[TowerTalk] ? Yagi Interaction on 15m: KT34XA and XM240

[Todd Dravland]

Dave Hachadorian wrote:
> 
> On Thu, 26 Oct 2000 10:24:59 -0400 "Jeff Maass" <jmaass@columbus.rr.com>
> writes:
> > What are the signs of interaction between tribanders and 40
> > meter yagis?
> >
> > I recently installed a KT34XA at 69' and an XM240 at 79' (~10 feet
> > separation). I'm seeing differences between the measurements
> > made with my MFJ-259 and a transmitter (90W and 1000W) with
> > wattmeter. You can see the measurements at:
> >
> > http://www.zing.com/album/?id=4293852949&p=4282913117&idx=69
> >
> > Is it possible that the interactions are not measurable with the
> > low-power
> > signal of the MFJ-259, but clearly evident when running more power
> > for the measurement?
> 
> I can't think of any mechanism which would make swr power-dependent,
> other than balun saturation, which shouldn't be happening, especially
> at the 90 watt level. Are you measuring swr at exactly the same point,
> such as through any antenna switches or other accessories?
> 
> Normally, I would suspect that the low power analyzer was being thrown
> off by a local BC transmitter, but in your case, the analyzer shows
> the lowest swr, and it looks like the KLM double minimum curve.
> 
> Here's a good interaction check to make:
> While transmitting 90 watts into the XA on 15 meters, disconnect the
> 40 meter feedline, and intermittently short its center pin to the
> shield. Also, intermittently ground and unground the 40 meter shield.
> Observe if the 15 meter swr changes. If it does, you have
> chosen an unlucky length of feedline for the 40, which is making
> the 40 meter driven element resonant on 15. Change the length of
> the 40 meter feedline to solve the problem.
> 
> Dave Hachadorian, K6LL
> Yuma, AZ
> K6LL@juno.com
> 
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I have noticed on my TS940s wattmeter/ vswr that when running 5 watts
the vswr is typically lower than with 100 watts.. what I am getting at
is that I don't believe my feedlines are ideally decoupled from the
antenna, due to either not good enough balun isolation or perhaps the 50
ft. horizontal run of feedline to the shack.  In any case, have seen
VSWR change with increasing power.

73, God Bless!
todd,WD0T

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