Mathew,
The Maggiore EV-1 is a very basic, crystal-controlled exciter that has none
of the filters, power control, or SWR protection features of commercial
units. I have one that I took off the air because of its tendency to
produce spurs. It is perhaps not a good idea to use a relatively unstable
exciter to feed a 200 watt (!) power amplifier, which itself may not have
all of the controls and protections of a commercial amplifier. I've never
heard of Vocom, so I don't know where it falls in the quality lineup.
I find it interesting that the fundamental frequency of your exciter,
12.1175 MHz, when mixed with your output frequency of 145.410 MHz, produces
a product at 133.2925 MHz. This product is in the aviation band and could
be slope-detected with ease by an AM aircraft radio on a nearby frequency.
I'm not suggesting that this is the cause, but it's a coincidence.
You might check to see that the box containing the exciter is tightly
shielded and its power and audio leads are properly bypassed. Check to see
that all connectors are clean and tight. Use a spectrum analyzer to see if
your PA is generating spurs. Try running without the PA, or use a
different and lower-powered PA for comparison.
Another brute-force measure which might be helpful in troubleshooting this
problem, is to put a bandpass cavity between the exciter and the PA, and a
second bandpass cavity at the output of the PA.
Finally, it may be helpful to install a ferrite isolator at the output of
the PA. There may be intermod issues at work here, and an isolator will
help to make that determination.
73, Eric Lemmon WB6FLY
Mathew Quaife wrote:
> Ok, the situation is that my repeater is being heard by commercial
> aircraft on 132.950 MHz. At first they were able to identify one user
> of the system, this was in June. Then again the repeater was heard on
> June 22, this time stating they could hear several users on the system.
> They did not give any other specifics other than this. This comes from
> Pilots who reported the occurrence to the FAA in Chicago. I'm told the
> interference is only to the planes, and not the ground tower. I've
> looked at the transmitter on the SA, there is one small spur there, but
> this same spur is there if I use just an ht, or any other radio for that
> matter. This is hooking it to the repeater antenna directly, or through
> the duplexer. My question is, could it be something in the antenna
> system, or where might I look? My system consists of a DB224 retuned for
> the ham bands, up 92' fed with 7/8" hardline. There are two horizontal
> antennas below the repeater antenna, a Cushcraft 5 element 6 meter beam
> and a Hygain TH7 below that. The tower is a guyed tower, with two sets
> of guys, one set at 65' and the other at 35'. There are no breaks in the
> 1/4" steel cables. The deviation on the repeater into my SM only showed
> 4.5 to 5 kHz wide, although the audio was tinny. Over the past few days
> I have been working on the tinny audio, for which I believe I have fixed,
> as it sounds much better. The transmitter is the Maggiore Hi Pro EV1 and
> the amplifier is a Vocom 200 watt amp. 90 Watts is what is being fed to
> the antenna from the duplexer.
>
> Mathew
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