Subject:	Re: TVI
Date:		Thu, 22 Jan 1998 20:10:01 -0600
From:		kf0m@juno.com
To:		the.hiltner.family@sympatico.ca
References:	1


I have expenenced plenty of grief from 2 meter RFI which has increasingly
gotten worse over the last 20 yrs The electronics in todays equipment has
gotten faster and capable of responding to vhf freqs.  The operating
voltages have gotten lower making them easier to disrupt.  I now have
plenty of RFI into audio amps because the devices used have gain at VHF
where older devices used for audio did not.  Manufactures shielding is
non existant as well as no RF bypassing.  It is very dificult to add
filtering after the fact at the board and device level in todays
equipment.

TVI can be a major problem because todays equipment is cable ready which
means the front end is wide open to amateur signals because every vhf ham
band is also a cable channel.  In the old days with mechanical tuners The
tuner switched a single channel bnadpass filter in and out with each
channel.  Ham band VHF signals were blocked out.  The best success I have
had with TVI is installing notch filter tuned for two meters in the
feedline .  I used a design that came out in Ham radio magazine that used
a short section of 75 ohm coax with a capacator across the end  to tune
it as a quarterwave stub tuned to notch two meters.  Of course, If they
are trying to watch cable it block the cable channel.  

I have had poor success in getting help from equipment  makers and the
cable company even when demonstrating how strong the leak from their
cable was on 145.25.   People don't seem to understand that it is their
equipment that has the problem not mine even when demonstrating that my
own TV is clean.

Conversly the noise floor here on two meters has gooten high enough to
ruin vhf operation and the noise all across the band from poorly shielded
equipment puts birdies all over the place makes it very hard to find
clear frequencies.  I operate very little now because all the fun is gone
due to RFI to my radios and others equipment.

The first thing you need to find out is if your signal is just getting
into audio of picture is it on one particular channel.  You can also
terminate the antenna input to the TV to see if it is comming in the coax
or directly into the set.  If they have a VCR the RF switching diodes in
it that switch it from off the air to the tape can cause big problems.  

IN my experience six meter is even worse than two meters for RFI and with
the latest equipment the devices are now getting sensitive to 432 and I
am even having RFI problems on that band.

I sure miss my old TV with the mechanical tuner and vacum tubes It was
bullet proof but the tubes finally got to expensive to replace and the
picture tube went bad and couldn't be replaced.

John Lock
--KF0M--
Wichita Ks EM17
kf0m@juno.com