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RE: The crossed yagis on the Dayton AO-40 demo station



Hello Tom

> I had one comment/observation.  I noticed that the Cheap Yagis mounted on
> Howard's umbrella dish for the AO-40 demo station did not appear to use
> the 17 inch, 72-ohm coax power divider described in the article in CQ-VHF.

It does have a phasing harness made out of two 11cm lengths of RG59 (which
was white coax so that might be why you didn't notice it). There are 75 ohm
BNC's on each end going into a BNC T-piece. I guess that this is the
expensive way of doing it, but I wanted to be able to use them linearly as
well as ease of reassembly.

> So, I guess I'm asking what the phasing harness does, if I'm feeding them
> in phase anyway.

Each of the two yagi's has a designed 50 ohm resistive impedance. The
quarter wave 72 ohm coax is an impedance transformer making the impedance
look like 100 ohms for each antenna. Parallel the two 100 ohm resistive
impedances together and you get 50 ohms.

Note that if each antenna is not well matched to begin with, just like
paralleling up two different resistors, more power will go into one antenna
compared to the other, so you'll end up with a less-than-circular result.

Also take care over the T piece you use. I found a number of thin ethernet T
pieces to be pretty bad at 435MHz. Get a good one if you do it that way.
Alternatively Kent Britain WA5VJB's article in Spring 2002 CQ VHF doesn't
use BNC's at all and the coax is simply attached directly to the elements.

73 Howard G6LVB

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