106.9MHz notch filter
 

Hi,

I am a newbie here and have just dropped in to ask a single technical
question.

Living as I do in a suburban area, I use an FM antenna to draw the stations
located in
a certain vector from me. A station of interest is located at 107.1MHz but
is interfered with by an off-axis stronger signal at 106.9MHz

The FM antenna has a "standard" 300ohm screw connection for the lead-in (to
which I connect
a 300/75 xfmr and use coax down).

I would appreciate anyone's help in designing and implementing a notch
filter that would suck
out a major part of the interfering energy.

It seemed to me that a reasonable attempt would be to use a piece of 300ohm
flat-lead and short
it at an appropriate distance from the screw-terminals of the antenna. None
of my attempts have
yielded any observable improvements. It may be that the filter must be
"deeper" (higher Q?) than
what my attempts provide.

Any ideas?

Thank you.

....Bryce

On Fri, 10 Sep 2004 03:50:44 GMT, "A-Tech"
wrote:

It seemed to me that a reasonable attempt would be to use a piece of 300ohm
flat-lead and short
it at an appropriate distance from the screw-terminals of the antenna. None
of my attempts have
yielded any observable improvements. It may be that the filter must be
"deeper" (higher Q?) than
what my attempts provide.

Any ideas?

Hi Bryce,

Length can be very critical as you can imagine. Trying to find the
sweet spot, over and over, can become taxing if not simply fruitless
(undoubtedly your experience to this point).

A simple method is to connect the twin lead in the usual manner.
Measure out a little more than 1.5M to work with (this will be too
long, but it is better than being too short). Take a razor blade (old
style used for scraping paint) and working from the far end cut
through the insulation of both leads at once to short them out.
Repeat at 1/16" intervals (or 1mm intervals if we are sticking with
metric) and note results. You don't want to cut through completely,
just enough to nick the copper and make a good short with the blade.

Keep in mind that while you are handling the line doing this, you are
part of the tuned circuit (and possibly detuned circuit too). It will
give you a quick ball-park and reduce the cut-and-try.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Fri, 10 Sep 2004 03:50:44 GMT, "A-Tech"
wrote:

It seemed to me that a reasonable attempt would be to use a piece of
300ohm
flat-lead and short
it at an appropriate distance from the screw-terminals of the antenna.
None
of my attempts have
yielded any observable improvements. It may be that the filter must be
"deeper" (higher Q?) than
what my attempts provide.

Any ideas?

Hi Bryce,

Length can be very critical as you can imagine. Trying to find the
sweet spot, over and over, can become taxing if not simply fruitless
(undoubtedly your experience to this point).

A simple method is to connect the twin lead in the usual manner.
Measure out a little more than 1.5M to work with (this will be too
long, but it is better than being too short). Take a razor blade (old
style used for scraping paint) and working from the far end cut
through the insulation of both leads at once to short them out.
Repeat at 1/16" intervals (or 1mm intervals if we are sticking with
metric) and note results. You don't want to cut through completely,
just enough to nick the copper and make a good short with the blade.

Keep in mind that while you are handling the line doing this, you are
part of the tuned circuit (and possibly detuned circuit too). It will
give you a quick ball-park and reduce the cut-and-try.

73's
Richard Clark, KB7QHC

Unfortunately, you will find that the -3dB BW of a single stub is in the
neighborhood of 5% or 5MHz at your frequency of interest. The
discrimination between your desired and undsired channels will be
negligible.
We manufacture a line of small multiple cavity symmetrical and asymmetrical
notch filters- getting under 0.5% BW with small topology filters is nearly
impossible at VHF.

Dale W4OP

I agree with Dale that you're not going to get anywhere near sharp
enough a filter by making something out of twinlead. Since the signals
are coming from different directions, it might be more practical to make
a null in the antenna pattern in the direction of the interfering
signal. This is most likely to work if the desired station is reasonably
strong, so it doesn't have to be in the direction of the antenna's
maximum response. A two element phased array can be designed to achieve
a deep and fairly narrow null, and you get a deep null off the end of a
very symmetrical dipole or the side of a Yagi. If the directions are
fairly close, an antenna with wider beam, like a 2-element phased
cardiod pattern array or a dipole, would give you more of the desired
signal when the undesired one is in the null, than a Yagi would.

You might have to use some sort of feedline decoupling (common mode
choke, also known as a "current" or "choke" balun) to prevent the
feedline from being part of the antenna and reducing the depth of the
pattern null.

Roy Lewallen, W7EL

You could also put the filter not at the antenna, but back on the lead
in just before the receiver. That way any 106.9 being pick up in the
feed line are also attenuated.

Have you tried calling the 106.9 station? Years back, many stations
freely (or very cheaply) provided filters to people experiencing
interference from their signal. I remember back in 1987 I had a customer
whose Muzak SCA reception was being trashed from a nearby broadcaster. I
was expecting to pay for a filter, but the station engineer literally
left one at their reception desk for pickup.

I would not expect free, but you could hope for inexpensive. Never hurts
to call.

Good luck,
B.

Hello,

Firstly what does "Q" stand for?

You could try and shield the antenna from the other transmitter depending on
where everything is located. You might be better buying a decent tuner with
a narrower filter built in, often Kenwood or Sony have this feature. I have
a DENON and it suffers from the same problem of having WIDE filters so it
can't select individual stations too well without getting hammered by the
one next door.

"A-Tech" wrote in message
...
Hi,

I am a newbie here and have just dropped in to ask a single technical
question.

Living as I do in a suburban area, I use an FM antenna to draw the
stations
located in
a certain vector from me. A station of interest is located at 107.1MHz
but
is interfered with by an off-axis stronger signal at 106.9MHz

The FM antenna has a "standard" 300ohm screw connection for the lead-in
(to
which I connect
a 300/75 xfmr and use coax down).

I would appreciate anyone's help in designing and implementing a notch
filter that would suck
out a major part of the interfering energy.

It seemed to me that a reasonable attempt would be to use a piece of
300ohm
flat-lead and short
it at an appropriate distance from the screw-terminals of the antenna.
None
of my attempts have
yielded any observable improvements. It may be that the filter must be
"deeper" (higher Q?) than
what my attempts provide.

Any ideas?

Thank you.

...Bryce




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Many years back, either Radio Electronics or Radio and TV news published a how-to article which described using two
identical Yagis horizontally "stacked" and gave formulas or charts to determine the spacing for creating an off axis
notch to do what Bryce needs to do. Otherwise, either Dale's filters or some from an outfit named Eagle may be the only
answer.

--
Crazy George
Remove N O and S P A M imbedded in return address

On Fri, 10 Sep 2004 03:50:44 GMT, A-Tech hath writ:
Hi,

I am a newbie here and have just dropped in to ask a single technical
question.

Living as I do in a suburban area, I use an FM antenna to draw the stations
located in
a certain vector from me. A station of interest is located at 107.1MHz but
is interfered with by an off-axis stronger signal at 106.9MHz

The FM antenna has a "standard" 300ohm screw connection for the lead-in (to
which I connect
a 300/75 xfmr and use coax down).

I would appreciate anyone's help in designing and implementing a notch
filter that would suck
out a major part of the interfering energy.

It seemed to me that a reasonable attempt would be to use a piece of 300ohm
flat-lead and short
it at an appropriate distance from the screw-terminals of the antenna. None
of my attempts have
yielded any observable improvements. It may be that the filter must be
"deeper" (higher Q?) than
what my attempts provide.

I think, as Roy suggested, that your best attack will be to aim
the antenna to provide the deepest null to the "... off-axis
stronger signal at 106.9MHz". Peaking the desired signal is
only a secondary goal here.

Don't spend a lot of Big Bucks trying to solve this problem.
Marvin's Law (Murphy's brother) says that once you _have_ put
considerable money, effort, and time into this, the station's
(new) owner will change the format to Country or Jesus.
(Now, How's *that* for provoking thread drift?)

gl es 73
Jonesy
--
| Marvin L Jones | jonz | W3DHJ | linux
| Gunnison, Colorado | @ | Jonesy | OS/2 __
| 7,703' -- 2,345m | config.com | DM68mn SK

On 10 Sep 2004 16:10:57 GMT, Allodoxaphobia wrote:

I am a newbie here and have just dropped in to ask a single technical
question.

Living as I do in a suburban area, I use an FM antenna to draw the stations
located in
a certain vector from me. A station of interest is located at 107.1MHz but
is interfered with by an off-axis stronger signal at 106.9MHz

Hello Bryce,

I'm sure you're aware that these two frequencies are adjacent channels. The FCC
allocated channels such that adjacent channels stations would not be located in
the same geographical area, expressly fo the purpose of preventing such
interference. Obviously the two channels involved are not in the same area, and
it seems strange to me that you can even hear the station on 107.1 MHz.

I believe that your only hope is to orient your antenna to place the null in the
direction of the 106.9 station, as Roy suggested,, but I'm not sure that even
that will reduce the 106.9 station sufficiently to allow acceptable reception of
the 107.1 station. In addition, I firmly believe that attempting to filter out
an adjacent channel will be futile, and not worth the effort.

Walt, W2DU

"Walter Maxwell" wrote in message
...
On 10 Sep 2004 16:10:57 GMT, Allodoxaphobia wrote:

I am a newbie here and have just dropped in to ask a single technical
question.

Living as I do in a suburban area, I use an FM antenna to draw the
stations
located in
a certain vector from me. A station of interest is located at 107.1MHz
but
is interfered with by an off-axis stronger signal at 106.9MHz

Hello Bryce,

I'm sure you're aware that these two frequencies are adjacent channels.
The FCC
allocated channels such that adjacent channels stations would not be
located in
the same geographical area, expressly fo the purpose of preventing such
interference. Obviously the two channels involved are not in the same
area, and
it seems strange to me that you can even hear the station on 107.1 MHz.
.....
Walt, W2DU

When you are BETWEEN markets, this is common. Or, if you want to listen
to a distant station and the wrong one is close. I'm equi-distant from
Chicago, Milwaukee and Rockford and have this problem hearing WDCB 90.9,
west of CHi-town with WHAD 90.7, S-W of Milwaukee pounding in. They're in
different markets.

This was discussed not to long ago here. A notch in the antenna pattern
is not too difficult. Two vertical dipoles spaced (about) a half wave and
fed with the proper feed line so they are 180 degrees out from each other,
can make a nice null - actually two. They are perpendicular to the plane of
the antennas. I'd have to sit and tinker to remember the feed system, but
there has to be 1/2 wave more feed line in one dipole, compared to the
other.
It is also possible to do the same thing and get a one notch pattern
(cardioid). If I recall, the antennas are spaced 1/4 wave apart and the
feed is 3/4 wave of feed line between them. Again, the receiver simply
connects to one of them and the null is off one end.
--
Steve N, K,9;d, c. i My email has no u's.