>Scott,
>A couple of questions:
>1) Where are you measuring the impedance - at the
>feedpoint or through a length of coax?
>2) If through a length of coax, how long and what type?
We tried both a couple of short jumpers at the top of
the tower to the feedpoint and then a ~130 foot piece of
RG213 from the feedpoint to the base of the tower.
>3) Did you cross over the conductors of the parallel metal
>strip feedline on the 40 meter dual driven element to get
>the 180 deg phase shift?
Yes.
>4) Are you sure the metal strips that form the parallel
>feedline on the 40 meter dual driven element aren't
>shorted together?
99.9% yes. K4JNY checked that on the tower yesterday.
>If you are measuring thru a feedline, my guess is
>that you have a short somewhere on the parallel
>strip transmission line. The periodic variation of the
>impedance would be consistent with a short circuit
>being rotated in phase around the smith chart by
>a ~200' long feedline. The 5:1 (versus infinite)
>VSWR would be due to the finite loss of the
>transmission line (~1.7dB one-way loss). If the
>short is far enough from the balun (~1/4 wave),
>it will get reflected back to the balun as an
>open. This would explain why you might get good
>VSWR on 15 meters (the driven elements of the
>40M4 are resonant here) where the distance
>between the driven elements on the 40M4 is
>probably getting close to a 1/4 wave on 15
>meters. You can check for this by disconnecting
>the metal strip feeders from the balun so you
>are only using 1 driven element. If there is a short
>on the other end of the metal strip line, you should
>see a dip in VSWR at around 7 MHz when you
>disconnect it from the feedpoint.
>I doubt the 10 meter antenna is having much
>affect.
>W4EF
We doubted it, too - which is why I'm here asking
questions. It just didn't seem like that would be
the case.
Other suggestions from the gallery?
Scott W4PA
__________________________________
Do you Yahoo!?
SBC Yahoo! DSL - Now only $29.95 per month!
http://sbc.yahoo.com
|