Message-ID: <3C6AB520.7E4779AF@netscapeonline.co.uk>
Date: Wed, 13 Feb 2002 18:49:04 +0000
From: "gii3kev" <gii3kev@netscapeonline.co.uk>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Loop comments (again)
References: <003701c1b46a$2257d880$019c01d4@dave>
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Dave Sergeant wrote:

> >From Dave G3YMC
>
> My loop has been in use for well over 3 years now, so the following may clarify
> recent discussion and surprise some used to 'conventional' vertical antennas.
>
> 1.  Loops are largely unaffected by ground resistance.  There is a small earth
> coupling loss which reflects to a series loss resistance.  The actual mechanics of
> this I have never understood, but the effective resistance of this is no more than
> 0.5 ohms. There is no connection whatsoever between my loop and real ground (the tx
> is grounded to mains earth), and the lower leg of my loop is just one foot above
> ground.  This is the big advantages of loops - they are effectively independant of
> earth losses.
>
> On this basis there is a limit to how low you can sensibly make the wire losses.  My
> loop has a dc resistance of 0.1 ohms - it is constructed of 30A speaker cable, two
> conductors 2.5mm diameter paralleled.  Because of the reflected earth loss (0.5 ohms)
> and skin effect losses increasing the wire size or using Litz would be a law of
> diminishing returns.
>
> 2.  Matching is via a capacitive network, there is no lossy loading inductance in the
> system at all.  Q is high, and bandwidth is about 100Hz.  Contrary to what has been
> suggested, the resonance is stable and has stayed within a few hundred

I consider 20 hz too much and that is the tolerance to which I refer and certainly not
hundreds of hz.
When I was discussing variations of detuning recently between litz and insulated 2.5mm
wire I considered 50 hz too much with the litz coil. My antenna wires are tight but with
strong gale force winds there is obviously some movement and this is more noticeable with
the litz coil but still only around 50- 70 hz. Apart from these gales the system remains
stable for months and needs no readjusting.
Anyone that I have worked on a loop puts out a very poor signal. A number of stations
have tx loops as well as verticals and when they switch to the vertical the signals
difference is tremendous, many db's stronger on the vertical.


Dave, I am not an expert on loops but your signal is barely audible with me

> Hz all the
> time it has been up.  Match at resonance is always 1:1 (and an swr bridge is a far
> more relevant tool for loops than an rf current meter). Sometimes the wire sags after
> wind and resonance shifts by 300Hz or so - retensioning the loop quickly puts it
> back.  In heavy wind there is some evidence of match change but never anything
> dramatic.  The only effect seen is temperature coefficient change in the matching
> network capacitors - between a cold winter morning and midday summer the resonance
> moves up by a few hundred Hz.  All these changes can be accomodated by changing the
> switch settings on my match box.  In a word, my loop is 'rock stable' and has needed
> no attention for 3 years. It is always available for transmit with no messing about.
>
> Not so my vertical.  Bearing in mind it is not helped by a 300 ohm earth loss, and
> the fact that the tuning is very broad anyway, in the short time it has been in use
> it has gone off resonance/match after any small change in conditions - rain, frost,
> wind etc.  It is infinitely more effected by the elements than the loop, and needs
> constant attention to keep it in a useable state.
>
> Hope this clarifies a few things.
>
> Dave G3YMC
> dsergeant@iee.org
> dsergeant@btinternet.com
> http://www.dsergeant.btinternet.co.uk