John and company,
I think you are missing some of the points on the matching and
modification of the antenna in question. I wont address the
conversion from a trigonal etc. to a single reflector since I wrote
that up many times over the years.
The "T" match in this antenna configuration first matches the driven
element of the antenna to a balanced 200 Ohms. The combination of the
length of the driven element and the Tee sections etc. perform this
transformation. It almost doesn't matter what's the impedance of the
antenna. The 13 Ohms you mention is the impedance at the exact center
of the driven element if you were to cut the driven element in half
and measure it there (a tough task considering the frequency and low
impedance etc.) but seems slightly low. Not to worry. Adjusting the
position of the straps etc. on the T rods will tune out the reactance
and step the driven element impedance (whatever it is) up to 200 Ohms balanced.
Next, let's discuss the half wave 4:1 balun. It converts the balanced
200 Ohm of the T match down to 50 Ohms unbalanced. At 2-Meters it
usually consists of about 24" or so of RG 11 coax. The impedance of
the coax in a half wave 4:1 balun is NOT very important. If you think
about it, you are converting 200 to 50 Ohms or vice versa so 100 Ohm
coax would be ideal coax in the half wave section. 100 Ohm coax is
not readily available. Therefore 75 Ohm coax (like RG 11) is usually
used for the half wave line and yields a broader bandwidth than a 50
Ohm coax (which could also be used ) but it all comes out in the wash.
Finally, many of the antennas on the market today (especially at 6
meters and above) do all these awkward gyrations for matching. They
add lots of hardware, gamma matching, T matching, stepping up and
then stepping down etc. and consequently add more chances for failure
than needed. I prefer to design the antennas for a true impedance of
50 Ohms (at the center of the driven element). Modern modeling
software like EZNEC can assist. Then all you have to do is feed the
antenna directly with 50 Ohm feed line with a few large (0.5-1.0"
diameter 1" long) ferrite beads on the feed line near the feed point
as the balun. The only trick is to insulate the center of the driven
element. This is all explained in my Communications Quarterly article
(Yagi/Uda Antenna Design) in the Winter 1998 issue.
At 07:05 PM 5/11/2008, firstname.lastname@example.org wrote:
>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.
>From: "Scott MacKenzie" Date:
>Sun, 11 May 2008 16:00:12 -0400
>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
>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.
>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
>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
>207 Gibbons Place
>Marietta Ohio 45750
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