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LF: Re: Re: Receiving Setup

To: [email protected]
Subject: LF: Re: Re: Receiving Setup
From: dj9dw <[email protected]>
Date: Sat, 27 Mar 2004 12:21:21 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Hello Jim &LF-group,
is the TX-antenne detuned, while you're receiving on the loop? (so ther is
no nearfield-coupling from the big one to the loop)?

Regards, Peter, dj9dw
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Friday, March 26, 2004 6:31 PM
Subject: LF: Re: Receiving Setup


>James can we have a quick reminder of the set up you used last night.

>thanks
>peter G8AFN

Dear Peter, LF Group,

The receiving setup here is the same as it has been for the last couple
of winters - the antenna is a 2m x 2m "bandpass" loop with preamp as
described in the article downloadable from G3YXM's web site -
http://www.wireless.org.uk/bploop.pdf
Also on YXM's site is an article on the "lazy loop",
http://www.wireless.org.uk/lazy.htm
which is capable of similar results when used with a fairly big wire
loop.

The loop was aligned to null out Loran noise coming from a SW direction
at my QTH - this also gives optimum pick-up for USA/Canadian sigs coming
from the NW. The TX long wire antenna is not much use on receive here,
due to  the level of 50Hz-related noise it picks up from the house
wiring. The loops are located at the bottom of the garden, which is
electrically the quietest place at my QTH. Audibly, the Loran clatter is
eliminated, although there is quite often still some mains-related noise
present. Using separate RX antennas located in the best available
low-noise site seems to be the factor which has the greatest influence
on results in my case. It does mean I run around in a panic every time a
new buzzing noise appears...

The receiver is an old Racal RA1792, which possesses decent sensitivity
(about 0.1uV for 10dB SNR in 300Hz BW) at 136kHz. The internal OCXO
frequency standard is also very handy for LF narrowband modes. The
sensitivity vs. frequency issue is quite important - O.K., if the
receiver goes deaf at LF you can get the required sensitivity back by
using a bigger antenna, or a preamp. But this will increase the
sensitivity of the system in the MF range, where it is already much
higher than at LF. So it is then important to add sufficient
pre-selection as well as increasing the signal level, in order to avoid
assorted intermods, reciprocal mixing, and cross-modulation due to
broadcast signals. Life is therefore a lot easier if the receiver
sensitivity is fairly constant over the whole LF - HF range, like the
RA1792. Especially if you live in Brookmans Park!

I mostly use DL4YHF's Spectrum Lab for copying QRSS - It is considerably
more complicated to set up and use than Argo, but I like the facility to
change all the display parameters whilst looking at different types of
signal and noise. Having said that, Argo is much simpler for the
operator, and gives just as good results with weak QRSS signals, which
it is optimised for.

Cheers, Jim Moritz
73 de M0BMU







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