5 Sep 2013

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.

2 comments:

John GM4SLV said...

Hello Roger,

I my experience there is little to be gained by adding a pre-amp after a Wellbrook loop - the loop's own amplifier effectively sets the system noise factor. I did a lot of tests on MF on weak daytime NDBs and a tuned pre-amp and didn't see any improvement in received S/N ratio with a tuned pre-amp after the loop. The same tests with a simple wire antenna showed a big improvement in S/N ratio when using the same tuned pre-amp. I didn't consider the protection from RX overload that may be afforded by using the pre-amp, and this may be more significant than the gain itself.

John
GM4SLV

Roger G3XBM said...

Thanks John. Just a bit concerned that the FT817 is a bit deaf on 136 and 472kHz. As you say, adding a bandpass filter to give protection from a full 30MHz of signals hitting the front end of the RX may be the more important benefit of a tuned, high dynamic range preamp between the loop and RX.