John,
I have built a number of antennas from 7MHz to 432MHz using YO and find the
same impedance shift you mention. Apparently the algorithms don't accurately
deal with this issue even though patterns and other aspects of the design do
model very closely with the finished antenna.
If you go through the series to parallel conversions involved with hair pin
matches, you will find that the longer hairpin is consistent with expected
results. A longer hairpin suggests higher inductive reactance with resulting
lower current flow at a given power level. Lower current would imply lower
losses which is a good thing but we are splitting hairs on that.
I see no down side for a longer hairpin (other than needing more aluminum
and that is good!) ; maybe someone else does.
73,
Gerald K5GW
Gen Mgr/Texas Towers
In a message dated 1/1/2009 10:25:42 A.M. Central Standard Time,
john@kk9a.com writes:
Several months ago I designed and built a 6el 20m beam with a 60 foot long
boom. You can see a picture of it on http://www.qrz.com/w4aaa . I designed
the antenna using K6STI's YO and according to the program the impedance
should be 29 ohms / max. I have use this program and AO many times with
great success. To match the antenna to 50 ohms I built a hairpin match,
however I could not get a good match until I made the hairpin very long ,
about 40 inches. This was about twice as long as I expected it to be, so I
removed the hairpin and checked the antenna impedance with no match. It
measured 36 ohms using 1/2wl of RG-213 and a MFJ-259B analyzer. The antenna
is fed with a new Array Solutions 1:1 balun and I tried a different balun
and had the same result. Since I was running out of time to play with it, I
just installed the antenna as is and I have used it in the ARRL SS Phone and
CQWW CW contests. The antenna appears to work very well, it has a very
sharp pattern and over 20dBs F/B. The 1.3-1.4:1 SWR across the band
confirms my impedance measurements. In this antenna design, the reflector,
driven element and first director are pretty close together which I suspect
it the reason for the high impedance. I have imported the design into AO
and it also showed a much lower impedance than actual. So, is it possible
that these programs are not correctly modeling my antenna?. I would like to
adjust the antenna further, but it is difficult to know what to do when the
software does not match the model. My second question, is it OK to use a
very long hairpin match on a 36 ohm antenna? BTW, I know that there is
nothing wrong with 1.4:1 SWR, I am currently building another identical
antenna and I would like to resolve this issue.
73,
John KK9A
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