[TowerTalk] Re: gamma's

[Frank Donovan]

Hi Chuck,

It's not necessary to use capacitors in a T-match.  I've always been able
to find a combination of driven element length and T-match rod length,
spacing and diameter that produce excellent VSWR bandwidth with 20, 15 and
10 meter monoband Yagis.  

Matching a 40 meter Yagi over the entire 7000-7300 bandwidth is a
different story because the driven element impedance varies radically over
the 7000-7300 kHz range.  K3LR developed an excellent solution to this
problem by adding a tightly coupled element a few feet from the driven
element.  Proper choice of spacing and length of this additional element
can produce very good VSWR bandwidth over the entire 7000-7300 kHz range

Unfortunately, the current generation of antenna models do not accurately
model driven element length when a T-match is used.  However, I included
my experimentally derived T-match and driven element length parameters in
the YO notes file for the antenna models I published on here a few months
ago.

The T-match has significant advantages in mechanical strength (no break in
the middle of the driven element), electrical integrity (no capacitors to
break down), balun robustness (a nearly bullet-proof half-wave coaxial 4:1
balun), and phase matching in a stacked Yagi array (no phase
shift in capacitors in series with the phasing harness).  Serious VHF/UHF
antenna manufacturers and experimeters adopted the T-match long ago for
its high efficiency and mechanical/electrical integrity.

On Sat, 22 Feb 1997, C. Logan Dietz (KE5FI) wrote:

> I seem to remember you saying you do not use capacators in your T 
> matches.  How is that?  I use gamma matches to eliminate the balun, but I 
> am open to any feed that would eliminate the caps as long as the element 
> is grounded (not split)...> 
> (The Antenna Handbook was no help in this regard.)
>  Chuck, KE5FI


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