Changes in signal strength from one frequency to another

[novascotian]

I have often noted that when an aircraft changes from one frequency to another there are major changes in the signal strength. I just observed it again a few minutes ago. Moncton ACC directed Alitalia to change to
Boston ARTCC. AZ608 replied to Moncton on 135.2 with a great signal, but when calling in to Boston a few seconds later on 134.95, it was way down and only identifiable due to me already waiting to hear the flight number. I suppose it works the other way too, weak, then strong, but I suppose I wouldn't have taken in the weak signal when it occurs first. So what do you think? I am thinking that it is a different radio, but then again why would it be? Even if the next frequency is dialed up already, isn't that in the same radio, and the standby frequency becomes the active? I think the secondary radio is left on guard or company type frequencies.

When I gave that example, it is only an example and I see this commonly in this region when sector boundaries are crossed and frequencies are changed.

 

[AirScan]

When you are monitoring 135.20 can your hear the controller or are you just hearing the aircraft ?

 

[invergordon]

I've observed flight crew programme the next frequency into box 2 and when directed on box 1 to change frequency they just turn the knob between box 1 and box 2 so this corresponds to a different radio. It depends on the aircraft type but I would suspect most modern airliners today have three VHF radios, 2 for voice/guard and a third for datalink.

 

[invergordon]

Oh and BTW, the antenna if switched may also introduce variations in gain.

 

[novascotian]

First off, on those frequencies I mentioned I am hearing only the aircraft side of things.. In the example mentioned, the plane would be about 200 miles away from me.

I had not thought of the antenna also being changed when the radio is changed. I had been thinking that the next anticipated frequency would be dialed up in the same radio that is currently being used, as the standby frequency, and then it is selected as the active frequency when directed by ATC, but I do understand that it could be a separate radio, with as was suggested, a different antenna.

This may also explain why some aircraft at similar flight levels and distances from me are received with significantly variable strengths.

 

[robertmac]

Would there be a cut in transmission power as well? I'm thinking some frequencies may be shared with other airports and with an aircraft being high, would possibly be picked up by another airport if power not cut. Not knowing much about aircraft radios, not sure how many power settings are in an aircraft radio.

 

[AirScan]

I was thinking if you were hearing the controller it might have been a frequency cross-coupling thing, but not the case if you are not hearing the ground side.

Modern airliner radios have a standby window you select the next frequency in, then you press a button to switch the standby to active. Airline standard operating procedures are very specific as to what radio is used for what, it's very unlikely any airline would continuously switch back and forth between radio 1 and 2 for ATC.

I'm guessing it has something to do with your radio or setup ?

 

[novascotian]

Looks like it will remain a mystery. If the situation is that it is the same radio with the same radio and only a very small difference in frequency, and I am using the same radio on my end with the same antenna, and the aircraft has only moved a short distance and with no change in altitude, then what can it be? I had thought about directional antennas on the aircraft ... in the example I gave and I suppose in many sector changes, the ground station they are communicating with is in a direction much different from the last one they were with, but I don't think directional antennas are practical on aircraft, unless it is as simple as being on different sides of the aircraft fuselage, but I figured that the antenna was along the middle of the bottom, something like that, so would be not favouring any one direction.

 

[looknlisten]

While this may not answer all your questions, I think it may help somewhat.
Lots of people don't realize that in outlining areas, there are repeater stations setup to help with range / coverage.
In this case, at CYQI, they have several repeaters setup (don't ask me what freqs they are because I'm not sure). If you disregard the recent cellphone antennas, you can see that there are way more antennas than a Unicom station would need for their own use. I do recall talking with a Navcan tech (many years ago) and he confirmed that Boston Ctr was one of the frequencies relayed. This could account for some of the signal drop if the original freq you were listening to was actually a re-broadcast and not directly from the aircraft you were listening to. (I don't know if Halifax is setup this way or not)
Hope this helps.

 

[AirScan]

looknlisten,

This could account for some of the signal drop if the original freq you were listening to was actually a re-broadcast and not directly from the aircraft you were listening to. (I don't know if Halifax is setup this way or not)

Moncton Center is set up this way, depending on the sector configuration the controller could be working multiple frequencies were aircraft transmissions are re-broadcast on all frequencies being used (frequency cross-coupling). However this would only be a factor if you could hear the ground station, which is not the case here.

Novascotian,

I figured that the antenna was along the middle of the bottom, something like that, so would be not favouring any one direction

Each radio on the aircraft has its own antenna, generally the #1 is on the bottom of the fuselage, while the #2 is on the top. In my experience, mostly on Boeings, it can make a difference in some situations when receiving weak signals. On a few rare occasions I've had to switch radios to get better reception. However as a general rule we don't switch between radios when talking with ATC so it's not the reason if you are hearing it on a regular basic.

Moncton Center has a repeater in Halifax on 133.950. I'm curious if you can hear the controller on this frequency from your location in Halifax ?

 

[nr2d]

As far as I know when an aircraft switches frequencies the power out of the VHF Comm radio remains the same, about 25 watts. If the crew uses the same radio signal strength should stay the same. If they use a different radio it could be a case of whether the new radio's antenna is on the top or bottom of the aircraft.

If the same comm radio is used then maybe the radio or antenna might have a problem.

 

[krokus]

There is a chance that the difference in freqs is enough that there is a cancellation, and the signal is weaker.

Sent via Tapatalk

 

[novascotian]

There are a few comments to reply to..

I do listen to the Moncton PAL on 133.95 located in Halifax. I receive it very well, and because it is always simulcast with 125.25 and often with several other frequencies transmitted from Sydney such as 133.3 and 133.7 and others to the north, I can hear a lot of both sides just by scanning. The funny thing is that 133.95 itself only very rarely has any aircraft on it.. so few that I often wonder why they have it. I assume it is meant for very far offshore if an aircraft goes out of range of the base stations in Sydney and Yarmouth, but yet, as I said I don't know when I last heard anyone on it.

I listen very consistently to high level Moncton traffic and the aircraft side of Boston (on 133.45, 134.95 and 128.05) and I really do not ever think I have ever heard any re-broadcast of the aircraft side of things. In fact I have never heard of such a thing, and doesn't seem like any purpose. The aircraft only has to be heard by the controller for the zone it is in, so who would it be rebroadcast for? I really don't get that there would be a rebroadcast of Boston traffic from Yarmouth, and feel that the tech is off base. Besides, if it was being rebroadcast from a ground station in Yarmouth, that is way way beyond my range of hearing it. It would make more sense that Boston had a frequency transmitted to aircraft from Yarmouth to help with coverage in the Gulf of Maine near the boundary but I don't think they need it, due to the transceiver in I think Rockland, Maine being adequate.

The relay concept must be a confusion with the PAL system that all centres have. Peripheral facilities that transmit and receive from remote sites far from where the control centre/center is located. So Yarmouth has these for Moncton on 123.9, 135.2, 132.975, 128.375, so maybe this is what is being confused into "relay" but they don't relay the aircraft side, at least not on air, that all goes via landline/fibre/microwave to the ACC/ARTCC..

I really do think that I am hearing different antennas being used...

 

[AirScan]

Re: 133.950

The only times I've heard them using 133.95 is for flights coming up from the south out of New York's Oceanic airspace on the handoff from HF.

The relay system I'm talking about, called frequency cross-coupling, allows aircraft transmission on one frequency to be relayed or heard on another or multiple frequencies. This allows pilots to hear all other aircraft transmissions even though they are on different frequency. This prevents pilots from talking over each other when the controller is using more than one frequency.

For example by monitoring 133.950 only, you would be able to hear aircraft transmitting on 125.250. The last time I was in Halifax I noticed this was active, although they do have the option to turn it off, and don't seem to cross-couple all frequencies. Are you hearing this on 133.950 ?

I've never heard of the output power being variable on radio transmissions from the aircraft radio, and pilots do not change radios/antennas when talking with ATC. I suspect it has nothing to do with the aircraft but is some anomaly with your setup ?

 

[novascotian]

Okay I get what you mean by cross coupling, but I must say I have never ever heard this happening. I have been listening from here in the Halifax area for many years, though not consistently, so may have missed this coupling if it happened only occasionally. So no, I do not hear aircraft that are transmitting on let's say 125.25 being rebroadcast on 133.95. I get what you mean about stepping on each other due to many different frequencies being monitored by one controller but have never encountered this way to overcome it.

There are times when the controller who is transmitting on 133.95 is also on maybe 6 other frequencies, several of which are out of my range to even hear the aircraft side, and it is not like I get to hear them on 133.95. As I said, I have hardly heard any aircraft on that frequency, so no relays and not even any that are actually on the frequency to start with.

Now that you mention it, it does seem like there would be a lot of stepping on each other. This area is extremely rich in traffic passing over and nearby in steady streams, at least at some parts of the day. So for example one controller can have three tracks on 125.25, 133.7, 133.3... he or she simulcasts on all three but the aircraft are strictly on their individual track frequencies. On the other hand they don't really transmit all that much, other than for sector changes, requests for altitude changes, deviations, and chat about clearances... Hey wait, that is what it is everywhere I guess LOL... Anyway, I do hear the controller say on occasion "you were stepped on:", but I guess on the whole it works out well without this coupling, otherwise really, why wouldn't they just have all the aircraft on one frequency to start with, I mean when the tracks are all within the range of one transmitter and frequency? In my example here, all of these three frequencies are used or are transmitted from the same site, so you really do bring up a great point, .... if one controller is doing all three, why not just have them all on one frequency? There must be a good reason for it.. I was thinking maybe it is good that pilots can hear the others that are on the track they are on but irrelevant and distracting to hear others??? just thought of that this minute...

Re an anomaly with my equipment, I cannot think what it could be. Same antenna, same radio on my end. Aircraft is on frequency A, changes to frequency B, has moved a mile or two in those few seconds. And not something to do particularly with the example I gave, so that any idea that my scanner is poor on 134.95 as compared to 135.2 cannot be a factor, as I have heard the same drop in signal strength when the opposite frequency change is made, so as I said it is a mystery for sure. But I certainly do appreciate the suggestions!

Also I think for others that I should clarify that when I spoke of PAL's, I think the US term is RCAG.

 

[majoco]

pilots do not change radios/antennas when talking with ATC

Of course they do. Nine times out of ten on a commonly flown route, the non-flying pilot who is working the radios has the next sector frequency dialled up and ready to go - all it takes is one click on the microphone switch on the audio selector panel and it's done. Each transmitter/receiver has it's own antenna and usually one is on the upper fuselage and the other on the bottom so there is a degree of isolation between the two.

I agree with nr2d...

As far as I know when an aircraft switches frequencies the power out of the VHF Comm radio remains the same, about 25 watts. If the crew uses the same radio signal strength should stay the same. If they use a different radio it could be a case of whether the new radio's antenna is on the top or bottom of the aircraft.

...and in novascotian's second post...

First off, on those frequencies I mentioned I am hearing only the aircraft side of things.. In the example mentioned, the plane would be about 200 miles away from me.

200 miles is much further than an aircraft would be talking to an ATC ground station in normally crowded airspace - company and discrete frequencies, yes, but a poor controller trying to control all the aircraft in a 200mile radius circle would be having a nervous breakdown!

Frequency paralleling is quite common here at night and quiet Sundays - two or more adjacent sectors have paralleled transmitters but I have never heard of any aircraft received signal being re-transmitted on another frequency. I usually have two scanners going at the the same time on the aircraft band and often hear the same ATC voice on two frequencies but never the aircraft reply.

 

[AirScan]

So no, I do not hear aircraft that are transmitting on let's say 125.25 being rebroadcast on 133.95

I was getting mixed up, it was some of the other Moncton high frequencies I've heard cross-coupled. The few times I have monitored 133.95 it has never been cross-coupled.

They seem to have ability to turn it on and off. I was actually flying through that airspace today using 125.25, and checked the other frequencies (132.75, 133.70, 133.30, 132.80) on the #2 and noticed the cross-coupling was off and all aircraft were on 125.25. This was in the morning before the Atlantic westbound rush had started.

Another time I noticed they had 125.25 cross-coupled with 133.70 but not 133.30 when the controller was working all 3 frequencies. Seems to vary depending on traffic volume and sector configuration.

if one controller is doing all three, why not just have them all on one frequency? There must be a good reason for it

I believe the primary advantage is it allows them to re-configure sectors without having to get all the aircraft in the effected sector to change frequencies. For example as the traffic builds and they need to split a sector in two, the new controller can flick a switch and already have all the traffic on the correct respective frequency for the sector that was split off.

Re an anomaly with my equipment, I cannot think what it could be. Same antenna, same radio on my end. Aircraft is on frequency A, changes to frequency B, has moved a mile or two in those few seconds. And not something to do particularly with the example I gave, so that any idea that my scanner is poor on 134.95 as compared to 135.2 cannot be a factor

I've noticed something similar with various scanners I have used which has usually been related to some sort of interference on one frequency not picked up on the other. Have you tried comparing it with a 2nd radio at the same time ? What type of radio are you using ?

 

[AirScan]

Of course they do. Nine times out of ten on a commonly flown route, the non-flying pilot who is working the radios has the next sector frequency dialled up and ready to go - all it takes is one click on the microphone switch on the audio selector panel and it's done. Each transmitter/receiver has it's own antenna and usually one is on the upper fuselage and the other on the bottom so there is a degree of isolation between the two.

They are using the SAME radio. Each separate radio #1, #2, #3 has it's own control panel with an active window and a standby window. When the pilot is given a new ATC frequency the new frequency is selected into the standby window on the #1 radio then the transfer button is pressed that makes the selected standby frequency now the active one and puts what was the active one into the standby window. The pilot is always communicating with ATC on the same radio.

Airline standard operating procedures are very specific as to what radio is used for what. Radio #1 for ATC and radio #2 for company or guard (121.5) and #3 for ACARS. It would make no sense to continually switch between different radios as you would have to continually re-tune the other radio as well, and reselect the respective transmit button after each frequency change.

Frequency paralleling is quite common here at night and quiet Sundays - two or more adjacent sectors have paralleled transmitters but I have never heard of any aircraft received signal being re-transmitted on another frequency. I usually have two scanners going at the the same time on the aircraft band and often hear the same ATC voice on two frequencies but never the aircraft reply.

The USA is way behind the times, this system has been in use in Europe, Canada, and Australia for many years now. It's just in the last few years I've noticed it being used in selected US Centers, as examples Cleveland and Indianapolis use it extensively while I've never heard it used in Boston or Miami. My understanding of the US system is that they can only cross-couple two frequencies together at a time while the rest of the world can cross-couple multiple frequencies. Montreal Center routinely cross-couples up to around 20 frequencies on their northern sectors on the overnight shift.

 

[ATCTech]

Frequency cross-coupling has existed in the Canadian ATC system since the late 1980s. Originally it was an outboard piece of hardware where frequency combinations (then called 'scenarios') were pre-programmed based on ATC requirements and could only be enabled or disabled in set patterns. Our current system has cross-coupling built in to the switch and virtually any combination of frequencies can be dynamically coupled by the controller at the sector.

Our controllers couple frequencies for a variety or reasons - as mentioned here earlier, when traffic volumes are low and sectors combine one sector will take the airspace from one or more other sectors. It also allows adjacent sectors and centers to hand traffic off to the same frequency at any time of day, knowing the frequency is being monitored. It's used in our towers as well, where a ground and clearance delivery frequency might combine late in the evening, then the air frequency will be pulled in to the group overnight when it's very quiet. The controller on duty knows everyone is hearing everyone else, so they won't talk over each other. Very handy.

Bob

 

[novascotian]

Looks like I have caused a lot of discussion, and I am finding it all interesting. But now I am not completely clear about cross-coupling. Is this the joining of various sector frequencies so that the controller (only) hears everyone, regardless of which specific frequency they are on in the coupled group? Or is there more to it, and the coupled frequencies are rebroadcast by the ground xmtr on all the other frequencies?

To illustrate my question, I will use the Sydney, NS frequencies for Moncton: 132.75, 133.3, 133.7, 125.25, plus the Halifax one on 133.95. Yes I know that Moncton sometimes joins others to the north as well into the group. I find however that in quiet times or depending on the traffic flow, they often do not use 133.3 and 133.7 at all, and actually combine the sectors into just 132.75 along the spine of NS and 125.25 offshore, but that is another story.

In the first premise, the aircraft are on whichever of these have been assigned, so that in busy times all five could be in use, with aircraft on whichever one, depending on position or track. [Except as I said previously there hardly ever is an aircraft on 133.95, but let's pretend that is not the case and it is just another frequency] The controller can hear each of them at his or her station, and when replying does not have to select a particular frequency, BUT the five signals from the aircraft are not rebroadcast out on the other three that the aircraft is not transmitting on, so the five different sets of pilots do not hear each other, but rather they all hear just the controller. This is what I commonly hear on the ground via the 133.95 PAL in Halifax. Never have I heard aircraft transmissions being rebroadcast on it.

In the second premise, it is the same except that whatever comes in to the controller on any of the joined frequencies goes back on all the frequencies, so that not only does the controller hear everyone, so do all the pilots. This is the concept that is totally new to me, Maybe it is because I only hear that Halifax 133.95 ground xmtr and for some reason they don't do that there, and maybe if I lived near the Sydney ground xmtr I would hear everything. This is the premise that I am frankly doubting, but really that is because I have never heard it, and must wonder why, if they do it, wouldn't they ever do it through the Halifax xmtr, but yet do it through the Sydney ones? and I have been listening for years... decades really, though my interest and listening is kind of up and down, though pretty consistent in the past couple of months.

So, which is it??? I am actually not sure how someone would know for sure without asking ATC the question... and here is another point I just thought of.... If an aircraft transmits on let's say 132.75 and the ground station retransmits it on all of the coupled frequencies, that means that it is also retransmitting it on the the same frequency it is receiving it on, and I am not sure that is possible. This is AM radio and wouldn't that be problematic out there for others who are supposed to be listening, in terms of out of phase competing signals and so on??