I don't think this is a dupe.
The amp IS a Henry 20in/200out job, brand new. It has
its own switching power supply mounted on the back. I
think they're using 24V MOSFETs now. My 10in/100out
amp uses ordinary 14VDC and bipolar transistors.
I didn't look directly at the PA output. I do remember
looking at signals on the duplexer's RX port at 449.5
MHz and had the span set for about 100 kHz and the
noise level was around -120dBm, probably limited by my
analyzer's settings. Keying the transmitter caused the
noise level to increase by 10dB (20dB with the 200w PA
in the circuit), so the problem isn't just present
with the PA. It may also have filtering problems, but
we haven't gone that far yet.
I found the transmit signal at 444.5 was down around
-60dBm, which makes perfect sense (20w = +43dBm, and
the duplexer has 100dB rejection; that gets me down to
-60dBm or so). I saw no other discrete signals (down
to the noise level) appear when we keyed the
transmitter.
The problem was not present when the repeater was
first put on the air about a month ago. It has gotten
progressively worse, to the point that it's nearly
20dB desense now on the tall antenna up near the cell
antennas.
The 7/8 coax was already at the site when the UHF
repeater connected to it. Whether it's bad or not will
be determined by someone with a site analyzer and/or a
visit to the top with a dummy load attached.
While there is a 17dB gain preamp with a bandpass
filter, it was NOT in the circuit during our tests
Sunday.
I think we also looked up near 890 MHz (2nd harmonic)
but only saw a rise in the baseline trace; no definite
signal appeared up there when the transmitter was
keyed.
We don't have any bandpass filters currently
available, but might be able to latch on to one if we
look hard enough.
I think we should go back up to the site and do some
more investigating with the spectrum analyzer to
answer some of the extremely detailed questions that
people are asking.
Bob M.
======
--- 'Jeff DePolo WN3A' <[EMAIL PROTECTED]> wrote:
>
> (having email "issues" today, so if this is a dupe,
> please ignore)
>
> RE: cell mixes and 440 repeater
>
> I had a 440 repeater at a site with no other UHF
> transmitters for
> probably a mile or two. On an adjacent tower was a
> cell site (this was
> back in the early 90's AMPS days). When certain
> cell channels were
> active I would get a mix product that fell on or
> near my receive
> frequency, manifesting as either feedback
> "squealing" or sudden
> increases in noise levels that made it sound like
> desense. My
> transmitter was a Micor driving a GE 1/4 kW tube
> amp, the duplexer was a
> 4-cavity Antenna Specialists pass/reject, and rx was
> a Micor with an ARR
> GaAsFET. I found that putting a harmonic notch
> filter (one of those
> little tunable Celwave jobs) on the output of the
> duplexer, tuned to the
> center of the cell site "B" carrier transmit band,
> got rid of all of the
> problems. I didn't investigate further to determine
> if the mix was
> happening in my Tx, in the preamp, or the Rx.
>
> Before spending money an isolator (which will also
> require a harmonic
> filter after it), you might try a cheap test using a
> shorted
> quarter-wave stub tee'd into the feedline at the
> output of your duplexer
> to see if it makes any improvement. I'd suggest
> using a piece of 1/2"
> Heliax for the stub.
>
> If you have a spare pass cavity (a real pass cavity,
> not pass/reject),
> you might experiment with it on the tx leg of your
> duplexer, and then on
> the rx leg, to help determine if you're experiencing
> a mix in your tx,
> or in your rx, or perhaps neither. Keep in mind
> that what appears to be
> an overall increase in the noise floor might
> actually be a mix involving
> wideband digital cellular (e.g. CDMA).
>
> FWIW, I've had substantial (and that's an
> understatement) problems with
> Henry SS amplifiers being unstable on VHF, UHF, and
> FM. I've also
> received amplifiers from Henry that had the wrong
> low-pass filter in
> them - an FM amp with a LPF cutoff around 210 MHz
> comes to mind. I also
> had a UHF 200 watt ham-band amp that was shipped
> with NO low pass filter
> in it. When I called the factory to complain, I was
> told that Part 97
> had no spectral purity specifications for anything
> operating above 225
> MHz so they didn't bother with a filter. The second
> harmonic was only
> about -30 dBc. Eventually they took the amp back
> and put in an LPF.
> Take a real close look at the spectral output of the
> Henry while it's
> operating into the antenna system before spending
> any substantial time
> or money trying to fix a non-existant problem
> elsewhere in the system.
>
> --- Jeff
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