Two nearby ASOS broadcasts on the same frequency: Why no squeal?

"Peter R." wrote in message
...
....

In any case, I am curious how it is possible for two transmissions to be
heard over an aviation frequency without the squeal (I forgot the
appropriate technical term for this).



The "squeal" that you refer to, is the absolute difference in frequency
between the two base carriers of the two different transmitters. This
"difference" is known as the "beat" frequency. If the two base-carrier
frequencies are exactly correct, then the difference would be zero and no
"beat" sound would be produced. Today's equipment... it is normal to
maintain transmitter frequencies very, very accurate.

The human ear does not hear very well below about 20 hertz, so a tiny
difference of 5 or 10 hertz would not produce any "sound", either...
although it may manifest itself as a quick "pulsing" in the volume of the
signals as your receiver attempts to handle the interfering signals.

The squeal heard when two pilots key up on the same frequency is called
a heterodyne. I assume this is similar?

Yes. The heterodyne effect occurs because both transmitters differ in
frequency by some audio tone. The allowable frequency error (from the
center of the design channel center) is +/- .003%. Assuming a transmitter
on 123.0 MHz. this means that one transmitter can be about 3700 Hz. (3.7
KHz.) high and one can be 3.7 kHz. low and still be within specification.
The beat note (heterodyne) between them is (3.7+3.7) 7.4 kHz. and this is
the tone you hear.

The odds of two radios being off to the limits at opposite ends of the
limits is very low; most of the heterodynes you hear will be in the 500-3000
Hz. range.

Having said that, since power consumption for a ground station is not a
primary concern, you can put the electronic device that controls frequency
into a temperature chamber (crystal oven) and expect that day in and day out
it will be within a few Hz. of where it was originally set. The same is
true of the other ground (ASOS) station, so the beat between them will be
very low and probably subaudible in an aviation headset. You may, if you
listen carefully hear a low rumble or (if they are very close) a wha-wha-wha
at one wha per second or so.

Wha is a technical term. You'll pick it up as you go along.

Jim



wrote in message
oups.com...
The squeal heard when two pilots key up on the same frequency is called
a heterodyne. I assume this is similar?

In article ,
"Peter R." wrote:

For some reason known only to the FAA, both Dunkirk, NY (KDKK), and
Wellsville, NY (KELZ), two small, uncontrolled airports in southwestern NY
state that are about 60 nm apart, share the same ASOS frequency.

For pilots flying to Dunkirk from the east, retrieving the current ASOS
broadcast is not possible until about 25nm east of the airport, due to the
more powerful KELZ broadcast. This is not so much a problem when
conditions are VFR, but when they are IFR I prefer to retrieve the ASOS as
early as possible in order to choose and prepare for a particular
instrument approach.

In any case, I am curious how it is possible for two transmissions to be
heard over an aviation frequency without the squeal (I forgot the
appropriate technical term for this).

It's not unusual for a chain of transmitters to broadcast on what
appears to be the same frequency. They are actually all set about 5khz
apart and aviation radios filter out the squeal. That filter also makes
it more difficult to hear the higher pitched female voice.

Say what? So far as I am aware, that is just not true.

Jim

"Ron Parsons" wrote in message
...

It's not unusual for a chain of transmitters to broadcast on what
appears to be the same frequency. They are actually all set about 5khz
apart and aviation radios filter out the squeal. That filter also makes
it more difficult to hear the higher pitched female voice.

Ron Parsons wrote:
That filter also makes
it more difficult to hear the higher pitched female voice.

Where can I get one of those?

Ron Parsons wrote:
That filter also makes it more difficult to hear the higher pitched
female voice.

Dave Butler wrote:
Where can I get one of those?

Does it work on spouses and kids?

You've got more than one spouse?

Jim




"john smith" wrote in message
. ..
Ron Parsons wrote:
That filter also makes it more difficult to hear the higher pitched
female voice.

Dave Butler wrote:
Where can I get one of those?

Does it work on spouses and kids?

"john smith" wrote in message
. ..

Does it work on spouses and kids?


If the plural of mouse is mice, why isn't the plural of spouse spice?

Dave Butler wrote:

Ron Parsons wrote:

That filter also makes it more difficult to hear the higher pitched
female voice.


Where can I get one of those?


Every man gets issued one at birth.