243.000 mHz is a valid ELT frequency. Also commonly referred to as "guard" for voice comms.
ELT beconing on VHF and UHF Guard
- Thread starter west-pac
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Has nothing to do with the second harmonic. 243.0 is the UHF Guard frequency. The transmission would take place simultaneously on both 121.5 and 243.0.
wtp
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kayn1n32008 said:
Hearing it on 243 would be the second harmonic. Even airborne I wouldn't expect to be able to hear it very far away.
like when the space station is only 250 miles up and a couple of hundred land miles away and i hear packet or voice ?
Hearing it on 243 would be the second harmonic. Even airborne I wouldn't expect to be able to hear it very far away.
like when the space station is only 250 miles up and a couple of hundred land miles away and i hear packet or voice ?
Don't overlook a certain brand of Swiss watch that is marketed to pilots and has a 121.5 MHz transmitter beacon in it. Useful to a professional, but often purchased by fools who unscrew a secondary crown and then activate the transmitter while bragging about the watch to others.
mmckenna
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kayn1n32008 may have been referring to some of the older EPRIBS that only ran 121.5MHz. Those could be sometimes picked up on 243.0 via the harmonic of 121.5MHz.
It would probably be pretty weak compared to the 121.5MHz primary signal.
It would probably be pretty weak compared to the 121.5MHz primary signal.
kayn1n32008
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Yes. And don't modern beacoustic TX @406.xxxMHz and have a low power 121.5 beacon for local DF'ing?
kayn1n32008
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No **** 243MHz is UHF guard. And it has everything to do with it being a second harmonic.
kayn1n32008
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Are you listening to the ISS cross band repeater on the fundamental frequemcy of 437.800Mhz or the second harmonic at 875.600MHz? Or listening to the packet/voice ops on a fundamental frequency of 145.800Mhz or the second harmonic at 291.600MHz. JFC, No **** you can hear the ISS 250Mi away when you have LOS and its transmitting at 5 or 10w. Ever tried to listen @ the second harmonic? Apples to ****ing oranges bro.
Maybe I'm wrong, but old civilian ELT's only transmitted @121.5MHz did they not? Is there not harmonic suppression required in Air band transmitters? Like the harmonic should be a small fraction of the power of the fundamental frequency.
nd5y
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Some analog 121.5 ELTs were designed to transmit the 2nd harmonic.
mmckenna
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Most of the older ones did, but I think they are slowly phasing them out. Spend more of the limited battery on 406 + GPS than driving a 121.5 beacon. A beacon is useful, but if the device is sending lat/lon down to a few meter accuracy, a separate beacon is arguably pointless.
mmckenna
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kayn1n32008
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Civilian ELT? At what kind of power level?
kayn1n32008
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Apologies for the atrocious fat fingering/no autocorrect. Can't go back and edit.
Again, not in the sense you're saying. It is not that the carrier is being broadcast on 121.5 and you just happen to hear the second harmonic on 243.0, thus affecting the range to where you can't hear it far away even if airborne.
Frequency doublers are used, and then the second harmonic is amplified to the same power output as the fundamental. So a 5 Watt ELT for example, is going to produce the same 5W on 243.0 as it does on 121.5. I promise you, an airborne ELT on 243.0 is going to be heard almost the same distance as 121.5.
kayn1n32008
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Double 121.5 and what do you get? The second harmonic... 243MHz.
Can we take a wild ass guess why UHF guard is 243MHz???
Now that we got that cleared up, can we take a wild ass guess why there would be a transmission simultaneously on both 121.5Mhz and 243Mhz from a 121.5MHz ELT???
I'd bet with an AM receiver, in close proximity to a 121.5Mhz ELT, we could even pick it up on I don't know, just a pick a random number out of a hat. Let's say 364.5MHz. I like that number. It's a good number. Rearranged, it can even be sequential: 345.6.
But some how I don't think we would hear the beacon on THAT frequency.
Here’s a current one, shows to me 100 milliwatts in both 121.5 and 243.
ELT 3000HM - ACR ARTEX
The all new ARTEX ELT 3000HM is a truly integrated, transport-grade Emergency Locator Transmitter (ELT) for the rotorcraft market. The compact form factor is coupled with the integrated NAV interface (ARINC429 or RS232) which omits the requirement and expense of installing a separate NAV...
www.acrartex.com
wtp
Member
kayn1n32008,
so does EVERY transmitter also transmit double the second harmonic ?
things would get kind of crowded.
so does EVERY transmitter also transmit double the second harmonic ?
things would get kind of crowded.
I understand that 243.0 is the second harmonic of 121.5. Again, you stated in your original post that you wouldn't be able to hear the 243.0 signal very far away because of it being the 2nd harmonic. That is 100% incorrect. That second harmonic is going through a frequency doubling stage, then amplifier stage, and is coming out fo the transmitter at the same output power as the fundamental. It is no different (output wise) than having two distinct transmitters on the different frequencies. Thus you would expect to hear an ELT roughly the same distance on 243.0 as you would on 121.5.
mmckenna
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I was wrong. 406MHz EPRIBS/PLB's do have 121.5MHz. Low power, as in milliwatt range.
The older 121.5MHz ELT's were 100mw. They did make 121.MHz -only- ELT's.
Here's an article discussing the end of use of them: ELTs: 121.5-only units no longer legal for sale, no longer being certified
The older 121.5MHz ELT's were 100mw. They did make 121.MHz -only- ELT's.
Here's an article discussing the end of use of them: ELTs: 121.5-only units no longer legal for sale, no longer being certified
mmckenna
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Most equipment specifically has filtering to lower any harmonics down to a point they can't be heard.
If you look at the HF ham bands, they were originally set up so the harmonics would fall in the higher ham bands: 3.5MHz, 7MHz, 14MHz, etc… Back in the old days, filtering out the harmonics on a home built transmitter wasn't easy, so the bands were set up that way.
There were absolutely 121.5MHz -only- ELT's. They were the more affordable options back then. Search and Rescue guys could home in on the second harmonic of the 121.5MHz transmitter (which falls on 243MHz) since it was a much lower power signal. It made it easier to home in on the location as you got really close to the beacon, since it was a much lower power signal. The 121.5MHz signal would be too strong to get an accurate fix as you got closer.
Amateur radio operators doing "fox hunts" will often do that if the primary signal from the "fox" is too strong.
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