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Sideslipper
11th Dec 2006, 12:14
Whenever I talk to civil ATC on VHF I am always "5s" on TX & Rx.
On frequency change to a military ATC it almost always drops to nearly unreadable. As soon as I go back to a civil frequency it's back up to "5s" again. It doesn't make sense to me. :confused:
Can't see it being a problem in my a/c. Is there anything different about military transmitters?

Piper19
15th Dec 2006, 19:12
Sometimes I have the same while flying. But not always. I know military radios can have a frequency coded system so that other people cannot listen to what is said, but that cannot be the case here since it is a civil known frequency.
(the system works by varying the frequency in a very fast way eg 120 mhz, 120,5 mhz, 119,5 mhz,...; the code of change in frquency is only known by the rx/tx).

onehundred
16th Dec 2006, 16:24
The freq skipping system u are talking about is known as "have quick" but as u said it's used only by military and in real ops or for training but that's not your problem.
Since military coms are done most of the part on uhf, could be that's the vhf receiver of the mil atc that has some problems. But i'm not convinced about that.

100

galaxy flyer
16th Dec 2006, 17:18
Being a cynic, I think the government simply doesn't, and doesn't need to, spend the money to have first class VHF radios. And UHF is used where clarity is an issue--tactical aviation. Money, the anwer when all other answers fail!

GF

BEagle
16th Dec 2006, 20:36
Nothing to do with HaveQuick.....that is UHF only.

It's probably that POS known as Mascot Mincom - and the awful RAF audio circuits rather than the transmitters and receivers.

Years (and years) ago, the RAF Flying Prevention Branch (aka Air Traffickers) defined a new ATC radio control system which was ever-so-whizzy and included rapid T/Rx switching etc. Quite a gucci system.

But then someone else decided on new headsets and microphones. Unfortunately the impedance of the new headsets didn't match the Mascot Mincom thingy - so, ever since then you hear phantom clunks on every RAF frequency as other transmitters are keyed through mutual inductance - and every transmission from an RAF ATCU sounds as though it's being made through cotton wool.....








...or a WAAF's knickers? RAF VHF comms are universally appalling when compared to any civil VHF comms.

chevvron
20th Dec 2006, 08:24
Beagle forgot to mention that the guy who signed the procurement contract then retired and walked straight into a well paid job with a certain comms manufacturer!

Sideslipper
10th Jan 2007, 15:07
Am I right in suspecting that in military zones ATC talk to military a/c on UHF, and civil a/c on VHF?
Going through one their zones once, ATC constantly asked me if I was still visual with a C130 climbing out, (I could hardly miss him), yet I never heard the C130 being asked if he could see me!
If so, it doesn't seem to make for good spacial awareness.

3 Point
10th Jan 2007, 20:52
Sideslip,

Yep you are spot on. I run a flying club at a mil airfield, mil aircraft on UHF and Civy on VHF requires all players (both pilots and ATC) to keep on thier toes! Very robust airmanship and dilligent lookout are essential.

Also, I think galaxy flyer is on the money; the VHF are not a priority for maintenance or funding.

Happy landings

3 Point

BEagle
10th Jan 2007, 22:32
At the Covert Oxonian Aerodrome, the normal way of operating is that everyone uses VHF, whether C-17, An 124 or PA28.

Except for some of the damn Alberts - who seem incapable of operating on VHF as it might require a frequency to be dialled up, rather than clicked over to on a UHF stud. But then I guess their copiglets aren't skilled enough to change radio frequencies manually.....

GOLF_BRAVO_ZULU
11th Jan 2007, 01:06
Wish I'd said that! (I will, I will)

H Peacock
14th Jan 2007, 08:38
Am I right in suspecting that in military zones ATC talk to military a/c on UHF, and civil a/c on VHF?
Going through one their zones once, ATC constantly asked me if I was still visual with a C130 climbing out, (I could hardly miss him), yet I never heard the C130 being asked if he could see me!
If so, it doesn't seem to make for good spacial awareness.

Not unusual for ATC to transmit on both VHF and UHF at the same time if appropriate. It can be a useful 'filter' to help minimize some of the R/T. If you go to Newcastle approach on UHF they have some sort of rebro; you can also hear the civil VHF transmissions from other aircraft.

As for hearing the C130, it was almost certainly on a different freq. At most busy military airfields that I have worked at the Departures has its own dedicated freq. Again filters out anyone else crossing or operating near the MATZ (Zone) and those recovering (Approach). Somewhere with excellent ATC (Linton) will always tell you about any conflicting traffic plus additional calls if they feel it would be prudent.

If the C130 was at Lyneham (FOT) then things will be different again. They don't have a MATZ but their own (class D?) airspace.

PS. I am not an airtrafficker!

chevvron
14th Jan 2007, 09:03
Of course, if MASCOT had been specified correctly in the first place, they would be able to cross-couple the UHF and VHF frequencies so that, from a pilot's point of view, it would sound like a single frequency. There is a 'coupling' button on MASCOT, but the problem we found was the time differential between the signal being transmitted and ATC actually releasing the Tx button was so great that you ended up with an 'echo' effect which built up until you deselected the coupling!
Many civil ATC units (all NATS airifelds) have this facility, and it works!

MrBernoulli
14th Jan 2007, 14:28
H Peacock,

That "Covert Oxonian Aerodrome" mentioned by BEagle has its own Class D airspace

Sideslipper
26th Jul 2007, 11:04
Having recently had an interesting transit through a military zone due to me being on VHF and the silent C130 I was avoiding presumably on UHF, I was moved to wonder they don't use VHF in their zones.
Having heard various military aircraft on VHF outside their own zones, I assume they must have the capability, so why not use it?
Is there a blindingly simple reason that has eluded me?:confused:

ianjb
27th Jul 2007, 13:08
The situation can vary widely from one country to another. Military comms on UHF and with cryptos are where the funding goes. Legacy systems on VHF and HF can be old equipment that is not serviced as effectively as it should be.

In some countries military VHF tx and rx equipment may be wide band valve-based kit. That means that the equipment uses channel bands that are twice or four times the width of current equipment and therefore incompatible.

A frequent problem can be antennae-based. Some military ATC has upgraded VHF to conform to exactly the same rules as civil systems including physical separation between tx and rx equipment and antennae systems. What can happen is that VHF antennae can end up with the worst position on the mast or tower and may be quarter wave aerials that are too close to the newer UHF aerial arrays.

Poor antennae location can lead to screening of signals in some directions and distortion of A planes and E planes. Thirty years back when computer modelling arrived for radio masts, most operators found that their masts would theoretically fall down in storm conditions and that changed the way that the antennae support structures were designed and monitored but did not change established practice for mounting antennae, resulting in heavy shielding by support structures that creates large blind spots.

Cabling may not have been replaced since Marconi was a boy and water ingression is a common problem with old cabling. When metered, water-logged cable shows a varying resistance as the transmitter goes to tx.

One further problem that can be suffered by some military radio systems is harmonic distortion that results from the wealth of equipment transmitting signals in close proximity. If the VHF systems are located at a military airfield, there will be many different ground comms systems on E band and A band VHF, T band and U band UHF, microwave systems in the 1-3 GHz range, together with all the HF, VHF and UHF land mobile and air mobile radio nets and radar systems on the ground and in the air. Older VHF radio and aerial systems are particularly vulnerable to interference form these other systems.

Brian Abraham
28th Jul 2007, 05:47
Used to find the military frequency tolerances were so bad that often had to experiment by tuning .05 up or down to get better reception. This was some years ago and sounds like it hasn't improved.