N353SK
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Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 6:07 pm

I thought about posting this in the military forum, but it seems that all of us true airplane geeks hang out here in tech/ops.

Simply put, why does the military communicate on UHF frequencies as opposed to the civilian VHF? Since it's the US military we're talking about, I'm sure UHF is about 10% better than VHF and 10 times more expensive, but why? I understand the advantage of TACANs as they're much smaller and can easily be placed on something like an aircraft carrier, but what is the advantage of using UHF for voice communications?
 
nomadd22
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 6:21 pm

Smaller radios, smaller antennas, more predictable coverage, less likely to be affected by interference. It's not 10 times or 2 times more expensive. It's just more capable so you can add expensive data overlay and encryption features that wouldn't work as well on VHF.
UHF is more likely to get where you want it to if you have your coverage figured right and less likely to be heard at great distances since it doesn't refract or bounce off the ionosphere as much as VHF.
Anon
 
KELPkid
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 6:21 pm

Back in the 1950's(?) when the decision was made, the parts (oscillators?) needed to construct a UHF radio receiver were not widely available and were super expensive, and the military chose a "security through obscurity" route. Today, of course, this is not always the case...

Of course, ever since the mid-1970's, a UHF radio scanner has been available at the consumer level..it was the advent of electronics (the transistor age) that made the technology affordable.

On a side note: sometimes you hear military aircraft on the airwaves VHF (Army National Guard helicopters being a primary example). Does anyone know which military birds get VHF radios in addition to the UHF radios?
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
nomadd22
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 6:30 pm



Quoting KELPkid (Reply 2):
Does anyone know which military birds get VHF radios in addition to the UHF radios?

Can't say I know, but I'd assume that almost all military aircraft have a standard aviation VHF so they can talk to civilian aircraft, officials and controllers
Anon
 
dragon6172
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 7:00 pm



Quoting Nomadd22 (Reply 3):
Can't say I know, but I'd assume that almost all military aircraft have a standard aviation VHF so they can talk to civilian aircraft, officials and controllers

I would say you are probably correct. I know the H-46's that I flew on had reciever/transmitters capable of both UHF and VHF.
Phrogs Phorever
 
MD11Engineer
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 7:15 pm

Civilian aircraft VHF radios are in a way obsolete anyway. They use AM, which is very much prone to static interference and wastes a lot of transmission power (about anybody else uses FM, which, while using a similar bandwith, is less prone to interference, or single sideband, which uses less bandwidth and less transmission power), which is . I assume that the military UHF radios use FM.

Jan
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2H4
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 7:30 pm



Quoting Nomadd22 (Reply 1):
Smaller radios, smaller antennas, more predictable coverage, less likely to be affected by interference.

So what is the likelihood that the civilian sector will ever transition to UHF? When (if ever) might this happen?

2H4
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A342
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 7:37 pm



Quoting 2H4 (Reply 6):
So what is the likelihood that the civilian sector will ever transition to UHF? When (if ever) might this happen?

I have no clue, but if UHF is indeed that superior, the military won't want to lose their frequency bands.
Exceptions confirm the rule.
 
PhilSquares
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 7:54 pm



Quoting Nomadd22 (Reply 3):
Can't say I know, but I'd assume that almost all military aircraft have a standard aviation VHF so they can talk to civilian aircraft, officials and controllers

Not quite. All the controllers have both UHF and VHF. So, the military aircraft could have UHF only.
Fly fast, live slow
 
IAHFLYR
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 8:24 pm

Almost every military aircraft that I've talked with in the last year or so is using VHF for ATC communication, of course there are a few who don't but those are dwindling.

Quoting 2H4 (Reply 6):
So what is the likelihood that the civilian sector will ever transition to UHF?

I would have to guess that won't happen, if for no other reason that cost alone with equipping all civil aircraft with UHF radios, but ya never know!
Any views shared are strictly my own and do not a represent those of any former employer.
 
rwessel
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 06, 2008 10:53 pm

The military aircraft radio spectrum, 225-400MHz (which is AM), actually straddles the boundary between VHF and UHF. There are only modest differences in propagation properties between that and the 118–132MHz band used for civilian aviation.
 
jgarrido
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sat Jun 07, 2008 11:56 am



Quoting KELPkid (Reply 2):
. Does anyone know which military birds get VHF radios in addition to the UHF radios?

I couldn't even start a compressive list, but in my experience all the tankers/cargo have VHF and seem to prefer it when given the option (c-5, c-130, k35, etc). Fighters and bombers tend to prefer UHF, but I've talked to b-52's, b-2's, f-16's, f-22's and f-18's on VHF. When I was in the USAF at a training base the T-1's (basically a BE-40) had both, the old T-38's were UHF only, the new 38's had both. I don't remember if the T-37's had VHF but it seems as though they did.

Quoting Nomadd22 (Reply 3):
Can't say I know, but I'd assume that almost all military aircraft have a standard aviation VHF so they can talk to civilian aircraft, officials and controllers

Wouldn't be bad idea, but I'm pretty sure all ATC facilities (at least in the US) have UHF frequencies assigned to them, even FAA and contract run facilities.
 
dragon6172
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sat Jun 07, 2008 4:41 pm

As I said, a lot of military aircraft are equipped with VHF/UHF R/Ts. From the Rockwell Collins website on the AN/ARC-210 radio.

Quote:
30 to 400-MHz frequency range provides VHF and UHF in all radios

It is being installed in all U.S. Navy tactical aircraft, as well as in over 180 platforms in the U.S. Navy, U.S. Marine Corps, U.S. Air Force, U.S. Coast Guard, U.S. Customs, U.S. Army and international services. A sampling of domestic platforms include the F-16, F/A-18, A-10, F-117, F-15, MV-22, CV-22, AH-1W, AWACS, B-52, MH-53, CH-46, C-17, C-5, KC-135, RC-135, E-4B, C-130, AH-64, MH-60S/R, UH-60, US101, Global Hawk, Predator, J-UCAS, P-8, eight DDG-51s, Littoral Combat Ship (LCS), and two CVNs. Internationally, the AN/ARC-210 is installed in Canada on Utility Tactical Transport Helicopter, MHP, C-130, CP-140, and F/A-18; in Finland on the F/A-18; in Australia on F/A-18, C-130, Air87 Tiger helicopter, Wedgetail; in Switzerland on F/A-18s; on the AH-64 for Egypt & Singapore; on P-3s for Denmark & Netherlands.

Phrogs Phorever
 
buckfifty
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sat Jun 07, 2008 6:58 pm

Most of the time, travelling in international civilian airspace, the U.S. military will use VHF. Hear them all the time on the radio. Came back from Dubai just a little while ago, kept hearing the tankers trying to get a hold of Karachi FIR. I must say, the standard of R/T in the USAF is quite bad.
 
mark5388916
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sat Jun 07, 2008 9:56 pm

I know when the C-17s from RIV came into RAL (Riverside Muni) for the airshow they used VHF and according to the pilots was for the sake of the spotters  Wink

Mark
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Caryjack
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sun Jun 08, 2008 3:55 am



Quoting N353SK (Thread starter):
what is the advantage of using UHF for voice communications

For strictly voice communications there is no advantage of UHF over VHF. The UHF spectrum simply has (had) more room for expansion.

Quoting Nomadd22 (Reply 1):
Smaller radios, smaller antennas, more predictable coverage, less likely to be affected by interference. It's not 10 times or 2 times more expensive. It's just more capable so you can add expensive data overlay and encryption features that wouldn't work as well on VHF.

UHF radios are often smaller than their VHF counter parts but in reality very few parts of a radio actually operate at the assigned UHF or VHF frequencies. For example a 422 MHz receiver has an input stage and an oscillator that operate at UHF (422 MHz) but after those two stages the frequencies drop to intermediate frequency (IF) stages below 20 MHz and then to the audio stages. Transmitters start with lower frequency oscillators then use frequency doublers and tipplers to attain the desired frequency where only the driver and final amplifier stages are operating at the assigned UHF frequency. A UHF transmitter may require an extra stage or two more than a VHF unit to work up to the desired frequency but otherwise both units are functionally identical.
Antennas are a different story. These are resonating devices that must be physically proportional to the wavelength of the carrier frequency. In other words, the higher the frequency, the shorter the wavelength, so the smaller the antenna. For example, an antenna designed for a 120 MHz carrier might be 63 cm (1/4 wavelength) long but an antenna operating at 420 MHz would be about 18 cm. This means that a higher performing antenna (direction & gain) can be put in a much smaller package. Some types of antennas (Marconi) require a conductive ground plane such as the skin of an airliner to complete the circuit. The ideal radius of this ground plane (flat and unobstructed) is generally equal to the length of the antenna. So as frequencies increase, the size of the ground plane decreases, resulting in increased performance in a smaller area. Antennas typically have design resoultions of 1/4 or !/8 wavelengths.
UHF allows for the transfer of much more information because at the higher frequencies proportionally more information can be multiplexed onto the carrier (transmit frequency).

Quoting Nomadd22 (Reply 1):
.
UHF is more likely to get where you want it to if you have your coverage figured right and less likely to be heard at great distances since it doesn't refract or bounce off the ionosphere as much as VHF


Once you get into the VHF spectrum, propagation is pretty much line of sight. Directing signals at the horizon will cause a reliable over-the horizon scatter effect, and heat inversion layers can sporadically duct UHF signals several hundred miles, but actual refraction of VHF signals is very rare.

Quoting 2H4 (Reply 6):
So what is the likelihood that the civilian sector will ever transition to UHF? When (if ever) might this happen?

By civilian sector you are clearly referring to the airline industry. UHF has been used by other civilian sectors including police, fire, business, and by armatures for more than 35 years. UHF systems are inexpensive, reliable and available. From reading this thread the issue is not so much the carrier frequency but the method of imposing information onto that carrier - AM vs FM.

Quoting MD11Engineer (Reply 5):
Civilian aircraft VHF radios are in a way obsolete anyway. They use AM, which is very much prone to static interference and wastes a lot of transmission power



Quoting Rwessel (Reply 10):
The military aircraft radio spectrum, 225-400MHz (which is AM), actually straddles the boundary between VHF and UHF. There are only modest differences in propagation properties between that and the 118--132MHz band used for civilian aviation.

This is the part that I don't get. Frequency modulation (FM) has such a clear advantage over amplitude modulation (AM) that I don't understand why FM isn't used. It must be that airliners fly above the fray and that the line of sight advantage they have at 35,000' mitigates the inherent problems suffered with AM communications.
Cary
 
SlamClick
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sun Jun 08, 2008 4:04 am

I can tell you why I liked using UHF over Victor. Less chatter. I loved it that every transmission one heard was from a professional, for professionals. Never heard

"umm"
"this is"
I'm about"
"fiver"

It was all crisp and sharp. If you ever did hear anything out of the ordinary it was likely to be something memorable.

When I got off active duty and got in the guard we could receive-only on VHF. I made a point of listening up on both sides but could only transmit on Uniform. Being based at civil airfields and flying in and out of lots of others, I often wished I'd had VHF and certainly understand why it is so common now, but I liked UHF traffic better.

HF, not so much. Military FM, okay I guess. Kind of fun to be able to home in on the other guy.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
buckfifty
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sun Jun 08, 2008 4:05 am



Quoting Caryjack (Reply 15):
This is the part that I don't get. Frequency modulation (FM) has such a clear advantage over amplitude modulation (AM) that I don't understand why FM isn't used. It must be that airliners fly above the fray and that the line of sight advantage they have at 35,000' mitigates the inherent problems suffered with AM communications.
Cary

With the current VHF sets we use in the aircraft, typical useful range is about 180nm from a transmission tower, outside of which we have to rely on HF transmissions (notwithstanding the role CPDLC and SATCOM plays). It's not a lot of distance, really, and depending on where you're flying, coverage could be scant at best.

As the wavelengths get shorter, the ranges get shorter. Which means more towers need to be built for full coverage. In turn, this means more costs. If you look at this from a worldwide perspective, moving to a higher frequency band is not necessarily a good thing.
 
Caryjack
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sun Jun 08, 2008 5:44 am



Quoting Buckfifty (Reply 17):
With the current VHF sets we use in the aircraft, typical useful range is about 180nm from a transmission tower, outside of which we have to rely on HF transmissions

I had the VHF range about right and I envisioned an airliner crew switching from ATC to ATC as they crossed the country. I am aware of SATCOM but didn't realize that HF was still in use, which brings up a few questions. What frequencies and modulation do the airlines use - AM, SSB and CW in a pinch? wink  How reliable is your HF transceiver and what is the range? What brands are in use today?

Quoting Buckfifty (Reply 17):
If you look at this from a worldwide perspective, moving to a higher frequency band is not necessarily a good thing.

Agreed. There is no advantage in the quality of voice communications in UHF over VHF.
Thanks,
Cary
 
rwessel
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Mon Jun 09, 2008 10:03 pm



Quoting Caryjack (Reply 15):
This is the part that I don't get. Frequency modulation (FM) has such a clear advantage over amplitude modulation (AM) that I don't understand why FM isn't used. It must be that airliners fly above the fray and that the line of sight advantage they have at 35,000' mitigates the inherent problems suffered with AM communications.

I'm pretty sure the reasons are simply historical. FM offers only a moderate improvement in that kind of low bandwidth voice application, and requires somewhat wider frequency spacing, that there was never a point where a conversion from the old standard (set when AM was practically much easier to do than FM), would all that much sense. You generally need a significant improvement to justify that kind of migration.

Without doubt if you were designing an analog voice radio system today, you wouldn't use AM. Of course who in their right mind would actually design an analog radio system today. I assume that if there is a transition, it'll be to a digital system, with security, jam resistance, and whatnot, in addition to significantly improved audio quality.
 
bond007
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Tue Jun 10, 2008 12:34 am



Quoting Buckfifty (Reply 13):
Most of the time, travelling in international civilian airspace, the U.S. military will use VHF.



Quoting Dragon6172 (Reply 12):
As I said, a lot of military aircraft are equipped with VHF/UHF R/Ts



Quoting Jgarrido (Reply 11):
I couldn't even start a compressive list, but in my experience all the tankers/cargo have VHF

It's certainly changing towards less and less UHF only, and it'll be the old fighters that will be last to go. As mentioned, the transport/cargo aircraft predominantly use VHF when on civilian freqs. The old fighters generally only have UHF, such as the older F16s, F15s.

Most of the towered airports in the US have corresponding UHF frequencies, but certainly not all of them. All of the centers do.

Jimbo
I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
 
sprout5199
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Tue Jun 10, 2008 3:00 pm



Quoting Caryjack (Reply 15):
This is the part that I don't get. Frequency modulation (FM) has such a clear advantage over amplitude modulation (AM) that I don't understand why FM isn't used. It must be that airliners fly above the fray and that the line of sight advantage they have at 35,000' mitigates the inherent problems suffered with AM communications

One reason is that most aircraft had AM radios in them when FM really came into its own(about when the transistor was invented) so the cost of converting all those radios was to big. Also another small reason is that you can tell when being "stepped on" with AM. FM will just pick the stronger signal and reject the weaker one.

Quoting Rwessel (Reply 10):
The military aircraft radio spectrum, 225-400MHz (which is AM),

IIRC we used AM,FM, and PM on the ship I was on. AM was used when "clear" and AM and FM when "secure"(depending which crypto gear we used). PM was never used but was available(so was PSK and FSK for TTY data).

Quoting Caryjack (Reply 15):
and heat inversion layers can sporadically duct UHF signals several hundred miles,

When I was in the Persian Gulf(Looong time ago) we could actually talk farther with UHF than with HF when talking to the AWACS.

Quoting Caryjack (Reply 15):
A UHF transmitter may require an extra stage or two more than a VHF unit to work up to the desired frequency but otherwise both units are functionally identical.

Naa, most just use a PLL and produce the xmit freq or 45MHz above/below the rec freq and go through a mixer(sum/difference/two originals) to get to the IF stage.

Dan in Jupiter
 
Caryjack
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Wed Jun 11, 2008 12:40 am



Quoting Rwessel (Reply 19):
I'm pretty sure the reasons are simply historical. .

I agree that history has alot to do with it but the fact that these radios are used around the world and are beyond the controll of the FCC/FAA also contributed.

Quoting Rwessel (Reply 19):
FM offers only a moderate improvement in that kind of low bandwidth voice application, and requires somewhat wider frequency spacing, that there was never a point where a conversion from the old standard (set when AM was practically much easier to do than FM), would all that much sense. You generally need a significant improvement to justify that kind of migration.

I would think that FM would offer a considerable improvement over AM in audio quality but I'm not an end user. Otherwise I agree with you.

Quoting Sprout5199 (Reply 21):
. Also another small reason is that you can tell when being "stepped on" with AM.

Thanks, I didn't know that part.

Quoting Sprout5199 (Reply 21):
When I was in the Persian Gulf(Looong time ago) we could actually talk farther with UHF than with HF when talking to the AWACS.

It may also have had to do with the the types of antennas, tower height and the near line-of-sight path between the AWACSs and your base stations.

Quoting Sprout5199 (Reply 21):
Naa, most just use a PLL and produce the xmit freq or 45MHz above/below the rec freq and go through a mixer(sum/difference/two originals) to get to the IF stage.

OK, I admit it, I used to build vacuum tube superheterodynes.  smile  Did you mean to say 4.5 MHz?
 
NoWorries
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Wed Jun 11, 2008 2:04 am

While it's true that FM usually offers better performace for analog audio (voice/music), digital communications systems often don't employ FM. They often use Phase Modulation (for example phase shift keying) or an advanced form of AM (for example QAM). This is because digitally enconded information is sent out as discrete values with fairly narrow parameters rather than the continuous values that are seen with analog audio (voice/music) in more traditional FM.
 
buckfifty
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Wed Jun 11, 2008 6:19 am



Quoting Caryjack (Reply 18):
I had the VHF range about right and I envisioned an airliner crew switching from ATC to ATC as they crossed the country. I am aware of SATCOM but didn't realize that HF was still in use, which brings up a few questions. What frequencies and modulation do the airlines use - AM, SSB and CW in a pinch? How reliable is your HF transceiver and what is the range? What brands are in use today?

I can't tell you any specifics to be honest, because I wouldn't have any idea what brand of HF transceiver we use in our aircraft. The range depends on atmospheric conditions on the day. For example, crossing the Pacific, we only talk to two centres, Tokyo and San Francisco radio. In that case, we could be at least 1500nm to 2000nm from either centres at maximum range.

The frequencies we use are generally between 3000khz to 16000khz band. The lower frequencies we use during the nightime, and higher frequencies during the day, due to the atmospheric propogation effects from the sun.

Interestingly enough, ACARS also works off the HF bands going transpac, with SATCOM being the backup.
 
sprout5199
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Wed Jun 11, 2008 1:59 pm



Quoting Caryjack (Reply 22):
Did you mean to say 4.5 MHz?

No, most radios use 45MHz as a IF freq across board(VHF,UHF,800 and so on) for their radios for parts commonality.

Quoting Caryjack (Reply 22):
It may also have had to do with the the types of antennas, tower height and the near line-of-sight path between the AWACSs and your base stations.

Mainly had to do with the crappy rf propagation in that area, we would be down in the straits of Hormuz and it would be up by Kuwait.

Dan in Jupiter
 
nomadd22
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Wed Jun 11, 2008 11:31 pm

45Mhz is the TX / RX spacing for a good part of the commercial 800 band. I've never seen that much in the lower bands for a regular channel. It can be the first IF in a dual conversion receiver.
Anon
 
Caryjack
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Thu Jun 12, 2008 10:43 pm



Quoting Sprout5199 (Reply 25):
most radios use 45MHz as a IF freq across board(VHF,UHF,800 and so on) for their radios for parts commonality



Quoting Nomadd22 (Reply 26):
45Mhz is the TX / RX spacing for a good part of the commercial 800 band. I've never seen that much in the lower bands for a regular channel. It can be the first IF in a dual conversion receiver.

Commercial VHF & UHF receivers (about 50 MHz to 480 MHz) are FM and generally use 10.7 or 11.7 MHz IFs. Modern receivers can demodulate a 10 MHz signal but if a second IF is necessary, it is normally 455 KHz. Tx/Rx spacing for duplex systems is generally 5 MHz. A frequency of 45 Mhz doesn't ring a bell but it seems very high for an IF.
In my experience parts commonality is manufacture specific (Motorola & GE) rather then frequency specific.

Quoting NoWorries (Reply 23):
digital communications systems often don't employ FM. They often use Phase Modulation

Any time you shift the phase of a signal, you change the frequency so phase modulation is essentially frequency modulation. In PM the modulator is placed after the oscillator and subsequently demodulated like any other FM signal.

Cary
 
sprout5199
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 13, 2008 6:42 pm



Quoting Caryjack (Reply 27):
Any time you shift the phase of a signal, you change the frequency so phase modulation is essentially frequency modulation. In PM the modulator is placed after the oscillator and subsequently demodulated like any other FM signal

Here at the Sheriff's office where I work we used GE-PEs a LONG time ago, and the were PM but our system was FM and they worked fine.

Quoting Caryjack (Reply 27):
Commercial VHF & UHF receivers (about 50 MHz to 480 MHz) are FM and generally use 10.7 or 11.7 MHz IFs. Modern receivers can demodulate a 10 MHz signal but if a second IF is necessary, it is normally 455 KHz. Tx/Rx spacing for duplex systems is generally 5 MHz. A frequency of 45 Mhz doesn't ring a bell but it seems very high for an IF.

The 45MHz was on Standard(HX series, and GX series radios), and the Kenwood's(TK series) used 34.4 MHz as the first IF. Both brands used 455KHz as a second IF. These were VHF High band radios(150-174MHZ). The radios I work on now(Motorola 880MHz trunked--MCS/MTS/XTS/XTL series) have every thing done internal to the slics(microprocessor).

Dan in Jupiter
 
WESTERN737800
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 13, 2008 6:51 pm

I know very little about military aircraft. I do know at our airport the Natl. Gaurd Blackhawks do transmit on VHF freqs. I would assume the vast majority of military traffic would have VHF capabilities if nothing else to talk to civilan aircraft in distress.
Bring back Western Airlines!
 
n234nw
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Fri Jun 13, 2008 10:11 pm

A disadvantage of FM is the "Capture effect". Only the strongest signal received by the receiver is demodulated (recovered + turned into audio).

I could see this causing problems hearing weak signals, since you wouldn't hear anything but the strongest signal in your area of reception. This is one reason why FM is relatively "Static/Interference" free.

With modern DSP filters, noise can be reduced in AM signals to adequate levels.
 
Caryjack
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RE: Why Do Military Aircraft Use UHF And Not VHF?

Sun Jun 15, 2008 1:16 am



Quoting Sprout5199 (Reply 28):
The radios I work on now(Motorola 880MHz trunked--MCS/MTS/XTS/XTL series) have every thing done internal to the slics

So you don't use a lot of pentagrid-converters?  biggrin 

Quoting Caryjack (Reply 15):
This is the part that I don't get. Frequency modulation (FM) has such a clear advantage over amplitude modulation (AM) that I don't understand why FM isn't used

As I said earlier, I don't understand why FM isn't used.

Quoting Sprout5199 (Reply 21):
Also another small reason is that you can tell when being "stepped on" with AM. FM will just pick the stronger signal and reject the weaker one.

 checkmark 

Quoting WESTERN737800 (Reply 29):
With modern DSP filters, noise can be reduced in AM signals to adequate levels.

 checkmark 

Quoting N234NW (Reply 30):
A disadvantage of FM is the "Capture effect". Only the strongest signal received by the receiver is demodulated

 checkmark 

I'm sure that this is all correct. It's looks like FM may not have the clear advantage over AM that I had originally thought. scratchchin 

Quoting N234NW (Reply 30):
This is one reason why FM is relatively "Static/Interference" free.

Lets not forget the 2nd limiter and a discriminator that doesn't know the meaning of the words "static & interference".  wink 

Quoting Buckfifty (Reply 24):
I wouldn't have any idea what brand of HF transceiver we use in our aircraft.

So it's not like in your car where you just read the brand off the radio?  smile 

Thanks,
Cary
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

RE: Why Do Military Aircraft Use UHF And Not VHF?

Sun Jun 15, 2008 4:26 pm



Quoting N234NW (Reply 30):
A disadvantage of FM is the "Capture effect". Only the strongest signal received by the receiver is demodulated (recovered + turned into audio).

That's actually a major advantage of FM. It allows you to isolate your signal from a much stronger one on the next channel because FM IF filtering can be much sharper. You can get 70 db isolation from a channel 12.5khz away a lot easier with FM than with AM.
And, anyone who has ever worked on data carriers can tell you that phase modulation is not the same as FM. With phase you can fit several bits of data into a single cycle of carrier. Good luck doing that with regular FM.
Anon
 
sprout5199
Posts: 1681
Joined: Sat Feb 26, 2005 8:26 am

RE: Why Do Military Aircraft Use UHF And Not VHF?

Mon Jun 16, 2008 1:57 am



Quoting Nomadd22 (Reply 32):
That's actually a major advantage of FM. It allows you to isolate your signal from a much stronger one on the next channel because FM IF filtering can be much sharper. You can get 70 db isolation from a channel 12.5khz away a lot easier with FM than with AM.

True, when using it for music,data and voice at certain times. However in aviation, you want to know when two people are "stepping" on each other, and if a radio is "off-freq" or broken, sometimes the voice will go through but just sound bad. Understandability trumps fidelity in aviation.

Dan in Jupiter
 
Caryjack
Posts: 312
Joined: Tue May 08, 2007 9:45 am

RE: Why Do Military Aircraft Use UHF And Not VHF?

Mon Jun 16, 2008 4:18 am



Quoting Nomadd22 (Reply 32):
And, anyone who has ever worked on data carriers can tell you that phase modulation is not the same as FM.

Agreed but as far as vioce communications are concerned the two are indiscernable and routinely combined.
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

RE: Why Do Military Aircraft Use UHF And Not VHF?

Mon Jun 16, 2008 11:43 am



Quoting Sprout5199 (Reply 33):
However in aviation, you want to know when two people are "stepping" on each other, and if a radio is "off-freq" or broken, sometimes the voice will go through but just sound bad.

Which reminds me, the New Jersey Turnpike Authority is stepping on my UHF channel again, and needs to be threatened. The busiest road in the nation uses a little cobbled together board that looks like something from the back pages of Amateur Radio, and drifts off 5khz every two years to control the signs and signals on the highway.
Anon
 
sprout5199
Posts: 1681
Joined: Sat Feb 26, 2005 8:26 am

RE: Why Do Military Aircraft Use UHF And Not VHF?

Mon Jun 16, 2008 3:51 pm



Quoting Nomadd22 (Reply 35):
The busiest road in the nation uses a little cobbled together board that looks like something from the back pages of Amateur Radio, and drifts off 5khz every two years to control the signs and signals on the highway.

And the recievers still work? so much for selectivity, the front end must be wide open. I remember working on the old Motorola MT500's and we had a cover with holes drilled into it to align it(if you did it with the cover off, when you put the cover on the TX would be 1 to 5 KHz off, and it was a plastic cover, go figure  confused   scratchchin  )

Dan in Jupiter

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