I’m experiencing some strong interference across the 6 meter band which
I believe is coming from a local (within 40 miles) TV channel 2. Where
can I get a filter to deal with this? Is there a low pass filter with
a cutoff at 54 MHz? I only work SSB and CW on 6 so I’m only interested
in the lower portion of the band, if I cut off 52 MHz and above that
would be fine by me…..

George

The most likely cuase is that your receiver front-end is broad enough
to pass the TV2 signal through the RF amplifier, and modulate everything
else being received.

It's difficult to get a low-pass or high-pass filter will a sharp enough
cut-off to do much good. Better solutions are to build a notch filter for
the TV2 video carrier at 55.25 MHz (using a T-connector and a length
of coax trimmed to length, or to buy or build a passband filter for
50-52 MHz.

I had this problem with channel 11 and a 220 MHz receiver, and ended
up building a bandpass filter with a pair of coupled LC tanks, tapped at
50 ohms. Totally eliminated the TV cross-modulation problems.

-- Tom




"GeorgeF" wrote in message
nk.net...
I’m experiencing some strong interference across the 6 meter band which I
believe is coming from a local (within 40 miles) TV channel 2. Where can
I get a filter to deal with this? Is there a low pass filter with a
cutoff at 54 MHz? I only work SSB and CW on 6 so I’m only interested in
the lower portion of the band, if I cut off 52 MHz and above that would be
fine by me…..

George

Tom wrote:
The most likely cuase is that your receiver front-end is broad enough
to pass the TV2 signal through the RF amplifier, and modulate everything
else being received.

It's difficult to get a low-pass or high-pass filter will a sharp enough
cut-off to do much good. Better solutions are to build a notch filter for
the TV2 video carrier at 55.25 MHz (using a T-connector and a length
of coax trimmed to length, or to buy or build a passband filter for
50-52 MHz.

I had this problem with channel 11 and a 220 MHz receiver, and ended
up building a bandpass filter with a pair of coupled LC tanks, tapped at
50 ohms. Totally eliminated the TV cross-modulation problems.



1) Where can one buy a passband filter for 50-52MHz?

2) When talking about coax trimmed to length, are you talking about a
coax stub? Such as cut to 1/4 wavelength of the offending freq? If so
would be ends be shorted or let open? never worked with stubs yet....

TNX
George

Don't know where you can buy a passband filter. Maybe be able to find some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way is
to
start with an open stub slightly too long and trim it to length. If you can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is difficult
due to losses
in the conectors, cables, etc.

Rough length of the open stub would be 1/4 wavelength at 55.25 MHz, about
1.35 meters. Accounting for coax velocity factor of 0.66, the stub would be
about
0.89 meters long. Start a maybe 5-10 CM longer than that and trim to
length.

-- Tom




"GeorgeF" wrote in message
ink.net...

1) Where can one buy a passband filter for 50-52MHz?

2) When talking about coax trimmed to length, are you talking about a coax
stub? Such as cut to 1/4 wavelength of the offending freq? If so would
be ends be shorted or let open? never worked with stubs yet....

TNX
George

"Tom" wrote in message
news:YBvjd.2272$bH2.1471@trnddc09...
Don't know where you can buy a passband filter. Maybe be able to find some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way
is
to
start with an open stub slightly too long and trim it to length. If you
can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is difficult
due to losses
in the conectors, cables, etc.

You may achieve -25dB loss at CH2 video, but unfortunately you will also
have high loss at 50MHz, and a VSWR that is out of sight.
Build one and see for yourself.

Dale W4OP

Well, probably true. Don't transmit through it! The computer with the
shorted-stub
spreadsheet on it is broken, so I can't look at the loss vs. frequency. It's
a simple
Excel sheet that models the complex transmission-line equation (including
loss)
of the stub against the feedline. Hoped it wouldn't be that bad at 10%
separation,
but am flying blind until I can get access to it and run the numbers.

-- Tom



"Dale Parfitt" wrote in message
news:jZAjd.2538$bH2.1351@trnddc09...

"Tom" wrote in message
news:YBvjd.2272$bH2.1471@trnddc09...
Don't know where you can buy a passband filter. Maybe be able to find
some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way
is
to
start with an open stub slightly too long and trim it to length. If you
can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is
difficult
due to losses
in the conectors, cables, etc.

You may achieve -25dB loss at CH2 video, but unfortunately you will also
have high loss at 50MHz, and a VSWR that is out of sight.
Build one and see for yourself.

Dale W4OP

"Dale Parfitt" ) writes:
"Tom" wrote in message
news:YBvjd.2272$bH2.1471@trnddc09...
Don't know where you can buy a passband filter. Maybe be able to find some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way
is
to
start with an open stub slightly too long and trim it to length. If you
can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is difficult
due to losses
in the conectors, cables, etc.

You may achieve -25dB loss at CH2 video, but unfortunately you will also
have high loss at 50MHz, and a VSWR that is out of sight.
Build one and see for yourself.

Dale W4OP


So you get a crystal cut for the channel 2 carrier frequency, and find
a place to connect it in the receiver chain. This sort of thing was done
apparently in repeaters forty years ago, to keep the transmitter out of
the receiver. I do think one would have to modify the receiver because
the crystal might not be able to handle the transmitter signal.

Michael VE2BVW

OK. got access to the spreadsheet. Setting the stub+short+connector loss
to 0.25 dB, and cutting the stub for 55.25 gives about 15 dB loss at 50 MHz,
and about 30 dB loss at 55.25. This probably is good enough to either
prove or disprove the hypothesis about what is causing the problem (video
carrier overloading the RF amp), and an inexpensive way to find out.

As a long-term solution, a bandpass filter would be easier provided it had
loss low and SWR (so it could be left in-line for transmitting).

-- Tom




"Dale Parfitt" wrote in message
news:jZAjd.2538$bH2.1351@trnddc09...

"Tom" wrote in message
news:YBvjd.2272$bH2.1471@trnddc09...
Don't know where you can buy a passband filter. Maybe be able to find
some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way
is
to
start with an open stub slightly too long and trim it to length. If you
can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is
difficult
due to losses
in the conectors, cables, etc.

You may achieve -25dB loss at CH2 video, but unfortunately you will also
have high loss at 50MHz, and a VSWR that is out of sight.
Build one and see for yourself.

Dale W4OP

Well, probably true. Don't transmit through it! The computer with the
shorted-stub
spreadsheet on it is broken, so I can't look at the loss vs. frequency. It's
a simple
Excel sheet that models the complex transmission-line equation (including
loss)
of the stub against the feedline. Hoped it wouldn't be that bad at 10%
separation,
but am flying blind until I can get access to it and run the numbers.

-- Tom



"Dale Parfitt" wrote in message
news:jZAjd.2538$bH2.1351@trnddc09...

"Tom" wrote in message
news:YBvjd.2272$bH2.1471@trnddc09...
Don't know where you can buy a passband filter. Maybe be able to find
some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way
is
to
start with an open stub slightly too long and trim it to length. If you
can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is
difficult
due to losses
in the conectors, cables, etc.

You may achieve -25dB loss at CH2 video, but unfortunately you will also
have high loss at 50MHz, and a VSWR that is out of sight.
Build one and see for yourself.

Dale W4OP

Well, probably true. Don't transmit through it! The computer with the
shorted-stub
spreadsheet on it is broken, so I can't look at the loss vs. frequency. It's
a simple
Excel sheet that models the complex transmission-line equation (including
loss)
of the stub against the feedline. Hoped it wouldn't be that bad at 10%
separation,
but am flying blind until I can get access to it and run the numbers.

-- Tom



"Dale Parfitt" wrote in message
news:jZAjd.2538$bH2.1351@trnddc09...

"Tom" wrote in message
news:YBvjd.2272$bH2.1471@trnddc09...
Don't know where you can buy a passband filter. Maybe be able to find
some
cavity type filters for 50 MHz, but probably only surplus. Building from
scratch
is easy but takes a network analyzer to tune.

A coax stub can be either open or shorted, your preference. Cheapest way
is
to
start with an open stub slightly too long and trim it to length. If you
can
borrow
a signal generator and power meter, then you can tune it that way.

It's practical to get about 25 dB of loss with a stub, better is
difficult
due to losses
in the conectors, cables, etc.

You may achieve -25dB loss at CH2 video, but unfortunately you will also
have high loss at 50MHz, and a VSWR that is out of sight.
Build one and see for yourself.

Dale W4OP