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Message-ID: <5.1.0.14.0.20021223183706.00ac2718@gemini.herts.ac.uk>
Date: Mon, 23 Dec 2002 19:49:22 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
In-reply-to: <000c01c2aaa4$3a7e2820$9ce8fc3e@l8p8y6>
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Subject: Re: LF: Loops again
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At 16:55 23/12/2002 +0000, you wrote:
>With all the loop talk that has been going on since before time began has 
>anyone found a design for a near perfect small loop, suppose its like how 
>long is a piece of string, obviously the bigger the better but then there 
>is the diminishing returns senario, it there much noticeable difference 
>between a 2 meter side square loop and a 4meter.

How much signal power a loop can deliver to the receiver depends on 2 
factors - the area, and the Q (or losses). How much you need  depends on 
the overall system design; how good or bad the RX is, whether you want to 
trade off sensitivity against bandwidth, have a wide tuning range, etc., so 
yes, it is a "how long is a piece of string" question. But as a rough 
guide, a 1m^2 tuned loop with a sensibly designed preamp, or with good 
matching to a low noise receiver will give a noise floor well below the 
band noise. The signal to noise ratio of the loop is good, but the absolute 
signal level is much smaller than with a vertical, so the performance of 
the receiver is much more important. If you double the size to 4m^2, the 
design becomes a lot less critical because you are picking up 12dB more 
signal to start with. With very large loops, almost anything will deliver 
more than adequate signal levels, but then you lose the advantages of a 
small loop - being able to rotate it for optimum peaking or nulling, and 
being able to move it to find a location with low noise levels.

>Which is better a single turn loop, several turns in series, several turns 
>in parallel and the best method of feed and matching at the loop.

It is largely a matter of convenience of design - Several alternatives have 
been mentioned; they all work if done properly.

>I have a lot of information about all of these loops little and large but 
>never used a very small loop and might try but there is a lot of 
>conflicting info about the various configurations and I do not want to re 
>invent the wheel so if someone has had good practical results it would be 
>useful.

Maybe it would be a good idea not to keep saying how they are all rubbish 
before you have actually tried them...

>I have been watching QRS activity lately while in the shack doing other 
>things and notice a lot of CQ'S from various amateurs around the UK and 
>Europe and sometimes all going at once, the screen is getting cluttered, 
>but only a few result in QSO'S so it would appear obvious to me that the 
>majority are not seeing each other, and that would indicate poor receiving 
>capabilities.

...Or that they worked each other a couple of days ago.

>I appear to be receiving all of the activity but my 90 meter perimiter 
>loop is reasonably large and is doing well on receive but not as good as 
>my vertical, however I would like to try a very small loop and compare 
>results, but I dont want to build several small loops varying in design, 
>if someone has already done this and knows which ones to forget about.
>I know location is also important, good ground, reasonable and poor ground 
>etc under the loop and some use loops to null out qrm but I am not 
>concerned about qrm, this location is rural and as qrm free as one can 
>get, this is why my vertical is also good on receive.
>Please dont tell me to read the Comic, I need todays technology.
>73 de Mal/G3KEV
>

If you have no QRM, and adequate sensitivity already, you can't expect to 
gain much from other types of antennas. Perhaps you need to install more of 
today's technology - light dimmers, home entertainment systems, compact 
florescent bulbs, computers etc. etc. so you can really put things to the test!

Cheers, Jim Moritz
73 de M0BMU