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

wrote:
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.

years ago, many cable pay-tv signals were scrambled simply with an
interfering carrier inserted at 2.25 MHz above the video carrier. The
'decoder' was simply a notch trap that removed the carrier.
It was quite easy to remove this 'scrambling carrier' with a stub of
coax, twinlead or even zip cord. Once you know the approximate length
its a matter of just trimming away in 1/4" intervals. I forget if the
qurater-wave stub remained open and the half-wave stub was shorted, or
vice versa, but you get the drift...it only takes a few minutes to try.
That said, its easy to do sitting on the floor behind the tv with a
steady signal but if you are trying to trap an intermittent signal in
the field it might be rather maddening without a test oscillator and a
means of measuring the signal. Then its placement into the 'system' of
iffy impedance may cause it to shift slightly.

IF you could find an old cable TV "Channel A" or "Channel 14" positive
trap it would be a cinch to retune it. I don't think any of the
commercial vendors would sell them as one-off, though...they only cost
about $6 new. Maybe somebody has one in a junquebox?

-Bill- I'm surprised my junkbox didn't yield one

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