[TowerTalk] Beta match adds capacitive reactance on 2 el Hy-gain 40m (reply)

[David Gilbert]

My Optibeam OB2-40 two element coil loaded shorty 40 uses the exact same 
thing for a match ... a small coil at the feedpoint wound with aluminum 
tubing ... and as you say, it works great.  I had measured the 
dimensions of that coil before I installed it, so I was able to 
reasonably estimate its inductance.  When I modeled the antenna with 
EZNEC+ using the element dimensions from the OB2-40 manual, the 
inductance of the "hairpin coil" required for a match turned out to be 
almost exactly that which I had estimated from the measured dimensions.

I suspect, therefore, that in many situations the required inductance 
can be calculated with enough dependability that the ability to adjust a 
true hairpin probably is not needed ... and in my opinion the coil is a 
simpler solution.

73,
Dave   AB7E




On 10/28/2012 7:26 PM, Scott Yost wrote:
> SM0HRP,
>
> Two years ago I built a 2 element 40 meter yagi.  I tried to use a DXE
> hairpin, but could not effect a match.  After consulting with DXE tech
> staff I abandoned it, as the device they sell did not offer adequate
> matching range.   I broke out my 1974 ARRL Antenna Handbook and used its
> charts to determine the reactance needed for the impedance transformation
> and wound an inductor to place across the feedpoint.  It worked like a
> charm.  I used aluminum clothesline wire to avoid dissimilar metal issues
> at the contact with the driven element, and sealed all connections well.
> Beyond the ARRL Antenna Handbook, the other key was my VA-1 antenna
> analyzer, which was very instructive.  And the magnifying lamp on the work
> bench.  Those charts were really small.   73 & GL, NM8R
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