R/T Break through
 

Guys, I was listening into ATC at LHR today asking inbound aircraft if they had "break through" on there 118.70 freq. Anyone know what this means? :sad:

Probably spotters listening in and causing interference..................................

Cheers..... Just like to point out, I'm no spotter. :)

Sudden short duration bursts of transmission coming through on the radio. Usually associated with high pressure air causing funneling of transmissions from a long way away. You would not hear on the ground.

If there is a local VHF FM station on 97.3MHz this could cause problems to some radios having a Local Oscillator Frequency 0f 10.7MHz. If memory serves its called Second Channel interference...97.3Mhz might just be a local commercial station...

As I recall Rockwell -Collins VHF-20's were particularly susceptible to this problem and is the principle reason why FM Immunity mods were introduced to some VHF Nav receivers...

Rainboe
Nah, mate, that is "Duct Propagation" on the same frequency.
Breakthrough is an irritation from an adjacent frequency.

Indeed it is, and can travel rather long distances.

Called "ducting" and sometimes the duration isn't so short! Classic example is trying to get London Volmet North on 126.6 MHz when approaching Berry Head from the south when it is blasted out by Santiago Volmet which is circa 400 n.m. away. Usually associated with high pressure/temperature inversion over the Bay of Biscay.

Radio Hams like this though so they can get extra long range on VHF!

It is indeed - into LHR you have LBC's FM service radiating a few KW on 97.3 from the Croydon mast. Although, knowing that paticular transmission chain very well indeed, I have to say that any breakthrough would have been due to tropo & not harmonics from 97.3 off of Croydon :)

No, you happen to be wrong. 118.70MHz would (if the Intermediate Frequency of the receiver is 10.7MHz) have a Local Oscillator frequency of 118.7 + 10.7 = 129.4MHz or 118.7 - 10.7 = 108.0MHz.

So, what you are really talking about is Image Frequency interference which can enter the receiver if there is insufficient front end filtering, i.e. poor rejection of out of band signals. The interfering signals will be at +/- twice the IF frequency from 118.7MHz, i.e. 97.3MHz or 140.1MHz. 97.3MHz has already been identified as the FM broadcast station LBC transmitting from Croydon.

Any decent Aeronautical VHF receiver will have high attenuation notch filters centered on the 88 ~ 108 FM broadcast band, and problems from that band should be extremely minimal.

The answer to the "breakthrough" question is that it is most likely tropospheric ducting caused by temperature inversions in the troposphere which can happen when a large high pressure system has been hanging around for a while. In this case the transmission breakthrough probably came from an aircraft using the same frequency with an ATC somewhere in Europe, e.g. Orly Approach.

An ICAO paper has been written on this subject and can be found at - http://www.icao.int/anb/panels/acp/w...0/wgd10_07.pdf

mm43