(one last try)
Recently I saw a posting about T matches which said that setting them up is
done by trial an error and that they can't be setup 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 of 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 a "trick" you need to know which I learned after 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 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 accurate
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 only be 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 point 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 should try different combinations
of tube to element spacing (try 4-8 inches for 20-10M and 20-10 inches for
40M) in combination with different T tube diameters. I suggest you start with
the same tube size as the element that you will be attaching to for 20-10M. For
40M, try starting with a 1 inch OD tube. Try different tube lengths ( tap
point ), separations, and vary the driven element length within 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 stated in the instructions, YO does not account
for the change in effective element diameter where the T tubes are parallel to
the driven element. To account for this, the driven element length has to be
increased. I found by experimentation that about 9 inches has to be added to
both tips of a 20M driven element. For the 17M beams I built, I scaled this to
18.1 MHz to be 7 inches. Doing this, I missed the 1:1 point on both 17M beams
by 25KHz. 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 had
expected to do a couple of adjustment at least. K5MR recently used 20 inches of
lengthening with his 3el full size 40M and had to increase this length. But, he
did not have to touch the T match itself.
YO can be used as a modeling tool to design T matches. 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 driven 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 starting with 4.5 inches of lengthening. I'll let you know how 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>
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|