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
instructions.
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.
73,
Dick / K5IU
<dickrts@texoma.net>
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