Newbie, Airband interference issue help!

dkcorlfla

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For the reason that commercial AM radio can be heard many miles away from the market where it originates where FM would fade long before that. If aircraft went to FM, there would have to be a complex and expensive network of repeaters.
I think some clarification is needed here, AM broadcast can skip off of the ionosphere and can go 1000 + miles but it is because of the frequency mostly and not the mode. AM broadcast run from about .54 to 1.6 Mhz while aircraft use from about 118 to 130 Mhz and frequency in that range are more or less line of sight. Line of sight is usually not a problem with aircraft. Repeaters are used for ground based station with very limited line of sight. Hope that clears it up.
 

Rod1989

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I'd be interested in your results if you disconnect your antenna system and try it with just the stock back-of-set telescoping whip antenna.
I tried that and i dont get any signal since i have lots of houses and buildings in the line of sight from my house to the airport, i can only hear airborne traffic, the roof antenna needs to be connected so i can listen the controllers. The main freq i get the noise is on the tower one which is in 118.3, the rest are higher than 121 mhz and receptions is way better. I tried disconnecting everything in my house and the issue is outside in an small office building, i live surrounded with office places.
Rod.
 

dkcorlfla

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I tried that and i dont get any signal since i have lots of houses and buildings in the line of sight from my house to the airport, i can only hear airborne traffic, the roof antenna needs to be connected so i can listen the controllers. The main freq i get the noise is on the tower one which is in 118.3, the rest are higher than 121 mhz and receptions is way better. I tried disconnecting everything in my house and the issue is outside in an small office building, i live surrounded with office places.
Rod.
One thing I can think of is it might be worth looking into a directional air band yagi. There is a big null on the side of a yagi and if the office that is causing the RFI problem is at 90 angle to the airport that you want to listen to it might help a lot.

Amazon list one

Sirio WY 108-3N 108-137 MHz Air Band 3 Element Yagi Antenna​

 

GROL

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or...
WABC in NYC has 50,000 watts output
my local NOAA has, as do many, 1000 watts.
ABC covers 38 states, my noaa guy barely makes it here at 20 miles.
OR it could be frequency related.
lower freqs go around the world
higher act like light and go straight.

the AM thing for planes is that a mayday can be heard through another plane transmitting.
FM did not exist at the time aeronautical radio began. In fact only AM was available until the early 40,s and not used much at all until after WWII. Even SSB wasn't practical back then due to frequency instability. With FM, the strongest signal wins, which is the FM capture. AM signals mix with two or more AM transmissions, although somewhat garbled you can still hear two transmissions and often figure out some of what is said. AM is considered somewhat safer for aircraft to use than FM, but there is a desire to go digital on aeronautical to get more channels which negates the benefit of hearing mixed AM. The problem is replacing all those radios being used poses a huge economical issue and therefore safety issue. I doubt we will see digital for a very long time. When aeronautical radio went from 100 KHz channel spacing to 50 KHz, 25 KHz and then 8.33 KHz, it allowed some attrition time for using older radios. Digital will be a much bigger transitional challenge.
 

GROL

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One thing I can think of is it might be worth looking into a directional air band yagi. There is a big null on the side of a yagi and if the office that is causing the RFI problem is at 90 angle to the airport that you want to listen to it might help a lot.

Amazon list one

Sirio WY 108-3N 108-137 MHz Air Band 3 Element Yagi Antenna​

A directional antenna may help. Hoping the interference is not inline between you and the transmitter. Also, you need to find out where that transmitter is. It may not be at the airport, but at a remote site. Which airport and what frequencies are problematic? Not the range but actual frequencies.
 

GROL

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Here is a little trivia question - why do Aircraft use AM? I was studying for the ham extra exam and one of the questions covered this. It has to do with FM capture. Can't have capture when flying an airplane, even the weaker signals need to be heard. I thought that was interesting and just wanted to share.
From experience, Air Traffic Controllers prefer squelch a bit tighter than just a little past threshold. Weak signals frequently breaking squelch is a bit of an annoyance for them. They have a huge amount to concentrate on and frankly I don't see how they do it so well. They cannot understand the weak signal anyway, so tighter squelch is preferred. Now the Guard Receivers (121.5) are pretty much threshold.
 

GROL

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FM did not exist at the time aeronautical radio began. In fact only AM was available until the early 40,s and not used much at all until after WWII. Even SSB wasn't practical back then due to frequency instability. With FM, the strongest signal wins, which is the FM capture. AM signals mix with two or more AM transmissions, although somewhat garbled you can still hear two transmissions and often figure out some of what is said. AM is considered somewhat safer for aircraft to use than FM, but there is a desire to go digital on aeronautical to get more channels which negates the benefit of hearing mixed AM. The problem is replacing all those radios being used poses a huge economical issue and therefore safety issue. I doubt we will see digital for a very long time. When aeronautical radio went from 100 KHz channel spacing to 50 KHz, 25 KHz and then 8.33 KHz, it allowed some attrition time for using older radios. Digital will be a much bigger transitional challenge.
Also going to FM would not have been nearly as easy a transition than decreasing channel spacing.
 

GROL

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If you could tell us which airport and what direction you are from it without pinpointing where you are, we may be able to provide a bit more help. Your profile says from FL. 118.3 shows up for Jacksonville, Miami and St Pete. You mention a hill in the path, and there aren't many of those in Florida high enough to matter, especially anywhere near those three airports. So I am guessing this isn't in Florida?
 

dkcorlfla

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Hi, I was going to try uploading a sample to see what happens but I tripped over a good link with lot's of RFI samples.
Sounds of RFI

Perhaps it will help, Dale
 

dkcorlfla

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Looks like the only options are 1, compress the audio file using the .zip format and upload it using the attach files tab below. Or 2, the media icon above list a couple of sites that may let you upload a sound file then link to it.

Perhaps there is another way but that's what I found on a quick look over.

Dale
 

Rod1989

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Guys, ive managed to upload some video samples. No chatting but squelch fully open, on 121.9 which is the ground freq i dont get much noise and if you notice the sound is normal, BUT on 118.3 which is the tower freq thats the problematic freq and weird noise if you listen carefully it sounds different. Both antenna transmitters are located in the same location. Hope you can tell me what could it be. FYI i use an FM filter from PAR, and an airband filter from AOR. Im located outside the US also.
Thanks Rod.


 

majoco

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IMHO ATC controllers aren't interested in DXing. In a previous occupation, we spent many hours flying around establishing minimum signal strengths on routes around NZ and the South Pacific on the VHF band for comms as well as navaids. I recall the the minimum acceptable signal was 10microvolts - if we found a distance where the signal dropped below 10µV then we circled back, went up 1000ft and tried again and if necessary established an "MRA" - minimum reception altitude - for that route. NZ being a rather hilly country many routes especially in the South Island have "MRA's" on them.
We used Singer/Ailtech NM37/57 field strength meters fed from a normal aircraft comms antenna on the belly of the aircraft.
So if you can't receive the signal you want - go up 1000ft!
 

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krokus

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Guys, ive managed to upload some video samples. No chatting but squelch fully open, on 121.9 which is the ground freq i dont get much noise and if you notice the sound is normal, BUT on 118.3 which is the tower freq thats the problematic freq and weird noise if you listen carefully it sounds different. Both antenna transmitters are located in the same location. Hope you can tell me what could it be. FYI i use an FM filter from PAR, and an airband filter from AOR. Im located outside the US also.
Thanks Rod.


The interference sounds like it could be a switch mode power supply. Does it change patterns?
 

dlwtrunked

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For the reason that commercial AM radio can be heard many miles away from the market where it originates where FM would fade long before that. If aircraft went to FM, there would have to be a complex and expensive network of repeaters.

No, it is definitely not the reason. It is a frequency/propagation effect that commercial AM goes farther ("ground wave". AM was used for aircraft traditionally and you can hear 2 aircraft at the same time in AM but not FM which gives you the advantage of knowing 2 aircraft are trying to talk to you. It has absolutely nothing to do with range.
 
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Here is a little trivia question - why do Aircraft use AM? I was studying for the ham extra exam and one of the questions covered this. It has to do with FM capture. Can't have capture when flying an airplane, even the weaker signals need to be heard. I thought that was interesting and just wanted to share.
I think aircraft communication’s have been around for along time. Maybe since commercial flights became available. And back then the main form of comm’s mode was AM for tower to craft. When aircraft’s became advanced and could endeavour longer flight, SSB was used. The reason why I don’t think all aircraft’s have gone FM for local tower, departures and approaches is the cost to change every aircraft, towers and centres to FM. Could you imagine this happening worldwide.
 

Rod1989

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The interference sounds like it could be a switch mode power supply. Does it change patterns?
It may be, i suspect it comes from a neighbor small office building, when they dont work the issue dissapears, its just present during work hours. I really dont know what to do, i tried filters, moving the antenna, etc but no luck.
Rod.
 

MUTNAV

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Also going to FM would not have been nearly as easy a transition than decreasing channel spacing.
There are other possible issues also, like if they wanted to keep the decreased channel spacing, what would be the effects of Doppler effect be on high speed aircraft, bandwidth would have to allow for both approaching and separating aircraft, and possible capture issues.

I have NOT done the math for any of this, just an idea.

Thanks
Joel
 

G7RUX

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AM has the advantage of longer range.
On the same frequency it does, a little, but not all that much. This is due to not having the need to enter limiting before the detector so a weak AM signal just gets quieter and a little noisy whereas a weak FM signal becomes extremely noisy and rapidly R1. Add to that the typically narrower channel filtering on AM VHF wrt FM and that another SNR gain, both on same-pwr Tx and on Rx.
 

G7RUX

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There are other possible issues also, like if they wanted to keep the decreased channel spacing, what would be the effects of Doppler effect be on high speed aircraft, bandwidth would have to allow for both approaching and separating aircraft, and possible capture issues.

I have NOT done the math for any of this, just an idea.

Thanks
Joel
Doppler shift at VHF isn’t huge, even when at Mach 2, so it should fall within capture without significant issues. However FM at narrow channel widths is typically troublesome and usually avoided.
 
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