This write-up will not be as scientific as I would like for it to be. This
is because of a lack of time before the 160M contest and the failure of
some of my test equipment. But if you are interested in my practical
experience of preparing and using the T antenna for the ARRL 160M Contest,
then read on:
The idea:
The week between the CQWW CW Contest and the ARRL 160M had me thinking
about improving my transmit antenna for 160M. Last year I used an inverted
vee antenna at 80 feet and made over 200K. However, that score was way out
of the top ten which is my on-going goal for the ARRL 160.
I wanted to put up a vertical antenna but the new station site has only a
few bald short towers up besides the 80 foot tower with the tribander where
the inverted vee is. I was looking at my 80/75 meter dipole which is
between the 80 foot tower and an unfinished 66 foot tower and I got an
idea. The dipole is about 133 feet across, is broadside to Europe, and had
jumpers in the ends for QSY from SSB to CW. The idea came to me that maybe
the 80 meter dipole could be converted to an 160M T antenna without too
much extra work.
Putting it together:
The dipole has pulleys at each end so it was an easy matter to drop it to
the ground. I replaced the coax at the center insulator with a PL259 that
has both sides shorted to a 63 foot drop wire, pulled the antenna back up,
and pounded in an eight foot metal fence post where the wire was close to
the ground. On the top of the fence post I installed one of the Lance
Johnson radial bases. This is a bracket that can clamp to a pipe and has
about 32 screws around the edge for attaching radials. It was an easy way
for me to start hooking up radials and I already had the bracket in my junk
box.
I ran 6 raised radials and attached them to whatever I could find in the
six, fairly evenly spaced, directions (this included trees, other tower guy
anchors, and a disabled pickup truck). Some of the radials had a lot of
droop, almost touching the ground. I installed a dipole center insulator at
the bottom of the vertical drop wire and then connected the ground side to
the radial distribution plate. The vertical drop wire was a little long but
that was no problem because it was light enough to spring up above the feed
point in a slight spiral.
Finally, I drove in a short ground rod and atached it to the radial plate.
Testing the antenna:
I have a MFJ249 antenna analyzer, the older kind without the resistance
meter. I tried it at the feed point of the new 160M T antenna and the
analyzer showed infinite SWR across the band. As a matter of fact, it was
on the pin across the band. I knew that the presence of a strong AM
broadcast station could effect the MFJ readings. I didn't know of any
nearby stations, but the terrain here is pretty flat and maybe there was
one close enough to effect me. Also, it was possibile that the batteries
were getting weak.
I connected the coax formally used for the 80 meter dipole to to the
feedpoint of my new 160M T antenna and headed for the shack. There I
checked the SWR using the meter on my IC765 and a Diawa meter and it read a
SWR of about three to one. The SWR seemed to improve very slightly as I
went lower in frequency. But the IC765 didn't go below 1800 KHz and the MFJ
stopped at 1700 KHz.
The MFJ249 was unpredictable and reading erratic; sometimes reading high
and sometimes closer to the other measurements. I switched to a AC power
supply for the MFJ and it still had problems. I decided then that the MFJ
wasn't going to be the tool for adjusting the antenna. Oddly enough the MFJ
still seems to be OK for checking antennas above 160M.
I decided to trim the antenna a little to see if I could bring the SWR in.
I cut a foot off the vertical part and it tested no change. I took another
foot off the vertical wire and still no change. So I decided that I would
wait to get a better match before cutting more wire off.
The antenna sounded like it was working because it seemed to hear
EVERYTHING better than the inverted vee. I tried loading the antenna with
my AL1500 amp using an MFJ-989B 3KW tunner. With a little more than 1KW
output, the tuner would start arcing. I tested the antenna before sunset
working K7CA in NV. He said the T antenna was about two S-units better than
the inverted vee. That was encouraging.
I did some more reading about T Antennas in the books I had and discovered
that the expected feed point resistance would be around 15 to 20 ohms. This
would explain why I was getting 3 to 1 SWR. It was at this time that I put
the request to the reflector for matching help.
Matching the antenna:
The antenna was built on Wednesday and on Thursday we attended my niece's
musical recital in East Texas (over 500 mile round trip). Therefore it was
Friday morning of the contest before I could get back to working on the
antenna. Meanwhile I had gotten a lot of E-mail with good suggestions for
matching.
The first thing I tried was a suggestion from N4KG connecting two 1/4
wavelength pieces of RG11 in parallel to match down to the lower feedpoint
impedance because I already had these pieces of coax in my junkbox. At the
same time I discovered that I hadn't soldered the wire on the vertical drop
to the center insulator. I had failed to do this on Wednesday because my
butane powered soldering iron had stopped working. Since then I had messed
with the igniting system on it and now it was working fine.
I went back inside to check the results of the coax matching and the SWR
had not changed a bit on either the IC765 and/or Diawa meter. However, the
AL1500 would now load to full 1500W output using the MFJ tuner without
arcing. This was some limited progress.
I then read an E-mail response from WX0B that I should disconnect the
ground wire to the ground rod and rely on the radial system alone. Jay felt
the ground rod only added to the ground losses on transmit. I clipped the
wire and tried it again and didn't notice any major difference with my
crude test equipment.
I had originally cut my radials a little long (140 feet), so I went out and
trimmed all six of them back to 130 feet and no change was noticed after
doing this. Now it was getting late on Friday and I tried one last thing. I
added six more 130 foot radials spaced around the radial plate bringing my
radial total to twelve. At this point it was time to stop and get ready to
start the contest.
The antenna still tuned just fine using the MFJ tuner, and I decided that I
would go with that combination. However, I had a sudden thought.
I have a TS940 that wasn't currently being used in the station that had the
modification for all-band transmit. I quickly hooked it up and found that
the SWR on the T antenna dropped to 1.2 to 1 at about 1585 KHz. Wow, it was
really low in frequency.
I quickly ran out and clipped another foot out of the vertical wire (yes, I
soldered it) and tried it again. Now the low SWR point was about 1600 KHz.
It didn't make as much of a difference as I thought. But at this point I
was out of time and had to start the contest.
The contest:
For the first time since moving back to Texas, I seemed to be able to work
everything that I could hear. My limitations were now the receiving
antennas which are a couple of short, temporary beverages. I started out
like gangbusters quickly going over a 100 QSOs ahead of last year. However,
I was getting some calls that I couldn't pull through because of the
receiving situation.
As it got later in the evening, my QSO totals dropped off until I was
actually behind last year. The same thing happened the second night with
the early hours being hot and dropping off later. But I ended the contest
with one more mult and a few more QSOs using the T-antenna than I had last
year (using the inverted vee only). Last year I had about 204K and this
year I finished over 212K.
Conclusion:
I know there were quite a few who called me that just couldn't hear. I am
sorry about that and I wll improve my beverage system before next year. I
believe that the general conditions of this years contest was down compared
to last year. Since I improved my score, that tells me that I was getting
out better. I have a remote chance of making the top ten this year, while
last year I was far, far down the list.
I feel that the T antenna was a successful experiment and greatly improved
my transmitted signal. I am convienced that this kind of antenna will
out-perform an inverted vee unless the inverted vee is over a 1/4
wavelength high (140 feet or higher). The beauty of a T antenna is that it
would be easy to install if you have two trees that are 50 to 70 feet high
spaced 100 to 120 feet apart.
I now think that my T antenna has much too much top hat for the frequency I
wanted to use it for. If the field drys out from the recent rains (it's
really muddy now), I will go out and remove the dipole clips making the top
hat about six feet shorter overall. The jumpers were still installed
because the last time I had used the 80M dipole was in the CQWW CW Contest.
Removing those jumpers may raise the resonant frequency closer to the 160M
band. When I try that, I will report any antenna changes here.
I would recommand a T antenna to anyone who has the supports available that
I suggested above. However, if you can put up a full-sized vertical or a
inverted vee higher than 140 feet, then you should do that instead. Also
the more vertical part there is of a T antenna, the better.
Experimenting with antennas is fun.
73, Richard
K5NA@BGA.COM
http://www.realtime.net/~k5na
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