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Message-ID: <533139D8E0B84583B84C4A46DE319CE1@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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In-Reply-To: <008901cc578f$a7f78c60$0401a8c0@xphd97xgq27nyf>
Date: Thu, 11 Aug 2011 15:09:11 +0100
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Subject: LF: Re: 137 PROBE
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Dear Mal, LF Group,

You wrote:
>Ref recent emails about probe antennas. How can receiving a simple cw 
>signal on 137 create such problems and technical >difficulties.
>Try throwing a random length of wire over a tree instead.

The important thing to realise is that the noise-coupling mechanisms are 
fundamentally the same for any antenna, although different designs of 
antenna may be affected to different degrees. So if you are having noise 
problems, throwing bits of wire around at random probably won't help in 
itself. The big advantage in using compact whip and loop RX antennas is that 
it is easy to experiment with different antenna locations - often the local 
noise level changes rapidly over distances of only a few metres, so it is 
often possible to find a localised low-noise spot for the antenna. On the 
other hand, a large wire antenna will be coupled to noise sources over a 
correspondingly large area, making high RX noise levels more likely. Any 
kind of antenna with a feeder can be affected by feeder noise problems.

The main advantages of a large TX-type tuned vertical are that it produces a 
high signal level at the receiver input, together with a rather narrow 
bandwidth. This combination helps to mask the poor noise figure and dynamic 
range performance of many receivers in the LF/MF range, particularly the 
receivers in amateur HF rigs. You could achieve the same benefits by using a 
high gain, narrow band preselector. I think a large proportion of  poor 
results reported with active receive antennas are due to this type of 
problem, especially when the operator often does not know how sensitive the 
receiver actually is - manufacturers and reviewers usually only give figures 
for the HF amateur bands, which is no guide. It is important to consider the 
whole receiving system - antenna, RX, preamps, filters etc. rather than just 
individual parts. This includes sound card audio levels too, where these are 
used.

Cheers, Jim Moritz
73 de M0BMU