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Re: [TowerTalk] Matching Sections - Modified Yagi

To: <>,<>
Subject: Re: [TowerTalk] Matching Sections - Modified Yagi
From: "Joe Reisert, W1JR" <>
Date: Mon, 12 May 2008 20:06:07 -0400
List-post: <">>
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.


Joe, W1JR

At 07:05 PM 5/11/2008, 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.
>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
>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.
>-----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
>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
>207 Gibbons Place
>Marietta Ohio  45750
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