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LF: Radiation Resistance of Transmitting loops

To: [email protected]
Subject: LF: Radiation Resistance of Transmitting loops
From: "john sexton" <[email protected]>
Date: Wed, 24 Nov 1999 07:44:00 PST
Reply-to: [email protected]
Sender: <[email protected]>
Hi Dave,

I've been reading your Home Page and came across a formula for the radiation
resistance of a loop antenna:

Rrad = 320*Pi^4*A^2/L^4

where A is the area of the loop in square metres and L is the wavelength in

There is a similar formula in the LF Experimenter's Source Book, in an
article on LF Transmitting Loops (page 2.25). No author is given, but it is
attributed to "Break In" July 1995. The formula given there (page 2.26) is:
Rr = 7.72E-30*f^4*A^2*n^2

where f is the frequency in Herz,
A is the loop area in square metres,
and n is the number of turns.

Putting n = 1 as in your case and this reduces to:

Rr = 7.72E-30*f^4*A^2

The problem is that this formula gives results twice as big as your formula.
A factor of 2 appears to have got lost somewhere in one of the two

My question is, which one is right? If the second one is right, your loop is
twice as good as you thought it was!

One more question. Where can I get 3C85 ferrite rings? I've been through all
the catalogues with no success.

I hope to be on the air before too much longer. I put up a G5RV a few weeks
ago for HF. Because the garden is short, the last 20 feet or so of each arm
are turned down to about 8 foot above ground. Having tried a few calls it
seems to be working quite well on HF. So now I have provisionally connected
the two ends with a good thick multistranded earth type wire and supported
it about 2 feet above the ground from one end to the other. The garden is on
rather a dramatic slope, rising by over 20 feet at the bottom, so climbing
boots were necessary.

Like Steve ALG, I quickly found that a 300 ohm conventional feeder has got
far too much resistance, so I must change that before I do much more, but I
don't want too much weight on the antenna. The total DC resistance was found
to be 1.8 ohms and after a few measurements and calculations I found that
this is made up of:

34 feet of 300 ohm ribbon               0.7 ohm
15 metres of RG58CU             0.63 ohm
102 feet of 16swg antenna wire  0.3 ohm
lower loop wire 0.17 ohm
So the main thing is to replace the feeder and the coax. I don't fancy
replacing the 16swg with 14swg, so I may have to live with that, but I may
go for even thicker wire for the lower part of the loop. I've got some very
heavy gauge 50 ohm coax with a 2.5 mm core, so this may do to connect the
bottom of the feeder to the shack, using a balun/matching unit at that

A loop of this type is attractive for me, because I can't really put up two
antennas, one for LF and another for HF. Obviously for HF the lower part of
the loop must be disconnected, but that doesn't seem too big a problem.

John, G4CNN, Locator: IO91ML

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