Hello John,
I have carefully listened to the recording of NDB's OZN-372 and ODR-372.
The upper sideband of ODR-372 is at 372.380 kHz. The upper sideband of
OZN-372 can be found on 372.405. Hence both stations are 25 Hz apart.
When you switch in your 10 Hz filter, both stations can be heard, hence my
guess is that your filter is a little wider than 10 Hz. Using an audio
filter with a 12 Hz 3dB bandwidth, I can listen to either station, without
knowing that the other is on.
Regarding the noise issue, I believe that the higher noise level without
your tuneable pre-amp is caused by intermodulation in the IC718. The
selectivity of the pre-amp does decrease the level of the strong "out of
band" LW/MW broadcast signals, protecting the IC718 front end from overload.
The IMD may also be caused by your local NDB.
I am also using a tuneable pre-amp in front of my selective level meters.
However the signal from my mini-whip antenna is directly fed into my AR7030
receiver. The AR7030 can be used with an outboard narrow audio filter or
with the 455 kHz output routed into a SPM-30 SLM (# 2) with a build-in
narrow audio filter. In either case, sensitivity is on par with the SPM-30
(#1) plus tuneable pre-amp.
To confirm if IMD is at stake, you could repeat the pre-amp experiment with
your AR7030.
I have used a R75 as well, which has a similar topology as the IC718. Most
of the time I had to back off the RF-gain a bit for optimum results.
Regarding your question on wide IF filters, 500 kHz is so quiet that I can
easily copy a number of stations at the same time on the speaker of my
AR7030 and 2.4 kHz SSB filter.
When stations are weak, narrow audio filters do help a lot. However this
askes for a different mind set to normal HF practice. On HF a 500 Hz CW
filter is regarded as narrow and 250 Hz as very narrow. However in weak
signal work 250 Hz is still very wide.
Roelof Bakker, pa0rdt
JO11tm
Middelburg, Netherlands
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