ATC radio bleed

[pugwash4x4]

A friend who lives in Warminster was complaining of massive radio bleed to me yesterday. Said that it was wiping out Radio 1. Clearly i told him that aircraft safety was slightly more important that his daytime listening, but i was little surprised it was affecting such a wide band.

What was happening atmospherically? is there any way of preventing it locally? are the RT frequencies anywhere near FM radio frequencies? Is bleed "restricted" and should he be making people aware?

Cheers

Pugwash

ETA- for spelling and accuracy- it was radio 1 it was interfering, not radio 2

[Pat42]

Sounds a bit odd, is your friend sure it was ATC communications that he was picking up rather than something else?

I suppose it is possible, both FM radio and ATC R/T sit within a similar area of the VHF spectrum. FM radio can go up to about 108 MHz and I believe aviation R/T frequencies start at around 117 MHz (with radio nav aids filling in the gap between the two)... but for obvious reasons the specific frequencies used in different areas are picked very precisely to avoid any interference - broadcast spilling into avaition frequencies could be potentially disasterous, and aviation spilling into broadcast would see some seriously pissed off comercial radio licensees.

I guess in particularly freakish meteorological conditions it might be possible though.

Is bleed "restricted" and should he be making people aware?

If it happens regularly then it may be worth whinging to the BBC and other affected broadcasters, if it's a serious problem they can take it up with Ofcom who regulate the bandwidth allocation and may need to reallocate a few frequencies.

[HEATHROW DIRECTOR]

I don't understand what you mean by "radio bleed" so please provide more information. If you mean that your friend's radio was receiving signals that it should not have it's almost certainly down to the radio. Low cost domestic radio and hi-fi equipment is very susceptible to interference from radio transmissions nearby irrespective of the "atmospheric" conditions prevailing.

Warminster is not near any major airport. Boscombe Down is about 25 miles away and I believe that military helicopters operate around that whole general area. However, these are unlikely to have caused any problems to radio reception - I get them over my house all day and have never experienced any breakthrough on my radios.

VHF broadcast stations are on FM and the airband is on AM and this should reduce any possibility of interference 'twixt the two.

[Someone_Else]

My flatmate often had this problem with his radio alarm clock.

Occasionally his radio would be interrupted with pilots chatting on LL_South. I lived on the SW corner of the LL zone at the time.

Didn't happen every morning. Just one of those things!

Just recently we've been having the reverse problem with some of our frequencies ie. music being heard on air traffic frequencies. Though when that happens someone gets out and investigates!

[pottwiddler]

Sounds to me like "Image" channel interference...

Although alarm clock radios are not the best receivers and interference from strong transmissions are quite likely. Civil ATC Transmitters are restricted to 5W at the antenna. Military can go up to 100W but are mainly in the UHF 240-400MHz band but can be on the VHF band of 118-138MHz and should be no more than 20W.

Basically they are "blasting" there way through the receivers.

Way to get around it?
Get a DAB radio!
You can listen to the Six Music that's much better than Radio 1:ok:

[HEATHROW DIRECTOR]

PW... Presumably you are a Tels man but I have to say that I'm amazed that ATC Txs only run 5 watts. Does that include the multicarrier stations which have to cover thousands of square miles?

[vintage ATCO]

Yep, 5w ERP.

I once sat in a barbers in Luton having what little hair I still have cut. A radio was playing what I believe was R1, I didn't recognise any of the music so it probably was. Often aircraft RT could be heard although the barber said 'it's the police again'. From what was being said it had to be 121.270 (TC NW) :)

Wouldn't happen on a Roberts. :cool:

[pottwiddler]

HD - Dunno about that lot but oceanic stuff use HF (3-30MHz) with single sideband transmissions, but then I don't know much about that- its not in my black book.

[HEATHROW DIRECTOR]

Pw... I know about HF, etc. LATCC sectors use VHF transmitters far remote from Swanwick, eg Davidstow Moor in Cornwall. These transmitters are used to provide service to aircraft over huge areas and it just struck me that 5 watts erp is a bit weeny! (I'm a licenced radio amateur as well as being an ATCO so have a fair knowledge of radio communications). I have an amateur radio 2m (145mHz) walkie-talkie which runs 5 watts and short of sticking its aerial into a portable radio I can't cause any QRM with it. That's why, even more so, I don't understand the person in Warminster getting ATC transmissions on his radio (if that's what they were).

[Comms Boy]

HD, En route ground ATC stations in the UK transmit considerably more than 5W ERP, I presume the 5W is from the Aircraft but have no knowledge of radios in aircraft.
For increased coverage of a specific ATC frequency a system of Transmitter Offsets are used. i.e for a 3-leg channel one ground station would use the exact frequency while the other two would be + or - 7.5KHz the centre frequency. Similar techniques are used for 2 and 4 leg channels.
Using this system increases the area coverage of a specific frequency.
Regards P

[HEATHROW DIRECTOR]

Comms Boy. Thanks - I know all about the multicarrier network but several people on here are adamant that ATC transmitters only run 5W erp. Now you dispute that so what is the true story please?

[Comms Boy]

HD, I only know what UK En route ATC ground stations run at and i know for a fact that it’s more than 5W ERP. I'm not prepared to disclose what the actual level is at the aerial common point on an open internet forum.
Hope you understand.
A search on Google for ATC ground station radio manufactures should give you a rough idea.

Regards P

[HEATHROW DIRECTOR]

<<HD, I only know what UK En route ATC ground stations run at and i know for a fact that it’s more than 5W ERP. I'm not prepared to disclose what the actual level is at the aerial common point on an open internet forum.
Hope you understand.>>

Well, not really! It's hardly a question of national security. However, I'll have a Google.

Cheers.

[berliner57]

This picking up ATC transmissions is nothing new to me, when i lived at Warton in the early 80's i reguarly got Warton on 124.45 breaking through on 97.00, as Warton was only across the road from me not really suprised.

As least FM radios are better screened nowadays.

[vintage ATCO]

This is what ICAO Annex 10 vol III says about erp . . .
2.2.1.2 POWER
Recommendation.— On a high percentage of occasions, the effective radiated power should be such as to provide
a field strength of a least 75 microvolts per metre (minus 109 dBW/m2) within the defined operational coverage of the
facility, on the basis of free-space propagation.
2.2.1.3 Modulation. A peak modulation factor of at least 0.85 shall be achievable.
2.2.1.4 Recommendation.— Means should be provided to maintain the average modulation factor at the highest
practicable value without overmodulation.PS Wassit mean? :confused:

:) :) :)

[forget]

Park Air uses a (VHF) ground transmitter of 250-watt carrier power to achieve the desired field strength for aircraft at long range.

Avionics Magazine :: Longer Legs for VHF (http://www.aviationtoday.com/av/categories/atc/Longer-Legs-for-VHF_780.html)

[Track Jitter]

For NATS people, if you go to "livelink" which you will find at the top of the natsnet home page, details of various things can be found.

[Fletchers Left Boot]

"I once sat in a barbers in Luton having what little hair I still have cut. A radio was playing what I believe was R1, I didn't recognise any of the music so it probably was. Often aircraft RT could be heard although the barber said 'it's the police again'. From what was being said it had to be 121.270 (TC NW) "

Sounds like a classic case of what is called "IF breakthrough" to me.
Domestic radios often use 10.7Mhz as an 'intermediate frequency' before being converted to the audio we hear.

Without being too technical, the 'image frequency' of 2x10.7, i.e. 21.4Mhz higher (or lower) is also recieved, but blocked by 60, 70dB or more (this is the 'IF image rejection' often seen in the technical spec of the radio).

However, this may not be enough if the transmitter on the image frequency above is close enough for the image signal to still be heard despite that 60dB (or one million times) reduction.

In the case above, 121.270 minus 21.4 = 99.87Mhz, plenty close enough to the upper part of the Radio 1 allocation to have potential for IF breakthrough.

[holyflyer]

Interesting I was flying into Ireland yesterday and was working Dublin who had band boxed frequencies. However, because some commercial channel was breaking into one of theirs - I think everyone ended up on 124.65 (memory might be wrong on exact freq) - female controller doing a fab job though.

Some years ago I lived directly under the ILS to 27R at LHR just by the outer marker. The radio in the kitchen always got break through if tuned to BBC Radio 4 - usually along the lines of "Speedbird 123 cleared to land" etc as the aircraft passed directly overhead. Changed radios but same thing occurred. No other radio's or TV's in the house were affected and only the aircraft transmission was picked up and only when tuned to Radio 4 FM.