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| To: | <[email protected]> |
|---|---|
| Subject: | LF: Re: Cardioid RX antenna for LF... |
| From: | "James Moritz" <[email protected]> |
| Date: | Sun, 13 May 2012 21:53:34 +0100 |
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| Sender: | [email protected] |
Dear Stefan, LF Group,I have used terminated loops of the K9AY and EWE type on LF and MF - they work well down to VLF also. They offer a simple way of obtaining a cardioid pattern; a null of 20 - 30dB can be obtained if the terminating resistor is carefully adjusted - this means it must be possible to distinguish between QRM and general band noise so that the null can be detected. The physical design is not critical - changing the shape or size of the loop does not matter much as far as directivity is concerned, although the terminating resistance will have to be adjusted for the best null, and the signal level will be greater for bigger sized loops. I found values near 200R were right in the VLF - MF range, which is a bit lower than the values given for HF use. The resistor value also seems to depend somewhat on the antenna environment. The main disadvantages are that the output signal level is quite low, so a low noise preamp will be needed with almost all receivers, and that these antennas take up quite a lot of space. If space is restricted, the same result can be achieved by combining the outputs of small loop and E-field antennas using amplitude and phase adjusting networks to achieve a cardiod pattern, but the terminated loops need less electronics, and require minimal adjustment over quite a wide bandwidth. Cheers, Jim Moritz73 de M0BMU |
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