AM receiver convert to ATC receiver

[Archimedes]

Hi all

If I change the tank circuit components (reduce the capacitance and
inductance) and change the transistors to appropriate VHF ones, will I
be able to pick up ATC (Air Traffic Conversations) using this
circuit ?

http://www.electronics-lab.com/projects/rf/006/index.html

Thanks
Shelton.

 

[Bob Eld]

Hi all

If I change the tank circuit components (reduce the capacitance and
inductance) and change the transistors to appropriate VHF ones, will I
be able to pick up ATC (Air Traffic Conversations) using this
circuit ?

http://www.electronics-lab.com/projects/rf/006/index.html

Thanks
Shelton.

Not likely. I doubt you'd ever get that circuit to work at VHF frequencies
no matter what you did with the resonant circuit portion. There are many
problems including wrong impedances for the various parasitic capacitances.

Secondly air traffic stuff is FM I believe.

 

[Michael Black]

Not likely. I doubt you'd ever get that circuit to work at VHF frequencies
no matter what you did with the resonant circuit portion. There are many
problems including wrong impedances for the various parasitic capacitances.

Secondly air traffic stuff is FM I believe.

Military may use FM, I don't know, but airplane related communication is
unique in that it does use actual AM.

A project that saw publication a number of times in the old days took
advantage of that, a "crystal radio" that tuned VHF. It was nothing
more than a tuned circuit and a diode detector feeding an earphone, not
sensitive but useful near airports and since it didn't radiate anything,
even useable (though maybe not legally) on an actual airplane.

The description of the circuit says it's a regen receiver, and those
were never popular at VHF, I'm assuming instability came into play.
You did see superregen receivers there. Either type will radiate, and
that's not a good thing in the aircraft band.

Michael

 

[Archimedes]

On Wed, 16 Apr 2008, Bob Eld wrote:





Military may use FM, I don't know, but airplane related communication is
unique in that it does use actual AM.

A project that saw publication a number of times in the old days took
advantage of that, a "crystal radio" that tuned VHF.  It was nothing
more than a tuned circuit and a diode detector feeding an earphone, not
sensitive but useful near airports and since it didn't radiate anything,
even useable (though maybe not legally) on an actual airplane.

The description of the circuit says it's a regen receiver, and those
were never popular at VHF, I'm assuming instability came into play.
You did see superregen receivers there.  Either type will radiate, and
that's not a good thing in the aircraft band.

   Michael- Hide quoted text -

- Show quoted text -

Hi

Thanks for all the reponses - yes i can calcualte the required
parameters to get this into the ATC band - but im now confused - some
say it will work some say it wont? So will it or will it not ?

 

[Phil Allison]

"Archimedes"

" Thanks for all the reponses - yes i can calcualte the required
parameters to get this into the ATC band - but im now confused - some
say it will work some say it wont? So will it or will it not ? "


** IMO - very poorly, if at all.

You will need to be very close to an airport to hear anything and then you
will regularly hear several voices together as TRF sets have poor
selectivity.

VHF communications listening really requires the receiver to be a superhet -
preferably a double conversion one.

A small " scanner " receiver is ideal.



....... Phil

 

[Archimedes]

Thanks Phil - very informative answer - and thanks to the others who
also responded!

 

[Claude]

Military may use FM, I don't know, but airplane related communication is
unique in that it does use actual AM.

A project that saw publication a number of times in the old days took
advantage of that, a "crystal radio" that tuned VHF. It was nothing
more than a tuned circuit and a diode detector feeding an earphone, not
sensitive but useful near airports and since it didn't radiate anything,
even useable (though maybe not legally) on an actual airplane.

The description of the circuit says it's a regen receiver, and those
were never popular at VHF, I'm assuming instability came into play.
You did see superregen receivers there. Either type will radiate, and
that's not a good thing in the aircraft band.

Michael

Here are the facts

Military ground troups and close air support
30 Mhz to 87.975 FM only

Air traffic control
117.975 to 156.000Mhz, AM only

Maritime
156.000 to 173.975 ( with some reserved for Sonobuoy operations) FM only

Military
225 Mhz to 399.975 Mhz AM or FM ( they can actually choose wich one for any
freq in this band)
Not interesting in this band as there is a lot of encryption, freq hopping
and other hush hush stuff.

Claude

 

[Joerg]

Military may use FM, I don't know, but airplane related communication is
unique in that it does use actual AM.

A project that saw publication a number of times in the old days took
advantage of that, a "crystal radio" that tuned VHF. It was nothing
more than a tuned circuit and a diode detector feeding an earphone, not
sensitive but useful near airports and since it didn't radiate anything,
even useable (though maybe not legally) on an actual airplane.

The description of the circuit says it's a regen receiver, and those
were never popular at VHF, I'm assuming instability came into play.
You did see superregen receivers there. Either type will radiate, and
that's not a good thing in the aircraft band.

Yes, and then you'd quickly have visitors coming with vehicles bearing
government license plates. A regen-receiver is most definitely not a
good idea in the aircraft band.

 

[Claude]

Yes, and then you'd quickly have visitors coming with vehicles bearing
government license plates. A regen-receiver is most definitely not a good
idea in the aircraft band.

Quite correct, it is most likely jail time where you will have hours and
hours of fun designing electronics such as movement alarms
that will detect the proximity of "fellow inmates" . Just swearing on an ATC
frequency will net you a $2,500 fine in Canada, I can't even imagine what
they would do to you if you jammed one of their frequencies.

Claude
Montreal

 

[Joerg]

One of you naysayers should estimate the amount of radiated energy from a
typical regen receiver.

Also, what is the distance from the intended regen location to the airport,
and what would you imagine the comparative strengths would be of the regen
signal and air traffic signals at air traffic receivers?

The regen receiver radiated signal would be lost in the noise.

The other party to air traffic communication are aircraft. Those happen
to roam about quite a bit

Seriously, disregarding the airstrip that's almost next to the office
here we are also roughly in the flight path for Mather Field. Altitude
above our building maybe 1500ft, give or take. If Fedex, DHL and other
pilots would report some weird shhhhht noise everytime they pass a
certain spot, guess what would happen?

 

[Bob Monsen]

The other party to air traffic communication are aircraft. Those happen to
roam about quite a bit

Seriously, disregarding the airstrip that's almost next to the office here
we are also roughly in the flight path for Mather Field. Altitude above
our building maybe 1500ft, give or take. If Fedex, DHL and other pilots
would report some weird shhhhht noise everytime they pass a certain spot,
guess what would happen?

One of the nice things about AM, and the reason they use it for aviation, is
that power wins. If two signals are colliding, the more powerful one will
always be heard.

There are various "unicom" frequencies around the US, which are used by
pilots at uncontrolled airports for announcing positions in the traffic
pattern. Since the frequency space is not all that big, they tend to
overlap. You can often hear folks announcing at airports up to 100 miles
away. However, there is never any problem, since the near transmitters just
blast over the far ones.

I'm guessing the regen receiver outputs less than one mW through its power
wires. That isn't going to be a problem for anybody unless the OP decides to
start messing with the design, and somehow manages to build a far more
effective transmitter. If he puts it into a metal box, he'll be safe.

OTOH, he'll never get it working in the AM aircraft band, since the circuit
itself really sucks.

He should instead buy something like this:

http://cgi.ebay.com/Blue-Emergency-...tcZphotoQQssPageNameZWDVWQQrdZ1QQcmdZViewItem

For 35 bucks buy-it-now, not a bad deal. It also does weather, AM broadcast
band, FM broadcast band, and shortwave, and even works from a crank so you
can work out while listening to the ATC folks vector jets around.

If the OP really wants to hack at something, I'd suggest an old Cessna
transceiver. Just don't plug in the microphone, and power it from 24V.
Should be fun to play with. This actually will get you arrested if you mess
up and jam a frequency, though. There is one on sale at eBay now for $10,
but it'll go for more.

I landed at Mather once when it first went GA. Very creepy. The runway is so
wide that it feels like you are much lower than you really are. A 172 feels
like a gnat on a surfboard.

Regards,
Bob Monsen

 

[Phil Allison]

"Bob Monsen"

One of the nice things about AM, and the reason they use it for aviation,
is that power wins. If two signals are colliding, the more powerful one
will always be heard.

** That is how FM behaves - ie the "capture effect".

AM does no such thing - " colliding " signals simply combine in the
receiver and are heard together.

Weak signals are heard in the background of strong ones.



....... Phil

 

[Bob Monsen]

"Bob Monsen"



** That is how FM behaves - ie the "capture effect".

AM does no such thing - " colliding " signals simply combine in the
receiver and are heard together.

Weak signals are heard in the background of strong ones.



...... Phil

Thanks for the clarification. However, I'm curious about this.

An FM signal is really just a frequency shift on the carrier. As I
understand it, the transmitted signal is the carrier frequency shifted in
proportion to the amplitude of the sound that will get transmitted (for
mono).

So, given two transmitters on the same frequency, you end up with two
varying carriers. If your receiver is just receiving both signals, mixing
them down to a lower frequency, detecting the shift, and converting that
into an amplitude, the detector must be doing this locking on. Do you know
how it works? How does it lock on rather than just outputting a mess?

Thanks,
Bob Monsen

 

[Phil Allison]

"Bob Monsen"

"Phil Allison"

Thanks for the clarification. However, I'm curious about this.

** Firstly - I am very impressed that you accepted my comments in the
spirit they were intended.

A rarity on usenet - my congrats.

An FM signal is really just a frequency shift on the carrier. As I
understand it, the transmitted signal is the carrier frequency shifted in
proportion to the amplitude of the sound that will get transmitted (for
mono).

So, given two transmitters on the same frequency, you end up with two
varying carriers. If your receiver is just receiving both signals, mixing
them down to a lower frequency, detecting the shift, and converting that
into an amplitude, the detector must be doing this locking on. Do you know
how it works? How does it lock on rather than just outputting a mess?

** Capture effect is almost entirly due to the " limiting " that occurs in
the IF amplifier stages.

Unlike AM, the IF stages of an FM receiver are normally operated very
heavily into overload ( ie gross amplitude clipping ) so the weaker of two
( IF frequency FM signals ) is completely over-whelmed by the stronger
ne - which them becomes the only signal present at the FM detector.

There is even a Wiki about it:

http://en.wikipedia.org/wiki/Capture_effect


This feature was seen as a BAD idea for aircraft radio comms - it being
preferable to have something more like a telephone party line, so the
weaker voice could still be heard and even if not read clearly, the pilot or
ground controller could ask for a repeat of the message.

This got screwed up once at Tenerife and two jumbo collided as a result.

http://en.wikipedia.org/wiki/Tenerife_disaster#Communication_misunderstandings




....... Phil

 

[Joerg]

I doubt that at 1500 feet the signal from a regen receiver would break the
squelch.

From a regen that misbehaves it certainly can.

 

[Bob Monsen]

** Capture effect is almost entirly due to the " limiting " that occurs
in the IF amplifier stages.


...... Phil

Thanks for the info.

Regards,
Bob Monsen

 

[Varactor]

"Bob Monsen"




** That is how FM behaves  -  ie  the  "capture effect".

AM does no such thing  -  " colliding " signals simply combine in the
receiver and are heard together.

Don't know abou that. When two stations broadcast over each other its
generally just distorted garbage. I speak as a pilot, not from theory.

 

[Bob Monsen]

Don't know abou that. When two stations broadcast over each other its
generally just distorted garbage. I speak as a pilot, not from theory.

As a pilot too, I know that if people step on each other's communications,
it ends up with distortion. Stuck mikes are always a problem. However, the
receiver isn't locked onto the stuck mike, and you hear something. ATC can
often blast over a stuck mike to get the offender's attention. Also, it is
possible for pilots to talk over other pilots at remote fields with the same
unicom frequency (very common here in California, despite the attempt by the
FAA to spread out the frequency assignments).

Anyway, enough said. I had always been told by instructors/pilots that AM
was preferred due to this effect, so I was parroting that (as they probably
were), without really examining the claim in detail. However, it does appear
to have some merit.

Regards,
Bob Monsen

 

[Joerg]

Yes, just about anything and everything that misbehaves can can cause a
problem. When I lived in Ketchikan, AK, I received one of those
salmon-colored FCC QSL cards from a listening station at Point Reyes, CA due
to a misbehaving multiplier in my HT9. Got a 579 on my second harmonic.

That one would be worth placing it in a frame

Problem with homebuilt regens is that unless they have a preamp they can
easily emit lots of RF power if something goes wrong, straight out the
antenna. "Dang, why doesn't it receive anything?" ... "Hey, Joe, uncle
Leroy said the ballgame just started." ... "Ok, coming." ... Meantime
the regen is forgotten and happily keeps humming until after the
overtime. Back when I was a kid people built their own RC electronics
because the stuff from companies such as Robbe was financially out of
range for most of us. More than once have I seen someone flick that
switch on the boat, followed by other boats instantly going out of control.

 

[Joerg]

I have tried to read and understand the posts in this thread the best I
can. Can someone tell me; Is there a problem for the aircraft industry, or
me, if I sit at the end of Regan National airport outside DC and use this,

http://tinyurl.com/3ayskq

As Chris said it's a superhet. Now if the oscillator leaks and the thing
runs the usual 10.7MHz IF the oscillator could still land on an active
ATC channel. That would mean trouble.

BTW $44.95 is a bit steep. Simple airband radios can be had for less,
sans soldering.