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Re: LF: Receiving

To: [email protected]
Subject: Re: LF: Receiving
From: "Mike Dennison" <[email protected]>
Date: Fri, 16 Apr 1999 12:29:16 +0100
In-reply-to: <[email protected]>
Organization: Radio Society of Great Britain
Priority: normal
Reply-to: [email protected]
Sender: <[email protected]>
DJ1ZB wrote:

But the blocking of the receiver is not caused so much by these near-by signals but by the sum of all stations delivered by the aerial, especially the LF broadcast stations of course, and this sum can be reduced considerably by such an input filter.


Very true. I found that the biggest problem was the 60kHz transmission from Rugby which, although it was not obviously audible, affected the general noise level and generated intermodulation.

By experimentation, I use a combination of attenuation and an on-
frequency tuned circuit before my Datong LF converter. I also found it useful to put a tuned circuit (actually an old ATU) between the converter and the "if strip" (an IC-706) at 28MHz because the output from the converter is flat over at least 2MHz. This is peaked at 28.136MHz, but gives a litte extra attenuation at 28.060 (MSF), 28.198 (BBC Radio 2) and various local medium wave transmissions at 28.5 - 30MHz.

In addition, although it does not affect the band noise I use the old trick of reducing the RF gain control so that the noise is just below the AGC threshold. This improves the signal to noise ratio. If your receiver does not have a continuously variable RF gain, adding a 100 ohm potentiometer to the antenna input will work.

Having dismantled my antenna loading coils to take to GW, I tried putting an odd untuned 15m of wire direct into the converter, and got a taste of what some newcomers must be experiencing. Certainly I could hear the big guns, G4GVC, G3KEV, G3YXM, but the general noise level was poor and - I cannot really explain this properly - it sounded strange. I presume this was because I was not hearing real noise, but the product of many out of band signals.

In a recent posting, I listed some strengths of commercial signals, compared to an S2-3 noise level. I forgot to list the Loran signals which should be audible all the time to anyone in the southern half of England. These are a series of pulses (some say like galloping horses) and are best identified by listening on their centre frequency around 100kHz. If you can't hear this all of the time over most of the band, you need to improve things - I can hear it only when the local TVs are off.

To repeat an earlier posting, if your own TV causes you QRM, try installing a braid breaker (make a crude one by putting 100pF capacitors in series with the inner =and= the outer of the coax).


Hope that helps someone.



Mike, G3XDV (IO91VT)
http://www.dennison.demon.co.uk/activity.htm


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