At 1/24/2005 07:30 PM, you wrote:
>On Jan 24, 2005, at 18:28, w9mwq wrote:
>
> >
> >
> > Ok, I know this sounds silly, but my repeater is now hearing better
> > than it transmits. Here's the setup, the the issue. I have a
> > Maggorie (no comments) HiPro transmiitter running 2 watts into a
> > Vocom Amp running 160 Watts into the duplexer, 130 out to the
> > antenna, fed with 7/8" hardline into a Diamond Dual Band Antenna at
> > 92'. VSWR is 1.1:1, with 130 watts forward and 1/10 watt reflected
> > at an impedance of 52 ohms, (MJF 259).
>
>I would vote for a failure of the Diamond antenna. I would bet one or
>more sets of the set screws holding the elements together have come
>loose. When this happens, the antenna will still present a perfect 50
>ohm load, but perform poorly. Diamonds are okay antennas when they are
>new, but they have to be taken down every year or two, disassembled,
>cleaned and tightened up. Really only a decent antenna if you can get
>to it conveniently for the annual repair. You will never see this type
>of failure with the wattmeter or impedance bridge.
Since this is a 2 meter system, it would make some sense that a partial
antenna failure would result in the TX being degraded more than the
RX. This is because of the relatively high noise floor on 2 meters. Any
loss in the antenna system will directly impact the TX signal of course,
but the same loss will cause both received signal & noise to be reduced by
the same amount. If the repeater RX has a very low noise figure, it's
likely that the first couple of dB of loss in the antenna system will
nearly equally drop both signal & noise as seen by the RX, resulting in no
perceived drop in RX performance. However, any mechanical failure of the
antenna would likely also cause a high VSWR and/or desense, neither of
which are present here.
Bob NO6B
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