[Top] [All Lists]

Re: [RFI] Fan Noise on Repeater

To: <rrath@charter.net>, <rfi@contesting.com>
Subject: Re: [RFI] Fan Noise on Repeater
From: "Jim P" <jvpoll@dallas.net>
Date: Tue, 9 May 2006 21:07:14 -0500
List-post: <mailto:rfi@contesting.com>
> > It is also possible that an
> > exceptionally "bad" fan may, in fact, create enough noise with harmonics
> > that extend into the 2 meter band that you really are getting the stuff
> > through the antenna input.

Harmonics through the 2M band?

Or, how about a fundamental that creates harmonics through
only about 600 KHz (the standard offset for a 2M repeater)?


2M xmit energy mixes with wideband hash around 600 KHz and
creats an 'on channel' signal at the transmit freq +- 600 Khz. The
antenna isn't that far removed from the equipment shack; only
about 18' at VHF Hi-band. ANd we don't know the antenna type,
some models demonstrate less susceptability to creating feedline
currents than others.

Is it possible this is happening? Yes. Is it probable? I don't know.

This mixing 'effect' is easy to demonstrate at UHF frequencies
using a 5 MHz (offset) signal into a nearby dipole or even a short
piece of wire (I would suggest trying a shunted 1N4148 diode and
6" of wire - this is most effective near a 5 MHz split system even
some 300' away), but the same thing can occur at 146 MHz with
a 600KHz source (or lower freq square or pulse waveform that has
a fast risetime or 'edges').

I've talked before on this list about the on-channel NOISE
created by a certain solid-state ballasted flourescent lamp when
a local 6 M repeater tranmitter was on the air, and it created a
signal on the receiver frequency ONLY when the transmitter was
on the air; I've also mentioned a certain digital wire-wrap project
that would create an on-channel signal on a UHF repeater ONLY
when it's tranmitter was live/transmitting. The antenna on this
UHF system was also about 20' above the area where this
digital project was being undertaken so the antenna doesn't
have to directly 'view' the affecting circuitry.

(BTW the solid-state ballast problem was solved with ferrite beads
inside the lamp base.)

Just my two cents; but I wouldn't overlook certain other 'mixing'
phenomonon that is outside the usual 'noise' that most people
experience. In a full-duplex system more 'things' are possible
insofar as 'noise' is concerned.

Jim P  // WB5WPA //

RFI mailing list

<Prev in Thread] Current Thread [Next in Thread>