[FoxHunt] gamma-matched quad antennas

[wolfbob]

I found that the Gamma match on my ten element mobile beam 
(5 el in each pol) would give a, azimuth skew of 5 degrees 
on the horizontal beam with respect to the vertical beam 
(and the boom). I now use a "T" match (or beta) for the hpol 
driver now and the boresight for the two antennas agree. I 
still use a gamma on the vert pol as this helps things by 
raising the elevation angle by a few degrees.

Bob, WB6JPI


----- Original Message ----- 
From: "Dale Hunt, WB6BYU" <wb6byu at arrl.net>
To: "foxhunt" <foxhunt at mailman.qth.net>; <HomingIn at aol.com>
Sent: Monday, April 09, 2007 10:18 PM
Subject: [FoxHunt] gamma-matched quad antennas


> I've used a gamma-matched 2-element quad for transmitter 
> hunting
> for many years, and we've built a bunch of them for other 
> folks
> as club projects in that time.  This started as the first 
> two
> elements of the stiff-wire quad from Joe's book, later 
> adjusted
> for better F/B ratio after following the rear lobe a 
> couple
> of times.  Now I use the W4RNL optimized quad dimensions
> (http://www.cebik.com/radio.html - including a spreadsheet
> where you can enter frequency and wire diameter and it 
> will
> spit out 2-, 3- and 4-element designs.  Ideal for when you
> need a quick antenna to DF on an odd frequency.)  I use 
> insulated
> wire for the gamma and wrap the end around the driven 
> element
> wire to form an inexpensive, weatherproof variable 
> capacitor
> that can be adjusted by unwinding turns, and the wraps can
> be slid up and down the wire to find the right tap point.
>
> In the last few months I've been modelling antennas with 
> EZNEC,
> including the quads.  It is great for optimizing designs 
> for
> maximum F/B ratio or minimum half-power beamwidth.  The 
> quads
> all looked good until I tried actually modelling the gamma
> match to see if EZNEC would give me the same length of the
> gamma rod that I had found experimentally.  Yes, within a
> quarter inch or so.  But the pattern looked a bit odd.
>
> Looking more closely at the currents in the elements 
> showed
> that the polarization had shifted - it was now about 45
> degrees from vertical, as if it were fed in one corner of
> the element rather than on the side.  Modelling programs
> aren't perfect, of course, so I hooked up a Microhunt
> transmitter directly to the BNC jack on one of my quads
> (to minimize radiation from the feedline) and checked
> the polarization with a yagi and my VK3YNG sniffer.  Sure
> enough, two different quads showed skew polarization,
> with the horizontal component slightly stonger than the
> vertical.
>
> Did the original design have this same problem?  I 
> modelled
> Joe's original 4-element design and, sure enough, there is
> a slight skew, but much less noticable than for my 
> 2-element.
> There are two reasons for the difference:  first, there is
> less difference between the horizontal and vertical half 
> power
> beamwidths of a beam as it gets longer, so any rotation is
> less noticable with the longer quad.  Secondly, the gamma
> wire is about half as long on Joe's quad than on the 
> single
> quad element that I've been using for investigation.   The
> maximum current point appears to be close to the top of
> the gamma rod, so a shorter rod (to match a lower 
> impedance)
> ends up with less skew.  But it looks like it may be 30
> degrees or so on the original, as nearly as I can judge
> from the plotted pattern.
>
> I'm not finished with my explorations yet - clearly there
> is something odd about the gamma match.  A balanced feed
> such as a T or a Delta seems to give a clean pattern and
> may be a better approach.  I've often used the delta with
> a 4 : 1 coaxial balun, both on quads and yagis, and that
> seems to give the expected pattern.
>
> Clearly such quads still work for direction-finding, but
> may have some unexpected quirks in the pattern.  For
> example, two such quads pointed at each other may be
> cross-polarized, but properly alligned when one is
> pointed the other way.  This does funny things to the
> effective F/B ratio!
>
> Has anyone else noticed this effect?  Anyone have a quad
> that they can test and share the results?  I like having
> a continuous driven element without a break at the feed
> point, as it makes it mechanically stronger.  So the next
> step is to come up with some sort of matching method that
> is easy and cheap like the gamma match but doesn't rotate
> the current in the driven element.
>
> Any thoughts?
>
>    - Dale   WB6BYU
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