View Full Version : Nav aids and UHF
9th Jan 2002, 00:40
Looking at US sectionals and low-altitude instrument charts (probably true for high-altitude too, but really applicable to me in my Cessna...) all the military frequencies seem to be UHF. Furthermore, the navaids are all referred to by channel number, which presumably represents a discrete UHF frequency.
I was wondering why this is. I can think of several reasons why this might be the case:
1. Channel numbers are easier to use and quicker to select than frequencies.
2. Potential use of common electronics for the transceivers (i.e. same frequency ranges, same internals, easier to replace in the field since there are fewer variations).
3. Better coverage/less prone to interference.
Am I on the right track with any of this?
Not channel number's per se for navaids, but TACAN channel numbers.
Mil may or may not have VOR, but always have TACAN, which can resolve heading to station (no to/from, typically displayed on a BDHI, thin of an RMI but with a range indicator and the heading card is slaved to a compass and you have it pictured :) ) and slant range (line of sight up to 300 mi, realistically up to 150 mi, altitude dependant, and which TACAN system installed).
UHF is clearer, and less congested than trying the VHF comm freq's. As far as UHF freq's go, most of the comm radios (ARC-186 springs to mind, can do both) have channel capability, each squadron has it's own list, but the radios are programmed for channels -> frequencies for ease of use. they also monitor guard 243Mhz, and on the ARC-186 will do 121.5 as well.
In addition, the main reason they are UHF: since the freq's are higher, the antenna's for such systems are smaller. Compare your VHF comm antenna sticking out of your cessna to a UHF blade antenna for:
Comms, TACAN, Transponder.
All smaller and on a high performance aircraft, produce less drag, etc.
[ 08 January 2002: Message edited by: RW-1 ]</p>
9th Jan 2002, 10:50
Makes sense to me!
Thanks a bunch <img src="smile.gif" border="0">
10th Jan 2002, 21:40
I thought the main reason UHF radio was used was because of the extended range ( 300 miles instead of about 120 miles for VHF ).
All the fast jets I've worked have used the same upper/lower antenna for V/UHF radio and sometimes homer anyway.
UHF power output is typically 30W max compared to only about 6W for VHF.
10th Jan 2002, 23:43
Correct me if I'm wrong; I don't have any books to hand. Tacan broadcasts it's DME info on UHF, same as the DME part of a VOR/DME beacon. Certainly on UK charts and books the relevant VHF frequency is shown alongside the Tacan channel numbers. This shows that the UHF frequency being used by that Tacan, equates to the UHF frequency used by a VOR/DME on that VHF setting.
This means that if you tune your civilian VOR unit to the Tacan VHF frequency, you should be able to receive DME from the Tacan.
<img src="rolleyes.gif" border="0">
11th Jan 2002, 05:20
I'd hazard a guess and say you're not an Avionics man !
If you're aircrew then you're forgiven.
If you're pondlife then it's time for retraining ! <img src="cool.gif" border="0">
VOR, VORTAC and TACAN:
<a href="http://www.flycyberair.com/Training/Tutorials/VorTutorial/vor.htm" target="_blank">http://www.flycyberair.com/Training/Tutorials/VorTutorial/vor.htm</a>
11th Jan 2002, 12:47
inginbridie - what you say is more or less true. The DME element of VOR/DME is indeed on a high UHF frequency, as is TACAN. A TACAN channel number equates to a particular DME channel which is available on a VOR/DME, paired to the VOR element's VHF frequency. So if ,on a civil DME receiver, you dial up the VHF frequency equivalent to a TACAN channel, you should indeed receive DME from that TACAN. Many civil DME receivers will give you the option of automatically selecting the appropriate DME paired with VOR selected on 1 or other of the VOR/ILS receivers, or the option of tuning to a separate DME frequency.
A DME frequency can also be frequency-paired with ILS, to give you ILS/DME. In this case it will normally indicate DME to touchdown, not to the DME beacon unless the approach plate warns otherwise. Except at Brize where the TACAN is on 42, the ILS is on 111.9. Dial up 111.9 and auto on a civil VOR/ILS receiver and you will get ILS info but DME to the TACAN, not to touchdown. There is a warning to this effect in Pooley's Guide though - military aircraft don't usually have 'civil' nav receivers and the DME would probably be selected on a separate TACAN receiver, hence the DME ranges on the aproach plate are considered sufficient information.
12th Jan 2002, 12:46
Thanks BEagle, I know you really meant me.
12th Jan 2002, 13:18
I did indeed! Finger trouble - sorry!!