[OZAPRS] TVI

Andrew Rich vk4tec at hotmail.com
Sat Dec 13 09:10:46 EST 2003


I have had similar problems, and the ONLY solution I could find to use is
to
move the 2m
away from the TV antenna. Seems to power density falls off greatly with
distance.


----- Original Message -----
From: "Chris Hill" <chris.hill at crhtelnet.com.au>
To: <ozaprs at marconi.ics.mq.edu.au>
Sent: Saturday, December 13, 2003 1:37 AM
Subject: RE: [OZAPRS] TVI


Hi Hamish,

You could try inserting a 2m notch filter inline with your TV antenna.

1.  Insert a "T-piece" where the 75 Ohm coax enters the first active
device
in your TV system (usually the back of the VCR)

2.  Plug a 42cm length of 75 Ohm coax into the "spare" port of the
T-piece,
making sure the far end of the coax is open circuit.


(The 42cm is based on a guestimate that the velocity factor of the coax is
80%)

This should give you approx 25dB to 30dB of rejection of your 2m
transmissions, but have minimal impact on your TV signal.

Fastest and cheapest way to try this, is to buy a 2m long "TV coax
extension
cable" from the local supermarket for $5 or so.

If you need more attenuation, you can make a 3 pole filter, which gives
approx 60dB attenuation...  but you really need access to a spec an and
tracking generator for that, in order to get the accuracy.

Oh, and as a free bonus, it will also attenuate amateur 70cm
transmissions...


Please let me know how you get on with it.



73 Chris vk6kch




-----Original Message-----
From: ozaprs-bounces at marconi.ics.mq.edu.au
[mailto:ozaprs-bounces at marconi.ics.mq.edu.au]On Behalf Of Hamish Moffatt
Sent: Friday, 12 December 2003 9:57 PM
To: OZAPRS
Subject: [OZAPRS] TVI


Hi all,

I am having some problems with TVI from the IGATE station. (Stupidly)
the 2m antenna is on top of the TV antenna. This has not been much of an
issue because we have Foxtel but lately we have been using the antenna a
bit more and the IGATE is even affecting digital TV reception to the
digital TV tuner card in my PC. It looks like it is just overload; I
thought it might be unclean signal (the radio is still a handheld)
but I just tried transmitting fairly low power from the FT-847 and also
had trouble.

I have another antenna located a few metres away from the TV antenna
but it has too much gain and even the FT-847 is badly affected by
pagers. How resistent to pagers is the FM900? I have one which I'm
not using and could put in, if it will cope with the pagers okay.

If I get around to it I might test the current radio on a scope to see
how clean the signal is - so it could be down for a bit.
Else I might be able to move the low gain antenna over the Christmas
break. If you have any suggestions they would be most welcome.


Thanks,
Hamish
--
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>
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