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Date: Tue, 12 Feb 2002 09:48:12
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Loop questions
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>Here are my questions.
>
>1) Does this "high impedance antenna?"  suffer from the same problems as a
>high "Q"  vertical such as absorption from trees, ground etc?  So that
>removing it from this environment might increase signal strength?
A loop antenna is a 'magnetic' antenna, it creates a strong local magnetic
field, while the electric field is build up 'on the fly'. Opposite to that
a vertical is an 'electric' antenna that creates a strong local electric
field while the magnetic field is built up on the fly.
It seems that the electric field suffers more from the local (dielectric)
losses in soil, trees, buildings etc. than the magnetic field (permeability
losses).
But a loop antenna will suffer much more from the loss within the antenna
(copper loss) as the currents will be much higher than in an electric
antenna. The result is that in most cases (and for dimensions that are
practical for hams) the efficiency of a short vertical will be a lot better
than from a loop with similar dimensions. Only those who suffer from
extreme environemental losses might be better off with a loop.

>2) Would I benefit from using Litz wire?  I have a quantity of  45/#36 litz
>that I could rewind the loop with?
Yes, litz wire will reduce the losses in the loop, this increasing efficiency.

>3) Does higher "Q" neccessarily mean more radiated signal?
Yes, as Q increasing Q will decrease losses.

>4) In most cases I don't belive we want the coil to radiate but in this case
>I wasnt to increase it. Any ideas on how to do that?
Anything radiates (even a wet string). It is just a matter how well
(efficient) it radiates, the larger the loop (coil) the better it will
radiate (assuming it is not screened). In most electronic design we want a
coil not to radiate, so we try to keep dimensions small or use other
'tricks' such as toroid coils to keep radiation low.

Some more remarks :
- Even more efficient than using litz wire might be to increase the
dimensions if the loop. Remember that the radiation resistance of a loop
will increase with the square of the loop area, so make the loop as big as
possible.
- It doesn't matter (for efficiency) if you use a single turn loop or a
multi-turn loop, as long as you use the same ammount of wire. This means
that a 10 turn loop will have the same efficiency than a 1 turn loop using
10 parallel wires. To minimize stay capacitance (between the turns) it
might be even better to use parallel wires.
- You mention that you feed your loop using a 110m long network cable. It
wouldn't surpise me if this cable makes a significant contribution to the
radiation, in particular if matching is not perfect.

73, Rik  ON7YD