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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