I don't know how long the site has been alive, nor do
I know the bands it's running in, but I do recall
seeing signals in the 850-890 MHz segment. My SA only
goes up to 1.5 GHz; other than a frequency counter, I
have nothing else that goes over 1 GHz.
We did put the SA directly on the antenna and observed
the 449-500 MHz range in detail. We saw several
carriers 25 kHz away that came and went, but we
attributed those to other area repeaters. Out here in
CT, machines ending in .X00 and .X50 transmit low and
receive high; machines ending in .X25 and .X75
transmit high and receive low. This repeater receives
449.5 so we would expect to see repeaters transmitting
on 449.25 or 449.75. But nothing appeared on our own
input.
There wasn't much equipment in this building compared
to other sites I've been at. A few tightly enclosed
cabinets and one that had plexiglass on the back and
plenty of continuously changing digital meters on the
front.
I'll be posting an update to this situation shortly.
Bob M.
======
--- Eric Lemmon <[EMAIL PROTECTED]> wrote:
> Bob,
>
> Depending upon the frequency band and the modulation
> being used, it's
> possible that the cellular transmitter is radiating
> a low-level spurious
> signal. All cellular telephone sites transmit one
> or more control
> channels continuously, and the associated mobile
> units home on this
> channel to receive calls and adjust their power
> levels. You didn't
> state whether this cellular site was 800 MHz, 1800
> MHz, or 1900 MHz, so
> let's assume that it is an 800 MHz cell site. Note
> that an "800 MHz"
> cellular base station actually transmits in the
> 869-899 MHz band. Some
> brands of 800 MHz cellular base stations use a
> doubler just before the
> PA, which means that a low level signal in the
> 434-449 MHz range might
> be leaking out of the site, and could be
> continuously desensing your
> receiver even if it is not exactly on your input
> frequency. Even if the
> cell site is in a different band, it still uses
> doublers and triplers in
> various configurations to reach the channel
> frequencies.
>
> To determine if this scenario is valid, shut off
> your repeater and hook
> a spectrum analyzer to your antenna lead. Look for
> any more or less
> continuous carriers on or close to your repeater
> input frequency or to
> its image frequency. It's also possible that a
> technician left off some
> shields or didn't tighten all the screws on a cover
> plate during routine
> maintenance.
>
> 73, Eric Lemmon WB6FLY
>
> "Bob M." wrote:
>
> > Hi. A local UHF repeater is having massive desense
> > problems. Everything is clean with a dummy load
> > attached to the duplexer, so we know it's a
> problem
> > with the connectors, coax, antenna, or jumper
> between
> > the two. This is a two-month-old system at a
> cellphone
> > site; nothing else there. The repeater is the only
> > thing on 444.5 MHz.
> >
> > I'd like to try a circulator before buying one,
> since
> > if it doesn't help the situation, it's a waste of
> > money to have it there. I would agree however that
> its
> > presence would be mandatory if this was a busy
> site,
> > but for now it's just his repeater and all the
> > cellphone equipment.
> >
> > He's running 200 watts out of a Henry amp, through
> a
> > TxRx 4-section duplexer. We see a 20dB rise in the
> > noise level at the Rx port when the transmitter
> comes
> > on, with the system connected to the coax/antenna.
> > Absolutely no change with a good dummy load
> attached
> > to the duplexer's output. We even have desense
> with
> > the 20 watts out of the exciter (power amp
> bypassed).
> > The antenna is a super stationmaster cut for the
> > 440-450 band. The coax is 7/8 and there's about
> 150 ft
> > of it.
> >
> > So if someone has a circulator and reject load
> that
> > would be rated for 200 watts to pass through it,
> > please contact me. If there's a way we could try
> it,
> > and buy it if it helps the situation, that would
> be
> > ideal.
> >
> > Thanks.
> > Bob M.
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