Problem of nearby transmitter breakthrough
 

Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training.
I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band receiver
(Air 8), boxed it up in a waterproof enclosure and piped the audio out to an
FM microwatt transmitter. The idea of this is that visitors to our airport
who like to sit in the parking lot and watch the airplanes can listen on
their car radios on FM 88.1 to the radio traffic on our Common Traffic
Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
Unfortunately, while this setup worked perfectly at home well away from the
airport, we have an Automatic Weather Observation Station (AWOS)
transmitting continuously on 120.60 MHz only 50-60 feet from the place I
need to site the receiver. Even though this is only a 5 Watt transmitter,
it overloads the front end of the receiver. As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed
to be a quick and dirty (gratis) job for the airport, and I've already spent
more time and money on it than I wanted to. Any suggestions as to how I
might fix this problem? Cheaply? Obviously a better receiver would work
(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't work?
I can't get it any farther away for several reasons. Anyone have a 120 MHz
preselector they can give me? Any really steep (and cheap) 120.6 notch
filter designs?

Thanks for any help you can offer.

--
Bob (Chief Pilot, White Knuckle Airways)

Are you absolutely sure the offending signal is coming in via the
antenna? Is it there if you disconnect the antenna or replace it with a
dummy load (termination)? That strongly influences what you need to do
to fix it.

Roy Lewallen, W7EL

Bob Chilcoat wrote:
Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training.
I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band receiver
(Air 8), boxed it up in a waterproof enclosure and piped the audio out to an
FM microwatt transmitter. The idea of this is that visitors to our airport
who like to sit in the parking lot and watch the airplanes can listen on
their car radios on FM 88.1 to the radio traffic on our Common Traffic
Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
Unfortunately, while this setup worked perfectly at home well away from the
airport, we have an Automatic Weather Observation Station (AWOS)
transmitting continuously on 120.60 MHz only 50-60 feet from the place I
need to site the receiver. Even though this is only a 5 Watt transmitter,
it overloads the front end of the receiver. As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed
to be a quick and dirty (gratis) job for the airport, and I've already spent
more time and money on it than I wanted to. Any suggestions as to how I
might fix this problem? Cheaply? Obviously a better receiver would work
(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't work?
I can't get it any farther away for several reasons. Anyone have a 120 MHz
preselector they can give me? Any really steep (and cheap) 120.6 notch
filter designs?

Thanks for any help you can offer.

Put a tee connector on the receive line coax a length of coax on it with
a short at the far end. It must be cut to be exactly 1/4 wavelength
(including connector spur.) This will appear t be an open circuit at
the resonant frequency, but will severely attenuate your nearby unwanted
signal.

Failing that, a series LC network across the receive line will do a
similar task. Try a 47pf trimmer cap in series with a coil made from 4
to 6 turns around a bic pen. (you can remove the pen after making the
coil... :) Tune your receiver to the unwanted signal and tune the
trimmer until you see the unwanted signal drop out. You may have to
experiment with the coil turns and spacing, but this design will also
help wipe out the unwanted station.

Fortunately your undesired signal is likely a constant-on transmission
so it's always there for you to play with. Secondly, you can make and
tinker with all this at home (provided you live within receiving range
of the airport AWOS) and bring the working product to the airport for
installation.


Good luck.


B.


Bob Chilcoat wrote:
Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training.
I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band receiver
(Air 8), boxed it up in a waterproof enclosure and piped the audio out to an
FM microwatt transmitter. The idea of this is that visitors to our airport
who like to sit in the parking lot and watch the airplanes can listen on
their car radios on FM 88.1 to the radio traffic on our Common Traffic
Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
Unfortunately, while this setup worked perfectly at home well away from the
airport, we have an Automatic Weather Observation Station (AWOS)
transmitting continuously on 120.60 MHz only 50-60 feet from the place I
need to site the receiver. Even though this is only a 5 Watt transmitter,
it overloads the front end of the receiver. As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed
to be a quick and dirty (gratis) job for the airport, and I've already spent
more time and money on it than I wanted to. Any suggestions as to how I
might fix this problem? Cheaply? Obviously a better receiver would work
(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't work?
I can't get it any farther away for several reasons. Anyone have a 120 MHz
preselector they can give me? Any really steep (and cheap) 120.6 notch
filter designs?

Thanks for any help you can offer.

Put a tee connector on the receive line coax a length of coax on it with
a short at the far end. It must be cut to be exactly 1/4 wavelength
(including connector spur.) This will appear t be an open circuit at
the resonant frequency, but will severely attenuate your nearby unwanted
signal.

Failing that, a series LC network across the receive line will do a
similar task. Try a 47pf trimmer cap in series with a coil made from 4
to 6 turns around a bic pen. (you can remove the pen after making the
coil... :) Tune your receiver to the unwanted signal and tune the
trimmer until you see the unwanted signal drop out. You may have to
experiment with the coil turns and spacing, but this design will also
help wipe out the unwanted station.

Fortunately your undesired signal is likely a constant-on transmission
so it's always there for you to play with. Secondly, you can make and
tinker with all this at home (provided you live within receiving range
of the airport AWOS) and bring the working product to the airport for
installation.


Good luck.


B.



Bob Chilcoat wrote:
Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training.
I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band receiver
(Air 8), boxed it up in a waterproof enclosure and piped the audio out to an
FM microwatt transmitter. The idea of this is that visitors to our airport
who like to sit in the parking lot and watch the airplanes can listen on
their car radios on FM 88.1 to the radio traffic on our Common Traffic
Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
Unfortunately, while this setup worked perfectly at home well away from the
airport, we have an Automatic Weather Observation Station (AWOS)
transmitting continuously on 120.60 MHz only 50-60 feet from the place I
need to site the receiver. Even though this is only a 5 Watt transmitter,
it overloads the front end of the receiver. As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed
to be a quick and dirty (gratis) job for the airport, and I've already spent
more time and money on it than I wanted to. Any suggestions as to how I
might fix this problem? Cheaply? Obviously a better receiver would work
(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't work?
I can't get it any farther away for several reasons. Anyone have a 120 MHz
preselector they can give me? Any really steep (and cheap) 120.6 notch
filter designs?

Thanks for any help you can offer.

I am making some assumtions here. You state that you only hear the AWOS
when a transmission occurs on the CTAF freq. I would think this means that
the receiver is NOT receiving the AWOS signal all the time or it would
trigger
the system constantly. If you listen to the receiver output while at the
airport
does it contain the AWOS audio or just the CTAF audio? My inclination is
that the AWOS sig is being picked up by the interface circuitry between
your receiver and the 88.1 transmitter. If this is the case then more
bypassing
and rf filtering is needed on the interface. Then again I could be barking
up the
wrong tree here, only some suggestions, hope it helps.

Cheers

"Bob Chilcoat" wrote in message
...
Sorry for the crossposting, but I'm looking for more expertise than I
have. I usually hang out in the aviation groups, but am an engineer by
training. I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band
receiver (Air 8), boxed it up in a waterproof enclosure and piped the
audio out to an FM microwatt transmitter. The idea of this is that
visitors to our airport who like to sit in the parking lot and watch the
airplanes can listen on their car radios on FM 88.1 to the radio traffic
on our Common Traffic Advisory Frequency (CTAF), which at our airport is
123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home
well away from the airport, we have an Automatic Weather Observation
Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet
from the place I need to site the receiver. Even though this is only a 5
Watt transmitter, it overloads the front end of the receiver. As soon as
anyone keys on 123.00 and the automatic squelch is triggered, all you hear
is the AWOS recording.

I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed
to be a quick and dirty (gratis) job for the airport, and I've already
spent more time and money on it than I wanted to. Any suggestions as to
how I might fix this problem? Cheaply? Obviously a better receiver would
work (my Yaesu aviation handheld works perfectly at the same location),
but I have no other (free) receivers handy. I can move the receiver
another 50 feet down the fence, which is my next option, but what if this
doesn't work? I can't get it any farther away for several reasons. Anyone
have a 120 MHz preselector they can give me? Any really steep (and cheap)
120.6 notch filter designs?

Thanks for any help you can offer.

--
Bob (Chief Pilot, White Knuckle Airways)

"Bob" wrote in message
...
Put a tee connector on the receive line coax a length of coax on it with a
short at the far end. It must be cut to be exactly 1/4 wavelength
(including connector spur.) This will appear t be an open circuit at the
resonant frequency, but will severely attenuate your nearby unwanted
signal.

Failing that, a series LC network across the receive line will do a
similar task. Try a 47pf trimmer cap in series with a coil made from 4 to
6 turns around a bic pen. (you can remove the pen after making the coil...
:) Tune your receiver to the unwanted signal and tune the trimmer until
you see the unwanted signal drop out. You may have to experiment with the
coil turns and spacing, but this design will also help wipe out the
unwanted station.

Fortunately your undesired signal is likely a constant-on transmission so
it's always there for you to play with. Secondly, you can make and tinker
with all this at home (provided you live within receiving range of the
airport AWOS) and bring the working product to the airport for
installation.


Good luck.


B.


While this approach looks good on paper, it often fails badly when the
desired frequency is so close in to the notch frequency. I just put a
quarter wave stub on our VNA and found that while it does diminish the 123
signal -33dB, it also attenuates the 120.6 signal by a whopping -22dB.
There is also an enormous VSWR upset -120:1 or so- this is perhaps not
important in your receive only application.
Each year we build hundreds of filters for this exact application-
AWOS/UNICOM separation. Typical insertion loss is under 1dB while the notch
is -40dB. The filter is about the size of a cigarette pack exclusive of the
N connectors.

W4OP

On Mon, 19 Sep 2005 18:07:25 -0400, "Bob Chilcoat"
wrote:


I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed

Bad thing to do and likely didn't reduce the signal as much as you'd
like. Try an attenuator, enough to kill the offending signal. Once
that is known the next step may be easier.

IF the attenuation needed is under 10DB and leving it in is acceptable
your done. Usually local signals are plenty strong enough.

If you need more than 10DB. Try a suckout stub tuned to the awos
at 120.6 as others have suggested. That should be enough as your
listening on 123.

(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't work?

I'd try that first, distance will always help the problem. Allowing
for a plastic case on the RX distance is more liklely helpful than
filtering. The problem is the case of the RX is plastic and there is
no shielding so any filter will be compromized by back door entry.
A metalic water proof box with filtered ins and outs for RX and fm TX
will be needed then.

It's possible to make a filter with a steep enough curve for that by
using a bandpass section for 123 and a notch section at 120.6 using
sections of UT141 (.141" copper jacket coax).

Allison
KB1GMX

Dale Parfitt wrote:

While this approach looks good on paper, it often fails badly when the
desired frequency is so close in to the notch frequency. I just put a
quarter wave stub on our VNA and found that while it does diminish the 123
signal -33dB, it also attenuates the 120.6 signal by a whopping -22dB.
There is also an enormous VSWR upset -120:1 or so- this is perhaps not
important in your receive only application.
Each year we build hundreds of filters for this exact application-
AWOS/UNICOM separation. Typical insertion loss is under 1dB while the notch
is -40dB. The filter is about the size of a cigarette pack exclusive of the
N connectors.

I think you get narrower selectivity if you use an odd integer
multiple of quarter wavelengths for the stub. The longer the line,
the greater the phase change with frequency. A 1/4 wave shorted stub
goes from open to short in a 1:2 frequency ratio. A 3/4 wavelength
stub goes from open to short in 3:4 ratio of frequency. Etc. I think.

The receiver is already in a (steel) waterproof box. Tomorrow I will try
moving the receiver as far away as I can. If that doesn't fix the problem,
I'll try the stub antenna notch filter solution. I have a BNC Tee. Can
anyone point me to the 1/4 wave length formula for 120.6 MHz? Does 0.591
meters (23 5/16") sound right?

I'm pretty sure right now that the interference is coming in on the antenna.
It's not enough to trip the squelch, but as soon as someone keys on 123 and
trips the squelch, the AWOS is on the audio. OTOH, I guess that doesn't
prove anything...

I suppose the wiring to the 88.1 MHz transmitter or its wiring could be
picking up the 120.6, although all that wiring is shielded (one audio line
with shield terminated at only one end, and one 3v power line with its
shield as return). The transmitter itself is in an unshielded plastic box,
but that's mounted flat against the bottom of a 10" ground plane for the
transmitter antenna. The last possibility is the 6v power line going into
the receiver box. It's not shielded and starts at a wall wart
transformer/psu very near the AWOS xmitter. I could put a few turns through
a ferrite toroid just outside the box, I suppose. Couldn't hurt. Gotta
find a suitable torroid.

Allison, I'm not experienced enough at this stuff to visualize how to make a
bandpass section out of coax. I can follow the stub notch filter, but the
bandpass isn't there. Could you explain a bit more?

Thanks to you all for all the help.

--
Bob (Chief Pilot, White Knuckle Airways)


wrote in message
...
On Mon, 19 Sep 2005 18:07:25 -0400, "Bob Chilcoat"
wrote:


I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed

Bad thing to do and likely didn't reduce the signal as much as you'd
like. Try an attenuator, enough to kill the offending signal. Once
that is known the next step may be easier.

IF the attenuation needed is under 10DB and leving it in is acceptable
your done. Usually local signals are plenty strong enough.

If you need more than 10DB. Try a suckout stub tuned to the awos
at 120.6 as others have suggested. That should be enough as your
listening on 123.

(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't
work?

I'd try that first, distance will always help the problem. Allowing
for a plastic case on the RX distance is more liklely helpful than
filtering. The problem is the case of the RX is plastic and there is
no shielding so any filter will be compromized by back door entry.
A metalic water proof box with filtered ins and outs for RX and fm TX
will be needed then.

It's possible to make a filter with a steep enough curve for that by
using a bandpass section for 123 and a notch section at 120.6 using
sections of UT141 (.141" copper jacket coax).

Allison
KB1GMX

On Mon, 19 Sep 2005 18:07:25 -0400, "Bob Chilcoat"
wrote:

Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training.
I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band receiver
(Air 8),

What is intermediate frequency (IF) of the receiver ? Some offending
signal (or mixing product) could fall on the image frequency and get
through that way.

boxed it up in a waterproof enclosure and piped the audio out to an
FM microwatt transmitter. The idea of this is that visitors to our airport
who like to sit in the parking lot and watch the airplanes can listen on
their car radios on FM 88.1

Is the interface present when this transmitter is turned off and the
signal is monitored through the speaker ? If the transmitter cables
are disconnected, does this change anything ?

to the radio traffic on our Common Traffic
Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
Unfortunately, while this setup worked perfectly at home well away from the
airport, we have an Automatic Weather Observation Station (AWOS)
transmitting continuously on 120.60 MHz only 50-60 feet from the place I
need to site the receiver. Even though this is only a 5 Watt transmitter,
it overloads the front end of the receiver.

One thing to try is to rotate the antenna, so that the receiver
antenna is in opposite polarisation than the transmitter position, one
in vertical polarisation, the other horizontal polarisation. This will
attenuate the offending signal by 10-20 dB. The null might be quite
narrow, so you would have to slowly rotate your receiver antenna to
find it.

As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

If you defeat the squelch, do you constantly hear the AWOS
transmission ? Does it matter if the 88.1 MHz FM transmitter is on or
not ?

Any other VHF transmitters on site (VOR?) that might take part in the
mixing process ? Do you hear any other background noises than the AWOS
recording ?

Paul OH3LWR