To: <towertalk@contesting.com>
>Date: Mon, 12 Apr 1999 17:58:04 -0400
>From: Joe Shuey NE3H <ne3h@access.webcombo.net>
>
>Eric Gustafson wrote:
>
Snip...
>> Does the ARRL book really recommend a 4:1 balun with a hairpin
>> match? I'd expect this to be a 1:1 balun.
>
>Yes, read section 18 of the ARRL Antenna Book. It is the 'Optimized
>Yagi' for 6 meters
>
I read the antenna book entry for this antenna. They do indeed
specify a 4:1 balun for feeding this thing. And, it is indeed
_not_ a T-match or floded dipole element. This discussion has
exposed my biases. I'm an HF kinda guy. It would never occur to
an HF antenna designer to match a Yagi driven element as they
have done for this one. Here is my guess as to how they arrived
at this configuration.
As I said, this is a _guess_. There are two possibilities. Both
may or may not have influenced the decision.
1. The antenna was built and optimized for pattern and gain
considerations. Then, when the matching system decision was
made, it was desired to match the DE feedpoint impedamce up
to 200 ohms with a hairpin in order to make it possible to
use a simple easy to build 4:1 balun which was already
familiar to most VHF enthusiasts.
2. The antenna was built and optimized for pattern and gain
considerations. Then, when the matching system decision was
made, an attempt was made to use a hairpin to get the
feedpoint impedance up to 50 ohms. But the hairpin
dimensions turned out to be small relative to the feedpoint
hardware and therefore fairly critical to get adjusted
correctly. And still left the feedline decoupling problem to
solve.
Either way, what they have there should work OK if losses are
carefully controlled and the thing is weather proofed adequately.
So my question now is why do you want to use a different
balancing device than they have specified and provided all the
dimensions necessary to construct? I can't imagine that the
ferrite 4:1 balun would be cheaper or more efficient than the one
shown in the book. If I was doing this and really wanted to use
a ferrite balun for the feedline decoupling, I'd set the DE and
hairpin to bring the feedpoint impedance to 50 ohms and use a 1:1
balun.
73, Eric N7CL
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