I am not at all familiar with the antenna you are referring to, however it
sounds like you have modified it into a conventional style yagi. If the
impedance really is 13 ohms and you are using a 4:1 balun to match it to 50
ohm coax, I don't see the need for any other type of matching. You should
be able to just connect the balun to the split driven element.
John KK9A
From: "Scott MacKenzie" Date:
Sun, 11 May 2008 16:00:12 -0400
Thanks Ralph.
I have the manual - I was able to download it. One issue that I have, is
that I substantially modified the antenna. The only thing I kept was the
matching section and the driven element. Everything else was changed except
for the spacing. All the other dimensions of the elements were changed.
Instead of the three reflectors that came with the antenna, it was replaced
with a single reflector. When I did the modeling, the impedance was
calculated to be about 13 ohms, so using a 4:1 step-up balun, I should have
it at 50 ohms. I am just not sure why they used the 75 ohm coax. Seems to
me, the impedance must have been higher. Also the length of the matching
section seems way too short. The balun that was on the matching section
when I got it, was 2 feet long. If I back calculate the frequency for the 2
feet of RG-11 and apply a velocity factor of 78%, the frequency works out to
be 191.9 MHz - it should be much longer!
I know I really should measure at the end of the coax - I just didn't have a
good way to measure it, without dragging out the rig and extension cords,
etc. I may have a handheld that I could use (I guess I could borrow the
wife's).
Just doesn't make a lot of sense. I guess I will just try making a new
balun and see what happens. But I really hate the "cut and guess" method.
Scott
-----Original Message-----
From: Ralph Matheny
Sent: Sunday, May 11, 2008 3:42 PM
To: Scott MacKenzie
Subject: Re: [TowerTalk] Matching Sections - Modified Yagi
I have some experience with that antenna. Normally the match is simple and
easy.
The original balun is a half wave of RG-11, 75 ohm cable. The match is a
"Tee" match with bars along the driven element set to some "proper" point
and then the driven element is shortened to get rid of some inductive
reactance. Some of those antennas had a plastic box with something inside,
I don't know what those were.
If you don't get the answer you need e-mail me back iand I'll see if I can
find the paper on the WB215 that I had and that may get you out of problems.
Don't measure at the end of 50 foot of cable, btw. Cut a half wave of cable
(or even multiple thereof) that is fairly short. I aim the antenna straight
up with the reflector 4 or 5 feet above ground for setup of mathcing. After
you get close, then you can aim at horizon and check things out. IF you use
long cable you run the risk of getting inacurate results because of cable
attenuation and length. You can tune the antenna to the cable length in
some cases.
Ralph Matheny
K8RYU
207 Gibbons Place
Marietta Ohio 45750
mathenyr@marietta.edu
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