[Antennas] Feeding Loops

[NPAlex at aol.com]

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: Sun, 17 May 2009 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:  Sun, 17 May 2009 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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