The use of 243 (guard)

[spanky15805]

The NAS currently has no systems that can cross connect frequencies. Nav Canada does.
I do believe the DoD uses the same system that Nav Canada does. So, if you are near a DoD RTR or RCAG
running 121.5 and 243.000 that would be my best guess.

 

[BinaryMode]

So, I currently use a BCD396XT and a BCD325P2. Both scanners monitor the two guard frequencies, and both scanners use their own independent outside antennas. The antennas are actually the old cellphone Amps antennas. I chose those because the frequencies I'm most concerned with are the trunked 800 MHz system. Never the less, I can hear aviation quite well indeed and when I do hear someone come up on guard I check my computer over here monitoring adsbexchange 24/7 to see where the plane is at. I saw the plane come up on 243 about 40 miles south of me. I don't hear the same traffic on 121.5. It's one or the other and it can be civilian.

 

[vagrant]

@BinaryMode - Are you hearing emergency traffic or just regular chatter?

 

[BinaryMode]

Regular traffic. It's like ATC is telling United or whatever to go to such and such frequency as they must have missed the hop over.

 

[alcahuete]

Regular traffic. It's like ATC is telling United or whatever to go to such and such frequency as they must have missed the hop over.

ATC will definitely broadcast on 121.5 and 243.0 simultaneously. You can isolate either, but that is the default.

Are you actually hearing civilian aircraft on 243.0, or is it just ATC looking for various aircraft?

 

[paulears]

The harmonic thing is a total red herring. The 2nd harmonic on aviation transmitters is tightly controlled by compliance standards, so is way, way down in level. In the workshop, a VHF frequency from a handheld is audible on double the frequency, but it’s gone within a few metres. It has to be otherwise, very VHF transmission would appear in the military band, causing havo, and it clearly doesn’t. If you are in an aircraft, with the usual AM squelch sensitivity, you only hear the fundamentals. If you hear 243 on a 121.5 transmission, then the transmitter is faulty! If a facility needs simultaneous transmission on both, they use two transmitters, not an unfiltered VHF one!

 

[zob-slantzero]

Sometimes when an aircraft misses a frequency change the controller will broadcast in the blind on 121.5 or 243 depending on the type of aircraft hoping that the pilot hears it and changes to the stated frequency. 121.5 and 243 are broadcast simultaneously which may explain what you are hearing.

 

[fireadioguy]

Still it has nothing to do with receiving the 2nd harmonic from an aircraft transmitter.

I could hear my old baby monitor on 149.xxxmhz 49.xx mhx tansmitter.

 

[alcahuete]

I could hear my old baby monitor on 149.xxxmhz 49.xx mhx tansmitter.

From 100 miles away?

 

[wtp]

i thought 49.845 was a stuck mic....maybe it is you...

 

[RFI-EMI-GUY]

I used to hears these Unknown Rider calls a lot near Miami//

 

[G7RUX]

I have been hearing civil aviation come up on 243 and a friend of mine who used to be an air traffic controller told me civil shouldn't have 243, but rather 121.5 which was what I suspected as well. But never the less I hear planes like United et al come up on 243. Do civil aircraft have this capability, or is there some kind of rebroadcast going on with 121.5?

You have a number of possibilities here.

Firstly, they could indeed be using the mil airband. This is unlikely for a civil aircraft, albeit possible.
Secondly, your receiver could be responding to internally generated harmonics. I would suggest this is much more likely. These could be generated in the antenna system or in nearby metalwork (rusty screws) but generally results in a mess of intermods and requires quite high signal levels.
Thirdly there could be a simulcast but this is rather unlikely. I have seen this capability in various places but it wouldn’t routinely be used for civil ATC; mil aircraft generally monitor civil ATC as well as their own for awareness.
Fourth, the transmitters could be generating significant levels of their second harmonic. This is highly unlikely as such a situation would indicate the transmitters were operating outside of their required specifications.

There are bound to be more possibilities but these are probably the most likely.

 

[BinaryMode]

I don't think it's harmonics as these transmissions are pretty far away. Sounds like they may be south of Denver and I'm way up here in northern Colorado. Attached are some examples of what I'm hearing. It's ATC calling Delta, United and SouthWest from what I can hear. It sounds reasonable to assume ATC is transmitting on both guard frequencies. I do monitor both guard frequencies on two scanners at the same time.

 

[paulears]

The stations are heavily licensed, inspected and tested. Aviation radios have all kinds of stringent restrictions because of the safety issues that faulty ones can cause. Harmonics are almost inconcievable in this context, because while faults happen, users and regular inspection would make it such a short term issue that it can be discounted. 121.5 and 243 are the most heavily monitored frequencies in the world, probably. With so many aircraft receiving ground stations, a VHF one misbehaving would be so obvious.

 

[BinaryMode]

Yeah, I read your first post and it sounds very reasonable.

Perhaps I should just kick ATC an e-mail and straight up ask. What could happen in this post 9/11 world...?

 

[alcahuete]

Yeah, I read your first post and it sounds very reasonable.

Perhaps I should just kick ATC an e-mail and straight up ask. What could happen in this post 9/11 world...?

No need. In your original post, you made it sound like the aircraft were transmitting on 243.0. With just ATC calling aircraft, yes, they transmit on both guard frequencies simultaneously by default. You can isolate VHF or UHF, but most controllers do not, so you will hear them broadcast on 121.5 and 243.0 simultaneously. That's every day standard protocol.

 

[BinaryMode]

Thanks for the clarification.

 

[autovon]

The NAS currently has no systems that can cross connect frequencies. Nav Canada does.

Is this for sure? A number of times this year I've heard a few frequencies (in ZMP & ZDC area) that sound like they are cross coupled. This includes the controller telling an aircraft to switch freq and hearing them when they were on the old and new one.

 

[spanky15805]

Yes, in our NAS, this is for sure. Multicasting is about the best way to describe it. Think one controller watching 3 different sectors, each sector having 2 different low enroute frequencies, one high enroute and one ultra high enroute. So for low you have 2 different vhf/uhf pairs, high has a vhf/uhf pair and ultra has vhf/uhf pair. When AT keys the mic ALL of those radios are keying at the same time. Headset is long gone, all of the traffic is coming out of loudspeakers. alcahuete could really draw you a picture. You are hearing the controller hand off the aircraft to the next sector, btw it's the same person who is typing into STARS they have the plane.

alcahuete, please correct what I stated incorrectly.

any of that make sense?

 

[paulears]

Multi frequency operation is common, but what we were originally talking about were the planes transmitting on the military frequency? ATC and the other control facilities listen and can speak on both if the equipment is fitted, but a civil aircraft does not have the antenna and radio installs for UHF. Military ones have both. Military aircraft needing to talk to civilian traffic come up on VHF - we have heard this. If you hear a civilian aircraft on 243, then somebody has cross band repeated it. As DF is usually fitted to both emergency frequencies, any repeat would be awkward, with the bearing to the repeater, not originator coming up?