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Re: FO-29 & bandpass filter



Bill,
It is necessary to determine the cause of the problem. Harmonic radiation
from the transmitting equipment, corrosion between metals making a diode
mixer, or just plain overload or a combination of all. Bandpass filters will
prevent the 70cm preamp/receiver from going into overload. A simple form of
a bandpass filter is a shorted 1/4 wave stub at 70 cm.

The old Motorola VHF Micors have a very nice low pass filter 70+db down at
70 cm with less than 1 dB insertion loss at 2m. Bandpass filters will pass
the third harmonic with little loss,and are of little value in this case.

If the third harmonic comes from rusty parts in the nearfield of the 2m
antenna taking care of those problems first is the best plan of attack.
Spraying water on the antenna and hardware will most always stop this type
of interference until the rust drys out. Many times this problem is a result
of two or more things, start with the easest to address and continue until
the problem goes away. Test after each change is made.

Art, KC6UQH



----- Original Message -----
From: "Bill Bruno" <billbruno@earthlink.net>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, July 05, 2004 4:26 PM
Subject: [amsat-bb] FO-29 & bandpass filter


> Working fo-29 at low antenna elevations my down link sig sounded very
> distorted. At higher elevations it was clear. I thought the 2m sig might
be
> getting into the 70 cm side. so I tried feeding the antenna coax to the
mast
> mounted preamp, thru the 70 cm side of a duplexer. On the last pass my
> downlink was clear as long as i could hear it. I have to try a few more
> passes to confirm its fixed.
> Question: If this did fix the problem what kind of filter should i be
> looking for to use in place of the duplexer,and where in the coax is the
> best place to install it?
> Bill
> PS  Thanks to all for getting back to me about my many many many other
> questions.
>
> ----
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