[OZAPRS] TVI

[Hamish Moffatt]

Thanks for the suggestions Chris and all.

I made up a notch filter as you suggested. (Unfortunately, nobody sells
things like T-pieces for the Belling Lee connectors used on TVs, so it's
a bit messy). It's fixed the digital receiver up completely - previously
all channels were affected, but now I can transmit 50W FM from my FT-847
without any affect on the receiver at all. Great!

It didn't seem to help the analog TV receiver elsewhere in the house.
However, I did change the transmitter from a Yaesu FT-411 to an ICOM
IC-02A (also a handheld) which seems to have helped. I'll investigate
the FM900/828 suggestions later, since the IC-02A doesn't seem that
great either. I have an 828 somewhere but without suitable crystals.

As to tuning filters, I have access to a network analyser which is ideal
for it, except that it's for 50 ohms and the results are quite confusing
when trying to analyse a 75 ohm network....!


Thanks,

Hamish


On Fri, Dec 12, 2003 at 11:37:48PM +0800, Chris Hill wrote:
> 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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-- 
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>
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