"Sal M. Onella" wrote in message
...

"ml" wrote in message
...
after my building changed to a new tv antenna and master amp
seems i am giving alot of tvi on strangly enough 2m

also some tvi from 10m too never did before

my antenna is physically pretty close to the antenna (thou it also
was previsiously )

for an experiment i was thinking about just making a coaxial stub
for 2m to see if that would 'notch ' out the interference

and using a T connector putting it just before the master amp (which
i think is just amplifing my 2m signal and mixing w/the tv channels
and or overloading the tv's)


should the stub be 1/4w on 2m or would say 1/2w be 'better'?

i'd cut it for the center of 2m


tnx

Open quarter wave, shorted half wave. I don't know which is better.
Remember to shorten the stub length by the velocity factor of the coax.

I had a similar problem years ago on a Navy ship. Nearby taxi radios
around
154 MHz overloaded the antenna amp. I bought a "Midband Trap" from a TV
shop downtown which fixed things. It went inline and attenuated those
non-TV signals between (approx) 120 - 170 MHz. It was only a few dollars
from one of the well-known TV reception equipment makers, Channel Master,
I
think.

Unfortunately, I haven't found it from any familiar names, but I did
locate
this page http://www.microwavefilter.com/pdffiles/pg27.pdf. The
3367-A/I,
3367-B/G, 3367-C/G and 3367-C/H should all do what you need for 2m
suppression. Prices unknown.

... and this page: http://www.atvresearch.com/overstk.pdf Find their
number 5KMT-A/I-TX Midband Trap. A clearance sale bargain if they still
have any left.

Here's a single-frequency tunable notch;
http://www.microwavefilter.com/pdffiles/pg28.pdf.

Good luck.


Professionally its often done with a pair of stubs shunting the transmission
line with a quarter wavelength of line between their junctions so the shunt
low impedance of each appears as a high impedance where the other stub is
connected. Sometimes the Q is increased by using resonators with lower loss
than co-ax cable (e.g. cavities or helical resonators). The resonant
frequencies can be staggered a bit to improve temperature stability.

An interesting homebrew affair that can yield an impressively narrow notch
is the bridged 'T' circuit, like
http://www.hobby-electronics.info/co...l/ch20s04.html, one of which is
explained at http://www.hobby-electronics.info/co...l/ch20s04.html.
The temperature stability is only as good as the (lumped) components but
using a preset variable resistor allows the loss resistance of the coil to
be cancelled out to some extent. For 2m I have used the centre-tapped L
version but with the input and output lines tapped into the coil, which
preserves more of its Q. It worked well for 'listen through' from a 2m
repeater with 600 kHz separation, using separate Tx and Rx antennas.

Chris