned preamp, which has been used to copy G, DL and OK amateur stations on 8.97kHz last year. The loop will simply be retuned to the new MF band.  Not sure whether series or parallel connection to the loop will be better as both should work depending on the FET stage configuration (common base or common source).

Proposed tuned preamp, but with values changed for 472-479kHz