Eric, here is a recap of existance with this problem. It started last July, I received a phone call from the FAA aviation department in Chicago, Illinois. He had explained that a pilot was receiving interference, and that they was able to identify one of the hams on the repeater, got his call, and he turned them to me. At that time his radio was messed up, extremely, so thus he quit using it. Thought the problem was gone, and received another phone call from the aviation department. They were quite nice about it actually. At that time, I looked at the output of the transmitter on a SA, and could see several spurs. Dave Vanhorn came down with his SA and confirmed this as well.
At that time, the exciter was a Maggorie Transmitter, that was indeed very spurious. This has since been changed to a micor repeater converted by Kevin. It looks excellent on the SA. The only thing
that was changed was that we went from a Diamond dual band antenna to the DB224 antenna at 92'. This antenna was then moved to a 130' tower at the same location, fed with 7/8 Andrew hardline. I had not heard anything from them until the other day, just after I retouted the hardline, and shortened the Andrew 3/8" hardline that goes to the repeater, by about 10'.
I hoped onto 132.950 and could indeed hear the repeater getting into that freq. While there was traffic on the repeater, I listened to the aircraft, and could actually hear the interference on their radio, not intellegent, but that there was indeed something there, and that they were stating that they were having the interference. I shut down the repeater, and it went away, as the pilots stated that the interference went away.
The only thing I have done at this point, was added a 6' piece of cable to the feedline, and have not
heard anything on 132.950 since.
My guess is that there is something with this DB224 antenna, and not resonating just right. I know the elements have been changed to 37 1/4" elements, and we had a new harness built, for what it's worth, there was no change in the behavior of the antenna, SWRs was the same, as well as the impedance of the antenna. To long ago now to remember what it actually was, but at 145.410, the SWR is 1.8:1, I will have to check the impedance.
As for the ground radios, I was told that they have not received any complaints of interference, just on the planes in the air. The Ham that brought this to our attention was from and Airport about 20 air miles from the repeater.
Closest ground that I will be able to get to the cable would be about 30' away, as there is a concrete pad directly under the point where the feedline enters the
home.
Mathew
Eric Lemmon <[EMAIL PROTECTED]> wrote:
Mathew,
Please elaborate on the exact nature of the interference. Is the issue one
of the tower receiving your signal, or the planes, or both? Have you gone
to the airport to listen to the interference, as it is occurring? Do you
have a recording of the interference? Is the audio clearly intelligible, or
a mixture of two or more voices? Does the airport have a remote A/G radio
at the same location as your repeater? Keep in mind that aviation radio is
AM, so it may be possible to receive your FM signal by slope detection.
There are a lot of issues to consider.
I don't think a tuning stub is the proper solution, assuming the
interference is due to a spur from your exciter or PA. Put a five-inch or
larger bandpass (not a pass/notch) cavity between your PA and the duplexer
PA input. This BP cavity should be after the isolator, if one is installed.
Do you have a ferrite isolator following the PA? Try the cavity with 0.5 dB
insertion loss, for starters. Also, check to be certain your feedline is
properly grounded when it enters the shack. It's odd that this problem
began when you changed the makeup and routing of the feedline...
73, Eric Lemmon WB6FLY
-----Original Message-----
From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of n9lv
Sent: Sunday, April 09, 2006 3:25 PM
To: [email protected]
Subject: [Repeater-Builder] Aviation Problem Returns, Need Some Help, or a
Tuned Stub
It appears that my problem with the 145.410 repeater has begun
interfering with the aviation frequency again at 132.950. It crept
out of no where, and the only change that has taken place with the
repeater is that I moved the entry of the feed line, and shortend the
jumper that comes inside, which is 3/8" hardline, same as before.
Someone back when all this was going on, mentioned they could build me
a tuning stub to hopefully make this problem go away. I dont remember
whom it was, but if anyone can help, I seriously need some help on
this one, as I hate to have to take the repeater off the air.
I know it is not the exciter, as it has been changed and is now a
micor. Please Help again.
Mathew
N9LV
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