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Re: LF: RX artillery

To: [email protected]
Subject: Re: LF: RX artillery
From: Peter Dodd <[email protected]>
Date: Tue, 20 Nov 2007 13:18:06 +0000
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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?




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