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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