Ken,
Please advise exactly what frequencies these pager transmitters are
operating on. There are a number of possibilities for intermod, but
knowing the offending frequencies is crucial to a solution.
Less than a mile from my home station, there is a hilltop site at which
two high-power (3,500 watts ERP) paging transmitters operate. One is at
152.480 MHz and the other is at 157.740 MHz. A classic third-order
intermodulation (2A-B) occurs when both are keyed up, resulting in a
product at 147.220 MHz, which nearly clobbers my reception of the K6SYV
repeater at 147.210 MHz. This is receive IM, where the mixing occurs in
my own receiver due to a wide bandwidth in the front end. I cured this
by changing to a Motorola CDM1550 radio, which has a very "tight" front
end that tracks the desired receive frequency.
You did not state whether the interference is on your repeater's input
or output, and corrective action will be different for the two.
Moreover, without having all of the pertinent information, it is almost
impossible to consider a solution. Please provide a complete
description of the symptoms.
Part of the problem (but not necessarily the prime cause) may be due to
insufficient selectivity in the front end of your repeater. The
majority of bandpass/bandreject duplexers have practically no bandpass
action; their operation depends primarily upon the notch action. A lot
of undesirable signal can sail right through the receive side of such
duplexers, ready to overload the RF input stage. In such cases, the
solution is to add two or three bandpass cavities between the duplexer
and the receiver.
But, please provide the necessary information so that others who read
this list can study the problem and recommend solutions. Who knows, the
solution may be the responsibility of one or both pager operators.
73, Eric Lemmon WB6FLY
kc4ih wrote:
>
> We have 146.04/.64 repeater on a nearby mountain top. It worked
> great for years with a range of 100 miles or more. Since the phone
> company and a pager company installed their high power transmitters
> near the site of the repeater (within 100 yards) the repeater is
> virtually useless. After much head scratching I believe that the
> difference in frequency of the pager transmitter of 600 khz is the
> problem but have no idea how to solve the problem without going to
> an odd split. The repeater coordinator for this area of Virginia
> won't even consider that as an option.
>
> The equipment that we are using is excellent. The transmitter and
> receiver on the repeater are both Motorola Micor and were modified
> with to the repeater frequencies by FCC 1st class licensed hams
> using Motorola parts. This is not an equipment problem. We are
> running a set of Wacom cavities which were bought new and are
> correctly tuned and the antenna is a Phelps-Dodge Stationmaster.
> When the intermod occurs it is dependant on BOTH pagers transmitting
> at the same time. If only one pager is transmitting there is no
> problem. This may at first sound unusual but the pagers are in the
> 150 mhz band and they are exactly 600 kc apart. These transmitter
> are both 250 watts or more output.
>
> My theory is that the 600 kc (difference of the 2 pagers) is mixing
> with the output of the repeater 146.64 and producing the 146.04
> signal, the repeater input frequency. We are using sub-audible tone
> for repeater access and as soon as a station working the repeater
> drops carrier the repeater drops. The intermod cannot hold up the
> machine once the tone is removed. This may be happening in the
> antenna or hardline connectors prior to the cavities. Every
> test I have run, and there have been many, supports this conclusion.
>
> We are not the only 2 meter repeater that has fallen victim of this
> problem and in every case we have found two pager transmitters
> situated 600 kc apart near the repeater. Most of the other machines
> have been taken off the air, others just put up with it. No one has
> been able to solve the problem and many technicians have studied it.
>
> Moving the repeater far enough away is not an option since the peak
> of the mountain is so small. Also we are using an existing tower
> which we would not have access to at other locations. The searches I
> have done on Google has turned up the stock answer of helical
> resonators which would apply to 2 meter radios but not repeaters. If
> you are familiar with the Micor equipment you know that the receiver
> has excellent helical resonators built in.
>
> Tonight I have considered the possibility of splitting the receiver
> and the transmitter of the repeater and linking the rx signal by a
> 220 mhz link. I am hoping that by reducing the level of the 146.64
> signal by 50-60 db would alleviate the problem. Maybe not, but I'm
> out of ideas. This split would be only about 100 yards but could
> that be sufficient to relieve the problem?
>
> Have any of you ever had this problem and solved it? Any input (pun
> intended) on this matter would be appreciated.
>
> Ken Sturgill, KC4IH
> Marion VA
> please reply to ([EMAIL PROTECTED])
>
>
> Yahoo! Groups Links
>
>
>
>
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