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

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?
========================
For the stub to work properly at the intended (notched) frequency , you must
take into account the velocity factor of the coax. For a number of common
coax types the velocity factor is about 0.67

For a freq of 120.6 MHz a quarter wavelength is 0.62 metres, so with coax
having the above velocity factor the physical length is 0.62 * 0.67 equals
0.42 metres equals 16.4 inches.
However ,as mentioned before in this thread you could use odd multiples of
this length

When you really want the stub 'as meant for the job' try to find someone
with a spectrum analyser (with tracking generator) , cut the coax a bit
longer than its calculated length and snip tiny bits off until the spectrum
analyser shows the notch at 120.6 MHz.

On a different but related topic ; interference to a UHF TV sig by a nearby
144 MHz tx sig , I made a high pass filter and a downstream quarter wave
stub which was subsequently optimised with the aid of a spectrum analyser.
As expected there was also a clear notch at 432 MHz ,since the stub is a 3
quarter (odd)wavelength for that frequency. BTW the actual notch was at
144.4xx MHz being the part of the 2m band used for SSB operation in the UK
, with SSB (varying RF amplitude) being a 'high risk' mode for interference.
This HP filter/stub combination now permits the relevant radio amateur to
operate on 2 metres ,be it with moderate power.

Frank GM0CSZ / KN6WH

On Mon, 19 Sep 2005 23:35:12 -0400, "Bob Chilcoat"
wrote:

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?

Wavelength is 300/F in meters. One quarter wave is 75/f in meters.
However your using coax cable and the speed of light (nominally 300 in
the examples cited) is slower depending on the coax used. for foam
types it's about .8 (varies by brand) and for solid types usually
around .66 (still varies some).

So for 120.6 a 1/4 wave section is 75/120.6=.62189M (24.4838").
A section of RG58 (solid dialetric) would be .66 that length or .4104M
(16.159").

To give you an idea of how critical that length is the 1/4Wave of the
same coax for 123mhz is .4024M (15.84").

So even small variations in VF (velocity factor for the coax) or
gutting error can make for big mistuning.

Generally speaking simple stubs work best if the two frequencies
are widely seperated such as 2m band and pagers at 153mhz
(5mhz minimum seperation).

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...

Sounds like intermod. The front end of the reciever is being grossly
overloaded.

The average airband radio the difference of 120.6 and 123mhz is so
small to the front end that overload is common.

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.

RF will com into the box anyway it can. Via power, antenna, audio and
even a poor connection between the cover and the box.

FYI: I'd bet anything that a few hundred feet of seperation from the
AWOS cures it or substantually improves it.

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?

If I had a way to post a schematic It would help. Suffice to say its
something that you'd need hardware to tune. The idea is that using
coax sections it's possible to get the equivelent of high Q tuned
circuits and couple them loosely for selectivity. Basically a open
ended 1/4 wave section of coax is a similar to a tuned cavity save for
a cavity at 123mhz is nearly 25" tall making coax less bulky.

So using an open stub creates a notch at some frequency, a shorted
stub is a tuned (resonant) pass at the same given the same length.
Visualize a feed from antenna, T with shorted stub(for 123), a 1/4
wave section for 120.6 section, another T with a open stub for 120.6.

Thats the starting point and it may be adaquate at that. However
to really make it work there is more and It's hard for me to describe
here. Sufficient to say VHF filters with that kind of selectivity are
not trivial to design and build.

Also obvious you need some instrumentation to tune the beast
for best results. Which is why there are people that build custom
filters for tasks like this.

Allison
kb1gmx

On Tue, 20 Sep 2005 02:00:13 GMT, "Dale Parfitt"
wrote:


"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.
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

The 1/4 wave stub works because of huge impedence upset it
introduces. However having a short (or nearly so) on the coax for
transmitting would be deadly for transmitters.

The problem is sections of coax have only moderate Q as resonators
and they are also resonant at harmonics.

I'd approach the problem by pulling the RF amp out of the radio and
loosely coupling to the mixer. Even a RX with 15uV sensitivity is
adequate for miles around an airport.

Allison

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.

--
Bob (Chief Pilot, White Knuckle Airways)

Rather than building a trap, why not use a narrow bandpass filter?
Also, if you are that close to both transmitters, try attenuating the
signal from the antenna to prevent overloading the receiver.

One other idea. if the signal is from a transmitter at the airport,
why not see if you can take the audio feed to that transmitter and feed
the FM transmitter.

--
?

Michael A. Terrell
Central Florida

BC,

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.

'
The technical solutions, so far presented, are about as good as
you're going to get while still keeping the cost reasonable. However,
you ARE dealing with a consumer grade radio that was simply not designed
to any particular standard of interference rejection. And that you've
packaged this with a transmitter operating within the FM band also
offers the potential for overload artifacts on a broad spectrum of
frequencies. Sony Air 8, however, was known to experience this kind of
problem even off-airport. It would even pass 11 meter CB on a good day
if the signal was close, or large enough. As did some Sony FM broadcast
rigs of the period when an aircraft was reasonably nearby.

I lived 2 miles off the approach end of 30L/30R at Lambert, St Louis
for years, and I'd get aircraft to tower chatter on my Sony portable at
103.3, 102.5, even 93.7 everytime an aircraft went overhead. The front
end was simply not robust enough to withstand a close-in assault of any
magnitude. Nor was the chassis shielded well enough to prevent leakage
through the IF's. Your real option may to select a different receiver.
If you're doing this as a favor to the airport, you may get a
decommissioned rig culled from a wrecked aircraft, or one that's been
sitting on the shelf at one of the FBO's, donated to the cause. In that
case, robust interference rejection is assured for your purpose.

There is, however another, and potentially greater issue at play here.

Rebroadcast of non broadcast radio frequencies may be in violation of
several communications acts. Brush up on your regs, and be sure you're
in compliance. In today's climate, there is always some asshat who has
to make trouble for his jollies bringing the letter of the law down on
fairly innocuous activities that may actually benefit someone.
And with overlapping laws written the way they are, today, you may be in
compliance with one law, while in violation of another.

One of my colleagues, downstate, got a spanking for a similar type of
service involving Air/Water show comms for a small airport near his
home. Someone complained. Even though there was no interference was
measured, and all hardware was in compliance for license free
application. Unauthorized rebroadcast of comm channels was the issue.


So, make sure your ducks are in a row.

BC - When All Else Fails . . .

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.

--
Bob (Chief Pilot, White Knuckle Airways)

Rather than re-broadcasting the signal why not try something simpler.
Just set up an Airport Traffic room within the airport and pipe the
audio in. Hang some old pictures and other memorabilia of the airport
on the walls to make it interesting.

In article , "Bob Chilcoat"
wrote:

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.

Bob-

By now you have probably solved your problem. If not, someone else
suggested that you insert attenuation in the Sony's antenna lead. I think
that approach is most likely to produce the results you want.

If you were to replace the Sony's antenna with a dummy load, there may
still be sufficient signal bleeding into the radio to make your system
work. The interfering signal would also bleed into the radio, but at such
a low level that the Sony's tuned circuits ought to be able to handle it.

If even this does not solve the interference, you may find the problem to
be audio rectification inside the 88.1 equipment, perhaps in the
oscillator stage!

Please keep us informed of what it takes to make it work.

73, Fred, K4DII

I'm guessing that because of all the new B.S. concerning Homeland
Security, the airport probably has a fence around it with a security
coded gate control so that non-pilots can't get onto the field...

Scott


John S. wrote:




Rather than re-broadcasting the signal why not try something simpler.
Just set up an Airport Traffic room within the airport and pipe the
audio in. Hang some old pictures and other memorabilia of the airport
on the walls to make it interesting.

Scott wrote:
I'm guessing that because of all the new B.S. concerning Homeland
Security, the airport probably has a fence around it with a security
coded gate control so that non-pilots can't get onto the field...

Scott


John S. wrote:




Rather than re-broadcasting the signal why not try something simpler.
Just set up an Airport Traffic room within the airport and pipe the
audio in. Hang some old pictures and other memorabilia of the airport
on the walls to make it interesting.


Most airports have a lounge of some kind....

All,

Thanks for all the suggestions, advice and information. I learned a lot
about VHF that I didn't know. Should have stayed with it way back when I
almost finished my "Novice" amateur license (couldn't manage the Morse code
part -- I guess that gives away my age).

Fred nailed it, although by the time I saw his note, I'd solved the problem.
After relocating the receiver as far away as I could (60 feet, given the
limitation of where the units needed to be and sources of power) which
didn't really help, I tried some simple filters, etc. During the course of
this, someone keyed a mike while I had the antenna disconnected, and it
worked fine with no antenna connected! A bit more experimentation indicated
that the 120.6 interference dropped out completely while the desired 123
signal could still be picked up from five miles away if I left the short
coax jumper inside the box from the receiver to the antenna disconnected at
the antenna end. I'm not sure I fully understand why I still get a pretty
robust signal this way, but I guess enough 123 MHz RF is leaking in from the
bare antenna or all the other wiring. Since the radio traffic of interest
is mostly local planes near the airport or in the traffic pattern, a five
mile radius is probably more than adequate, and this is certainly the
cheapest solution.

I will also look into the regulations about rebroadcasting non-commercial
radio signals. The transmitter satisfies the part 15(IIRC?) FCC reg about a
100-foot reception limit, but I thought the rules about rebroadcasting were
to prevent commercial rebroadcasting. The 100 foot limitation will keep
all reception on the airport grounds, but I will do some more research on
this. Thanks.

As far as an area inside the building where people can listen to the radio
is concerned, we already have that. This project is for the people who sit
in their cars in the parking lot and watch the planes on their lunch hour,
etc. We get quite a few of these. Anything that improves relations with
the general public should help alleviate some of the problems we have with
the neighbors. I still can't understand how anyone can buy a house near an
airport, and then feel that they have a right to complain about airplane
noise. The airport has been there since 1946, but that's another issue.

Again, thanks for all the help.

--
Bob (Chief Pilot, White Knuckle Airways)


"Fred McKenzie" wrote in message
...
In article , "Bob Chilcoat"
wrote:

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.

Bob-

By now you have probably solved your problem. If not, someone else
suggested that you insert attenuation in the Sony's antenna lead. I think
that approach is most likely to produce the results you want.

If you were to replace the Sony's antenna with a dummy load, there may
still be sufficient signal bleeding into the radio to make your system
work. The interfering signal would also bleed into the radio, but at such
a low level that the Sony's tuned circuits ought to be able to handle it.

If even this does not solve the interference, you may find the problem to
be audio rectification inside the 88.1 equipment, perhaps in the
oscillator stage!

Please keep us informed of what it takes to make it work.

73, Fred, K4DII

"Bob Chilcoat" wrote in message
...
... a rather old but serviceable Sony digital air band receiver
...piped the audio out to an FM microwatt transmitter.
... visitors ...can listen on their car radios on
.... Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
...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. Bob (Chief Pilot, White Knuckle Airways)

My turn...

Lots of really good ideas here. Make sure it is the Sony antenna that is
picking it up...

1- The stub-trap idea may work with some large number of odd-quarter waves.
As you stack more odd multiples of quarter waves on (extend the shunt trap
length), the dips get closer together and as a result the peak-to-null
distance shrinks. A stub that is 21 quarter waves long has peaks and nulls
very close together (you could calculate this). Of course the dips will be
less deep due to cable loss. This may make it easier to get the desired
"Pass / Null" response. You need to make sure the signal is getting into
the antenna and not the receiver itself. The 2.4 Mc spacing is quite close.

NEW IDEA:
2- Try tuning it on the image (for your 123 Unicom frequency). You will
probably need to do this away from the airport . You should hear the Unicom
in two places on the dial spaced _TWICE_ the IF frequency away. These
radios usually have pretty wide front ends There may be a schematic inside
the radio to help determine this. If the IF is 0.455, the image is 0.910
away. This radio probably uses low side injection meaning the LO is below
the 123 by 0.455. Therefore you must tune the radio 0.910 HIGHER than 123.
This places the on channel further away from the 120.6 AWOS. Not knowing
the IF, you have to hunt.
-Maybe doing this _AT_ the airport will yield a tuning that fixes it.
Just try "tuning around".
Related idea, you could also figure out where the RF stage stuff is in
the radio and de-tune it upward to get more attenuation at 120.6, but if
that works, then just some attenuation in the antenna line may work as well
and be easier.

3- Put the radio FAR away, but run the audio in some small coax to the 88.1
closer to the visitors.

4- I like the "cross Polarization" idea. Do a light saber thing with the
Sony antenna. There may be a sweat spot that just might get the levels down
and fix the IM or whatever is going on..


Mike T. Use the airport receiver...

Sounds good, but they probably don't want you messing with it.... and what
about the airport's transmissions...

5- HOWEVER, how about simply a microphone right next to this receiver and
within earshot of the airport's microphone---ON the 88.1 Tx ?? You may
hear other stuff in the FBO, but it may be ok too...

Related... I have a little thing I plug into my dual band rig speaker
output and it transmits the audio to my car FM radio. They're made for
listening to battery operated CD players in your car.

73, Steve, K;9.D,C'I

Around here, most "lounges" or "Terminal buildings" are inside the
fence, so the original problem remains that limits non-pilots (or pilots
who happen to be driving by a strange airport) from gaining access to
the field.

Scott


John S. wrote:

Scott wrote:

I'm guessing that because of all the new B.S. concerning Homeland
Security, the airport probably has a fence around it with a security
coded gate control so that non-pilots can't get onto the field...




Most airports have a lounge of some kind....