Message-ID: <4742DE8E.5060603@ukonline.co.uk>
Date: Tue, 20 Nov 2007 13:18:06 +0000
From: Peter Dodd <g3ldo@ukonline.co.uk>
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Subject: Re: LF: RX artillery
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When I heard John's recording of my signals with a 10Hz AF filter I was 
most impressed.

My own view is how various bits of hardware work depends a lot on the 
listening environment.

On 73kHz the receiver performance improved with insertion of front end 
attenuation or at the very least a reduction in RF gain. The front end 
attenuation was particularly important for those operators who were 
lucky enough to have a go at operating from one of the Decca stations 
before they were decommissioned. The signal from the large antenna 
completely overwhelmed the receiver front end. In this case a sharp CW 
filter was helpful provided the QRM level was not too high.

At this QTH 136kHz was badly affected by the sidebands from the the 
Loran station at Lessay. CW operation with a sharp filter was not 
practical because of the effect  the Loran pulses had on the filter. A 
wide filter was much better. This was the main reason most of my 136kHz 
operation was on QRS.

On 500kHz the antenna has a gain of -20dBi. Nevertheless I usually have 
the 20dB attenuator switched most of the time and this is only switched 
out when when the band is very quiet.

I use a FT-990 for receive which has the IF digital filters and I will 
try to use these more effectively.

Peter, G3LDO


> Have a listen to the recording here :--
> http://www.sighthound.demon.co.uk/gm4slv/OZN_372_RX_test.mp3
>
> In this test I tuned to 372.420kHZ, the upper sideband ident of the
> OZN-372 NDB in Greenland. At 09:30z it's not too strong here. In fact
> it was totally inaudible. The noise level was too high to make it
> worthwhile adding more gain in front of the RX....or so I thought.
>
> The recording starts with the RX in CW mode, 2.3kHz IF filter and no
> internal or external pre-amp. Pretty noisy and no sign of either OZN or
> the more local ODR. 
>
> I switch in my 10Hz AF filter. The wideband crud disappears, but still
> no OZN.
>
> Next comes the LF353 op-amp tuneable pre-amp. This shouldn't help, as
> all I'll do is amplify the noise, right...?
>
> Bingo!
>
> Both OZN and, a few Hz lower, ODR. 
>
> I have a local NDB (TL-376) and the keying harmonics (multiples of
> 400Hz) can be just be heard (wearing 'phones) - a final switch if the
> IC-718 internal 250Hz inrad filter cleans it up.
>
> My queston now is....
>
> Would more people benefit from similar, simple, additions to their
> 500kHz stations? 
>
> Many people are still trying to receive on wide IF filters and
> straightforward amateur rigs (the IC-718 is one such rig). My
> experience of being able to hear people perfectly Q5 yet be inaudible
> to them might be in part due to the extra hardware, not purely my
> superior location? 
>
>
>