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I have a shortwave receiver and I am using a dipole antenna.In order to receive week signals, I want to use a pre-amp between the antenna and receiver.
Any friend here knows a suitable pre-amp circuit or any link?
Do a google search on things like 'active antenna' or 'active antenna AA-7' and youll find a few things. 'dxing.com' or organizations like National Radio Club might also be good places to look.
I have lots of schematics though none in electronic form that I can post. I ran across a nice article by Joe Carr that discussed filters, pre-amps, attenuators - collectively - and what you might expect in terms of performance improvements.
My own experience suggests that if you live in or near a city and your receiver is not a high performace model that you might benefit from some filtering. Strong signals that are not on the frequency of interest can still affect the receiver. I built and installed a broadcast band filter and it eliminated a large amount of junk/noise that I heard from about 2 mHz to almost 7 mHz. My receiver is a superhet/moderate performance.
Another alternative is a tuned loop. This can work well on stuff from 10 mHz on down. I built one for broadcast band (550-1600 mHz here in US) and even without a pre-amp the results were impressive.
A pre-amp may not improve things but only way to know for sure is to try it. If strong out-of-band signals are causing problems they might get worse with pre-amp. If you construct the pre-amp you can then add filtering and other features - so all won't be lost.
Thanks very much Stevez. Your idea of broadcast band filter is a nice one. I will try it and see. Tuned loop is a bit of a problem since I do not have much real estate.
My receiver is a quite sensitive one. The area that I am interested in, is SSB signals. If you have schematics I would appreciate it very much if you send it to met through my e-mail ravi@ualink.lk
Meanwhile I will do a search on the web. Thanks again.
Ravi - a tuned loop that is 0.5 meters square is sufficiently large - and a smaller one will work. Sorry I can't point to a website but National Radio Club and other shortwave listening sites probably have better information.
The tuned loop is a coil of wire that could be 0.5 meters in diameter and 20 to 30 turns spaced about 5 mm apart. Mine is built on a square wooden frame - whether square or round the idea is the same. A capacitor is connected in parallel with the loop so that it resonates at the frequency of interest. Several turns of wire along side the loop turns and some feedline connect to the antenna. The axis of the loop is horizontal - and the frame is often mounted on a swivel so that it can be rotated. The loop is very directional. A preamplifier is often used. Less turns are required as the frequency goes up. I've heard from others that a loop as small as 0.2 meter is still quite effective. The ferrite bar antenna that is common in MW or AM broadcast band radios is simply a compact version of this antenna.
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