30 M Loop and Alternatives

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From: James R. Duffey (ji3m@maxwell.com)
Date: Wed Jul 16 1997 - 13:32:56 EDT


Brad - You don't need to build a new loop for 30M, your 40M loop should
work fine on 30M. It will be 1.3333333... wavelengths in circumference. The
easiest way to feed it is to add enough balanced line to yield a mostly
resistive feedpont. This will be (approximately) when there is an integer
multiple of halfwaves in the wire length used for the loop and feedline.
This should require you add about 2.5 M of balanced feeder to the 40 M
loop, which will add about 5M of total wire length (2.5 M for each side of
the feedline) for a total of 45M of wire or 3 half waves on 30M. The
feedpoint impedance will be resistive and should be about 150 Ohms or so.
You can get a reasonable match using a 1/4 wave series transformer made
from 92 Ohm coax; RG-62, which is obtainable, although not widely
available.

Alternately, if your feedline run is short, you should be able to load the
40 M loop on 30 M with satisfactory results with most tuners.

As yet another alternate you can supplement your 40M horizaontal loop with
a 30M loop oriented verrtically. You can load this on 40M by adding 5 M or
so of balanced feeder to bring the total length of wire in the
feedline/loop to 40M. The feedpoint resistance at this point will be 50 to
75 Ohms depending on many things, but it will be a reasonable match to 50
Ohm or 70 Ohm coax. You can also use this on 20M with a 1/4 wave
transformer. These two antennas should have different radiation patterns
and you can use either on either band so you will have your choice of
antennas for different situations. You can never have too many antennas :-)

These antennas will work on other bands either as is, or with some
additional wire length. I would be careful using them on bands where the
loop circumference is 2 wavelengths or greater as pattern breakup can put
much of your power in uninteresting directions. This is a problem with long
antennas in general, not just loops. If the main radiation lobe is in an
interesting direction when the antenna is used at low frequencies, the
lobes when used at higher frequencies are not in the likely to be in the
same interesting directions. I don't want to imply that long antennas don't
work, they do, just that one should be aware of their limitations,
particualrly when used as multiple band antennas.

As you know, I like loop antennas and encourage everybody to have one or
two. Let us know what you end up with and keep on looping. - Duffey KK6MC/5

James R. Duffey (505) 764-3143
Principal Scientist (505) 843-7995 (FAX)

Maxwell Technologies Incorporated/Albuquerque Division
Suite 300
2501 Yale Blvd SE
Albuquqerque, NM 87106


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