The use of 243 (guard)

[BinaryMode]

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?

 

[N9JIG]

Are you sure that you are hearing the commercial planes themselves or a controller talking to them? If the latter then the controller is likely multicasting on both 121.5 and 243.0. They often multicast on civil and military freqs.

 

[vagrant]

Harmonics

 

[ChrisP]

Indeed - 243.0 is 121.5 x 2

- Chris

 

[nd5y]

Harmonics

Not harmonics. If it was then why does nobody else hear civilian aircraft in the 236-274 MHz range?

 

[vagrant]

Not harmonics. If it was then why does nobody else hear civilian aircraft in the 236-274 MHz range?

The secondary and tertiary harmonic are progressively and significantly reduced from the fundamental frequency ( dBC ). Anyways, I think N9JIG pegged it with a controller simulcasting, but if it really was from an aircraft I am wondering how close the OP is to the aircraft and what they are using for an antenna system.

 

[ChrisP]

Not harmonics. If it was then why does nobody else hear civilian aircraft in the 236-274 MHz range?

Actually, I do. I'm near the flight path of a local airport and hear signals from the VHF frequencies multiplied by 2 in the UHF band. Granted, the aircraft signals are pretty strong, bu tit can happen.

- Chris

 

[spacellamaman]

Indeed - 243.0 is 121.5 x 2

- Chris

and 40.5 x 3 = 121.5

I believe 40.5 is Army aviation emergency.

 

[wtp]

and did anyone think it was for the antenna harmonic and not from a radio harmonic.???
a 121.5 quarterwave is a halfwave at 243.0.
edit.
OK vagrant, but i just got in and was tired. sooooo...
121.5 quarter wave is a 243.0 halfwave.
and thank you.

 

[vagrant]

and did anyone think it was for the antenna harmonic and not from a radio harmonic.???
a 121.5 quarterwave is a halfwave at 243.0

Perhaps rethink that for a while.

 

[KevinC]

Perhaps rethink that for a while.

Strike that, reverse it.

 

[wtp]

 

[RichardKramer]

Actually, I do. I'm near the flight path of a local airport and hear signals from the VHF frequencies multiplied by 2 in the UHF band. Granted, the aircraft signals are pretty strong, bu tit can happen.

- Chris

I do too, using a Stridesburg amp, RS discone 40' agl, and a 16 port tv splitter. I sometimes have to replay the recordings twice to be sure the tx was from a commercial ac.

 

[ATCTech]

It's possible that whatever the ATC facility you're listening to has enabled cross-coupling of multiple frequencies including 121.5 and/or 243.0 in error, especially if it's most noticable at repeatable times of the day or night. That would allow "perfect" reception of any regular ATC frequencies they have coupled. Without knowing what voice switch is in use I can't take it further down the rabbit hole but that's exactly what cross-coupling is for, minus using it with guard frequencies in "normal" operation of course. In Canada our enroute, TCU, Tower and FSS facilities all have that function available to them.

Just as an aside, decades ago we had a software "map" for one sector that accidentally put 121.5 in the radio module that was assigned to a regular en route frequency until the sector's midnight shift maps were activated. If the controller didn't notice, they would be transmitting on 121.5 for minutes or longer. One member of the public even filed a formal complaint about it.

 

[imonitorit]

I thought the only VHF air band image you could pick up easily was the freqency + (I.F. of the radio x 2) Example: I.F. 10.8, 133.000 + 21.6= 154.600.

We used this technique back in the early days of scanners when the Bearcat 210 and other radios didn't cover the 406-420 fed band.

 

[AFD103]

"Generally, civil aircraft are VHF radio equipped, and military are UHF radio equipped. However, both emergency frequencies are available to any aircraft properly equipped."

https://www.faa.gov/air_traffic/publications/media/atpb_feb_2023.pdf

 

[nd5y]

I thought the only VHF air band image you could pick up easily was the freqency + (I.F. of the radio x 2) Example: I.F. 10.8, 133.000 + 21.6= 154.600.

We used this technique back in the early days of scanners when the Bearcat 210 and other radios didn't cover the 406-420 fed band.

That's different. Images are caused by local oscillator harmonics inside the receiver.

 

[tvengr]

That's different. Images are caused by local oscillator harmonics inside the receiver.

Incorrect! In the mixer stage, frequencies lower and higher than the local oscillator frequency can both produce the same IF. For example, I used to monitor Channel 2 TV audio with my scanner. The scanner first IF was 10.8 MHz. If I programmed the scanner to receive 38.16 MHz, the local oscillator for that frequency was 48.96 MHz (38.16+10.8=48.96). The TV audio was 59.76 Mhz which was 10.8 MHz above the local oscillator frequency (59.76-48.96=10.8). As you can see those 2 frequencies below and above the local oscillator will produce a 10.8 MHz difference frequency and both will feed through the receiver IF. Rejection of the image frequency is only affected by the selectivity of the RF amp. In case you are wondering why our aural frequency was not the standard 59.75 MHz, our visual carrier was offset by +10 KHz to prevent a venetian blinds effect for viewers between Baltimore and New York. As a result, the aural carrier also needed to be raised 10 KHz to maintain the 4.5 MHz intercarrier (difference between visual and aural frequencies) to properly demod the audio in TV sets.

 

[nd5y]

Incorrect!

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

 

[prcguy]

From the description of the transmissions heard on 243.0 it doesn’t sound like a second harmonic of 121.5 as you should only be hearing emergency related traffic there or on 243.0. Sounds more like a 10.7MHz or similar image.