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[TowerTalk] T Match Design Using YO

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Subject: [TowerTalk] T Match Design Using YO
From: (Dick Weber)
Date: Thu, 11 Sep 1997 18:31:18 -0500
Recently I saw a posting about T-matches which said that setting them up is 
done by trail and error and that they can't be designed using models. My 
experience is totally opposite. 

To date, I have successfully used YO to set up the T-matches on the following 
antennas: two full size 3 element 40M beams, three 20M beams, and two 17M 
beams. On all these beams I never had to adjust the distance between the 
element and T tubes or the tap point on the element using the dimensions 
predicted by YO. On all, but one ( the very first one I ever did using YO) I 
adjusted the driven element lengths only once (or not at all) for each antenna. 
Now, there is one "trick" you need to know which I learned while setting up a 
T-match for the first time using YO. There are also a couple of things you must 
do in addition. These and the "trick" are actually covered in the YO 

After you've run YO and have your basic design established, select the T-match 
matching option. Set the impedance to 200 Ohms and the series capacitor to 
99999.0. Setting the series capacitor to this value essentially creates a zero 
reactance series capacitor making it a dead short. This needs to be done to 
eliminate it from the circuit. The next step is very important. You must make 
an entry for "lead length." This is the length of the leads from a balun, like 
a 4:1 Amidon  balun or similar,  to the T tubes. This may be only a few inches, 
but you MUST enter even a short lead length to get an accurate answer. Not 
entering the actual lead length will cause errors for the tap position on the 
driven element. If you're using a 1/2 wl coax balun with its ends hooked 
directly to the T tubes, you can enter a "lead length" of 0.  I had a 17M beam 
that was set up for a 1/2 wl coax balun that I switched over to a  4:1 Amidon 
balun. The VSWR changed drastically which I did not expect. I then remembered 
the YO instructions about "lead length."  I reran YO with the actual lead 
lengths. I then set the taps according to the new YO values and the VSWR went 
back to being good. I had to move the taps about 1.25 inches because the lead 
length changed from 0 to 5 inches. I would not have guessed the tap point would 
be this sensitive to "lead length."

 After getting the lead length inputted you now should try different 
combinations of tube to element spacing (try 4-8 inches) in combination with 
different T tube diameters. I suggest you start with the same tube diameter as 
the element that you would be attaching to. Try different tube lengths, 
separations, and vary the driven element length in the T-match dialog box. 
After playing with it a while you'll get a feel of how the variables are 
interacting. At first it took me a while to get the feel for the interaction of 
the variables. Now, after doing this many times, I can do a T match design in 
less than a minute.

Once you've got a combination of tap length, T-tube diameter, and separation, 
there is a manual adjustment you must make to the YO predicted driven element 
length. This is the "trick." As it states in the instructions, YO does not 
account for the change in effective element diameter where the T tubes are 
parallel to the drive element. To account for this, the actual driven element 
length has to be increased. I found by experimentation that about 9 inches has 
to be added to both tips of an 20M driven element. For the 17M beams I built I 
scaled this at 18.1 MHz to be 7 inches. Doing this I missed the 1:1 point on 
both 17M beams by 25 kHZ.  I then adjusted the element tips once and got what I 
wanted. On 40M I used 20 inches for the lengthening and hit it exactly for the 
two 40M beams I helped put together at GI0AIJ. Hitting it the first time was 
luck. I expected to do a couple of adjustments. Recently K5MR did this with his 
3el full size 40M and had to use more than 20 inches. But, he did not have to 
touch the T-match itself. 

T matches can be designed by modeling using YO. I've done it seven times. Make 
sure you've got your lead lengths correct, set the series capacitor to 99999.0, 
and add to the drive element tip lengths starting with 20 inches on 40M, 9 
inches on 20M, and 7 inches on 17M. In about a week I'll be tuning a 4 element 
10M using 4.5 inch of lengthening. I'll let you know how close it works out. If 
anyone has a question about setting up a T match feel free to email me direct.


Dick / K5IU

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