[Antennas] Feeding Loops

Sun May 17 22:32:06 EDT 2009

In the two messages that follow one proposes a two loop arrangement and 
asks what the "coverage" would be, and the response speaks to some of the 
characteristics of loops, particularly their driving impedance is in the 100 to 
 150 ohm range and requires an impedance transformation to achieve a near 
50 ohm match. In addition their results in coverage is described. I would 
like to clarify in my comments Polarity.

The functional error in the discussions below are the assumptions regarding 
 radiation polarity. The "wall" mounted loop depending where the feed point 
is located can be horizontal polarity (at the bottom) or Vertical when fed 
90 electrical degrees away (on the side ). The loop that is on the 
ceiling is usually horizontally polarized depending on the height, but at low 
heights it is classified as a NVIS (Near Vertical Incident Skywave) antenna 
whose characteristic is to send the signal nearly straight up, and with 
the return relatively close in. It will be essentially omni directional 
whereas the wall mounted loop will have the classic dipole bi-directional 
characteristic and its polarity a function of the feed point.

These characteristics I think are supported by the second writers comments 
regarding the 500mi signals best on the horizontal loop, and the DX best on 
the Vertical loop (physical orientation, not electrical polarity).

In Eric's case I would not use the "ceiling" mount loop, but plan on dual 
feed of the "Wall" mounted loop. In this way he could achieve either 
"Vertical" polarized signal (good for mobiles) or "Horizontal" polarity for those 
with beams. Remember that in a dual feed arrangement, the feed line must 
be decoupled from the unused feed point. Gamma matches do this well, or 
the cable can be isolated via an RF decoupling choke. Ferrite beads, or a 
solenoid wound coax choke can be used. Using 75 ohm cable for the choke will 
also achieve the impedance match needed.

Note, the ceiling loop would give a more omni directional coverage.

Regards,
Norm W4QN
===================================================
Message: 2
Date: 2009年5月17日 16:12:28 -0400
From: Eric Mynes <kc8wzb at gmail.com>
Subject: [Antennas] Delta Loop Question
With my lack of air space for antennas, I've decided that a delta loop
on 28mhz would work if attached to the wall. My actual thoughts was
to build 2. One on the wall and one on the celling. This would give
horzontal and vertical coverage based on which one I switch in. Every
design I've looked at shows the feed point at the bottom. I'd like to
keep the feed points to be close together. So, would a delta loop
work with the feed point at the top? To me, it should work just the
same, but I wanted to ask those who would know more than I do before I
start construction.
Thanks,
Eric
kc8wzb
==========================================================
Date: 2009年5月17日 20:27:22 -0400
From: "Merv Stump" <mstump at hvc.rr.com>
Subject: Re: [Antennas] Delta Loop Question
Full-wave loops are very forgiving. You can feed it anywhere on the loop
and it will work and work well. In addition you can make it basically
any shape you want - triangle, square, rectangle (so long as it is not
too narrow). The impedance will vary somewhat depending on all these
factors plus the environmentals but in general at resonance it will be
close enough to 100 ohms that if you feed through a 1/4 wave section of
75 ohm coax you can use it without a tuner. 
Your idea of one vertically mounted and one horizontal is right on. I
once did this on 75 meters and found that within about 500 miles the
horizontal loop was approximately 10 db better. Into Europe and further
out the vertical loop was generally 10 db better. 
Good luck, Merv - W2OE

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