The restricted SWR BW of single D.E. Hygain tribanders was mentioned in the
thread on TH6's. I also had a real problem with SWR BW (starting in 1978
when I bought my first solid state transceiver) and thought some might be
interested in the following.
I modified my Hygain tribander (an extended TH-3 built from combining parts
from a "TH-4" and TH-5) to increase the BW by eliminating the beta match and
off center feeding the D.E. This technique might work for other Hygain
tribanders.
I off-center fed the D.E. by making one side longer and the other side
shorter while observing the swr. I was not able to center the swr on 10 and
15 due to limits on shortening the elements and interaction with 20m with
which I was mainly concerned.
I used a simple RF choke (35 turns on a 1" PVC form) between the center of
the D.E. to replacethe DC ground between the elements that the beta match
provided.
The results were (measured with MFJ-259):
freq swr spec swr modified
14.0 1.8 1.3
14.15 1.1 1.0
14.25 2.0 1.3
14.35 NA, >2 1.8
21.0 NA,>2 2.5
21.1 1.6 1.8
21.2 1.3 1.3
21.3 1.8 1.2
21.45 NA, >2 2.0
28.0 1.5 1.0
28.3 1.2 1.0
28.6 1.7 1.4
28.9 NA, >2 1.9
I think this technique works because it eliminates the beta match
transformer which inserts another frequency sensitive element in the antenna
system.
73, Ed
PS: Re the thread regarding SWR meters. I have both the Autek RF-1 and
MFJ259 and don't use the RF-1 anymore to tune antennas. It's just too
unhandy to use. The MFJ is very quick and easy to use. The RF-1 is good
for measurements after the tuning process over though.
PPS: I don't like antenna tuners because they can have significant losses
and also slow down frequency changing.
PPPS: I also don't like traps because I think they have significant losses
when used in the resonant condition (every trap failure in which I know the
circumstances occurred when the trap was resonant, ie the 15m trap failed by
overheating (melting, charring, flaming) when the antenna was being used on
15m). I plan to replace my TH3x this summer with a beam that uses full size
elements. If anyone has first hand info on the little Tennadyne( LP5 I
think) or the C3S (I'm limited in the size of the antenna I can put on my
tower), I would like to hear it.
Ed Jensen, K5ED
ae110@rgfn.epcc.edu or ejensen@nmsu.edu
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