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|Forums | Rap N Tap Clubhouse - Basic Crystal Radio Fun! | Set for picking up air traffic control||Post Reply||Send Topic To a Friend|
|raypat||Posted - 29 May 2012 19:5 |
When we were kids my father built a crystal set for my brothers and I. It was a coil wound on an old toilet roll with matchsticks as a bridge. By moving the alligator clips we would get different stations. Our all time favourite was listening to Sydney Airport traffic control. We lived near the airport in Sydney Australia by the way. We would take turns and listen for hours. I was wondering if anyone knows if it is possible to build a crystal set that would receive air traffic control nowadays? I am a total beginner and thought it would be a great first attempt project . I have a nagging feeling that modern day air traffic frequency is not in the crystal radio band capability. Can anyone help with this? Thanks.
|raypat||Posted - 29 May 2012 19:9 |
The frequencies for Sydney Tower are listed as 120.5 and 124.7 Forgot to put them in original post.
|_J_||Posted - 29 May 2012 19:44 |
Well, you are not far wrong. Typical crystal radios work well for MW frequencies because long wire antennas can harvest a lot of energy at those wavelengths. Also, there is a lot of energy put into the short wave band.
But at 120 MHz, there are two difficulties. One is getting enough energy without a beam antenna. The other is if it is FM, (vice AM) that is difficult to demodulate with conventional crystal set approaches.
But certainly possible, an interesting challenge.
|raypat||Posted - 29 May 2012 20:58 |
Thanks John. But I get the feeling from your reply this might be a bit too much of a challenge for a beginner. Will have to do some reading and see if I can at least try and put something together.
|homebrew||Posted - 30 May 2012 7:36 |
In the sixties I could pick up barge traffic just below the broadcast band. Now most have switched to FM.
If you build a set with plug in coils you may be able to pick up traveler information and airport beacons around the lower AM broadcast frequencies and plane taxi information around the 6 MHZ shortwave frequencies. These are low power transmissions so you have to be really close to pick them up on a xtal set.
|Garry Nichols||Posted - 30 May 2012 7:43 |
Air traffic control communications is AM. That is so that folks trying to listen can hear more than one station transmitting in case there is a mix up. They may not be able to copy one or both, but it will be apparent that someone else is trying to communicate. There will be a heterodyne tone heard - the audible difference between the two transmitter's frequencies.
This can be handy when a transmitter gets a "stuck mic". That is, someone's mic transmit button gets stuck in the transmit position and the user does not realize it. Communication can often proceed on top of the stuck mic, but often with difficulty.
Listening with an FM receiver would cause the strongest signal to be "captured" and the other one would not be heard. (Or maybe heard much less well, I'm no expert on this part.) So a nearby plane with a stuck mic on FM might block all communications.
When I was about 14 I had a Remco Radio which was a crystal set with a one transistor audio amp. Compared to my modern well built crystal sets, it was not such a hot performer, thinking back, despite the transistor amp.
But, I could pick up the Hartford, CT VOR (VHF navigation station) on it when using my long wire and ground. It was not supposed to tune to that frequency, but because it was not very selective with only the single tuned RF stage, the signal managed to get in somehow. I was probably 5-10 miles from the transmitter and the terrain was a bit hilly with lots of forest.
So if you are close to the transmitter, I think you stand a chance of picking up aircraft VHF, but you may find that you hear multiple frequencies at once, depending on what is close by and strong. I think it would be difficult to get selectivity out of one stage at VHF.
|golfguru||Posted - 30 May 2012 17:45 |
Some info here:
|gzimmer||Posted - 30 May 2012 21:30 |
There are a number of FM Crystal set designs on the net. Because the Air band is just above the FM BC band, and because aircraft use AM instead of FM, these sets should work well with a little tweeking. The only problem would be that aircraft transmissions are much lower power than FM Broadcast, so you will need to be quite close to the transmitter.
Incidently, another form of Aircraft transmissions are the NDB beacons which are just below the AM broadcast band. When I was a kid, the local NDB was about the only thing I could hear on my first crystal set.
Edited by - gzimmer on 5/31/2012 6:46:21 AM
|raypat||Posted - 31 May 2012 4:53 |
Thanks for all the replies. I have just bought 200ft of Litz wire. Think I will take a shot at a crystal radio with a tuned antenna circuit. I have three dual gang variable capacitors in a draw somewhere 500and 480 pf I think so I will build a fairly simple set to start with and see where it leads me.
|Garry Nichols||Posted - 31 May 2012 6:54 |
For VHF, your coils and variables will be very low value. Coils should be out of solid wire, maybe 14 to 20 gauge. Not sure if you are thinking about litz, which would not be a good idea at VHF.
What are the specs on the litz you ordered? Most folks use #44, #46 or #48 size wire with the strand count varying according to desired performance and coins in the piggy bank.
Were you going to start with a MW set?
|raypat||Posted - 1 June 2012 0:48 |
Hi gary.Yes I have given up on the air traffic idea for now and intend to build a broadcast band set first. The Litz wire I bought from ebay is 175/46.
|SWsenior||Posted - 1 June 2012 9:28 |
I'm some what confused here, and need an explanation about receiving frequencies of a crystal radio. As I understand it, AM radio is 550 kHz to 1700 kHz. So how can a crystal radio set receive a AM signal of 108 to 136 MHz, and not a CB signal of 27MHz? Some one please explain this for me.
Edited by - SWsenior on 6/1/2012 9:29:36 AM
|Garry Nichols||Posted - 1 June 2012 12:45 |
A crystal set could receive 27 mHz CB. But those sigs would likely arrive at the set's antenna much weaker than from BCB transmitters, unless the CBer is fairly close. They are limited to 5 Watts, legally.
Frequencies that high do not propagate well as "ground waves", as do BCB stations. They would be "direct waves" and subject to attenuation from objects between the transmit and receive antennas.
Aircraft might be more line-of-site (direct waves), and I think the transmitters might be more powerful, but I am not sure. The ground stations would suffer the same problems as CB sigs, but might have their antennas on a tower, improving the situation.
In both cases, a single stage set would likely receive multiple frequencies at once. I think selectivity would be a problem at higher frequencies, but I cannot explain why . . . .
Did someone say that a CB crystal set could not be done? I think that it might yield a poor result.
A regen might be worthwhile, even a single stage one. But it would have to be well designed and built because the L and C for the CB band is quite low. Any tiny movement of components would shift the frequency. Hand capacitance could drive you nuts.
|_J_||Posted - 1 June 2012 18:50 |
The strength of energy in the wave is the main distinction. Crystal sets can receive CB, it is just that CB transmitters are supposed to be so weak that you would have to be next door, or have a real strange coincidence.
Commercial broadcast transmitters (MW or SW) by contrast are very powerful and they can skip, and their waves can have a lot of energy. FM Broadcast (FM) is high power too, but don't skip much and it is hard to collect power from their waves because they are so short. Aircraft transmitters are not much stronger than CB, and they are also very short. Hope that helps?
|Garry Nichols||Posted - 1 June 2012 18:56 |
I wonder if you could clarify "strength of wave" and "shortness of wave". I'm not following whether you are indicating that the low power of the transmitter causes CB and VHF to be difficult to receive on a crystal set or whether it has to do with the shorter wave lengths (compared to BCB). Or both?
|gzimmer||Posted - 2 June 2012 0:59 |
Oh dear, confusion reigns.
A crystal set has a tuned circuit which is intended the accept the wanted station and reject all others.
If you didn't have this tuned circuit you would (in theory) be able to hear many hundreds of different AM transmitters all jumbled up together (Medium Wave Broadcast, CB, Aircraft, NDB's, mobile phones, etc). However, because a crystal set is not very sensitive, you would only be able to hear the stronger ones (eg those nearby).
So to receive CB (for instance) on a crystal set it is only necessary to re-tune your Tuned Circuit to the necessary frequency (eg 27 MHz).
Of course if you have a CB'er next door, you probably will be able to hear him even on a normal crystal set. This is because the single tuned circuit in a crystal set has very poor selectivity.
So the bottom line is this: To hear any AM transmitter, you only (1) need to tune the circuit (including the antenna) to the appropriate frequency and (2) be close enough.
You can build a simple "un-tuned" crystal set (just a diode without any tuned circuit). But as before, you will only hear strong local signals.
have fun ......... Zim
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