Why so hard to pull in air freqs?

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adamgirard

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Hello,

I've been scanning for a year or two and I live near KALB in Albany, NY. The airport is right near an interstate and if I had my scanner on during a drive, I will usually only hear occasional responses from aircraft to the ATCC. However, for about the 2 miles or so near the airport, I can hear the tower broadcasts, although broken.

My question is, why is the signal from the tower only able to be heard clearly in such a small area on the ground but obviously such a large area in the air? The aircraft need to be able to talk with the tower from 15+ miles away. Does it have something to do with me using a handheld scanner and the antennae on the aircraft is just that much more sensitive?

Just something I have always wondered, thanks in advance!

Adam
 

eorange

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It's mostly because you're on the ground.

For simplicity, picture a funnel that represents the radio signals from the ground up to the airplanes. Unless you're right on top of the antenna (neck of the funnel), then you won't hear much. The signal radiates wide and high for the airplanes.

Things to try: open your squelch all the way, parked on a ground freq. I can hear some faint CLE ground traffic that way, but in general I can't hear nearly anything from CLE ground.
 

prcguy

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There are no special upward pattern antennas used at airports or for ground to aircraft comms, they are all fairly low angle. This includes fiberglass enclosed dipoles, the ground planes with lots of ground radials and Discones commonly seen at airports.

Its probably a combination of terrain surrounding your airport and the rubber antenna on your handheld that are at fault. I can pick up the tower at LAX 10-15mi away on a handheld scanner but only a few miles away for Long Beach airport a few miles from my work. The height of the airport antenna has a lot to do with seeing over local terrain.
prcguy
 

eorange

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I hear you, but this has always puzzled me. For instance, I visited my local RCAG a few years ago, and I didn't hear the corresponding ground traffic until I was a mile away. Understand that in this part of NE Ohio...it's pancake flat. I would have expected better - or any - reception farther away. And this was with a proven airband receiver (BR-330) and antenna (Diamond RH77-CA).
 

whiskeytango

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in my area its pretty flat, and the airport is only 15 mi away as the crow flies, i hear the ground to plane maybe 10% of the time, and the planes come in loud and clear...but it makes sense for the airport to focus that radio energy upwards
 

prcguy

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It doesn't make sense to focus the radio energy upwards. Even with an aircraft at 37,000ft directly over head its only 7mi away line of sight. You can pick that up with just the center pin of a PL-259 sticking in mid air and no antenna. At 37,000ft and a 45deg angle to the aircraft is 10mi away and so on.

At 37,000ft and 100mi out the aircraft is about 35deg off the horizon, 200mi its about 19deg off the horizon and dropping fast. At this point ground obstructions and ground bounce/signal cancellation are probably working against you and antenna height and gain becomes your friend. (The distant look angles will be lower than I calculated due to the curvature of the earth, if anyone wants to add that please do so!)

Aircraft comm antennas put the energy at the horizon where the energy is needed to communicate at a distance. I have a garage full of airport and FAA antennas and they are no different than what most of us use for scanning and they are all low angle antennas.

I see many small airports with antennas fairly low to the ground and some major airports with 10 story high control towers with antennas on top. I'm not up on airport transmitter power but its fairly low, maybe an airport radio guy can fill us in. I think the airport reception discrepancy has a lot to do with the listeners surroundings and how high the airport antennas are mounted.
prcguy


in my area its pretty flat, and the airport is only 15 mi away as the crow flies, i hear the ground to plane maybe 10% of the time, and the planes come in loud and clear...but it makes sense for the airport to focus that radio energy upwards
 
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kb2vxa

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OK, let's simplify this with the Golden Rule of VHF communications, height is might. On the ground a signal coming from atop a mountain or high tower goes farther than one from a low rooftop so expound on this and you have the reason why an aircraft at altitude can be heard over a wide area and the controller on the ground cannot. He doesn't need to be heard far on the ground so the antenna doesn't need to be high when the aircraft more than makes up for it.

As an aside so I don't confuse you, that funnel analogy has its place although wrongly applied in this instance. It is known as "the cone of silence" close in and over the ground antenna, as the signal radiates outward it radiates poorly upward resulting in a dead spot over the ground station. Being the ATC only needs to communicate in the glide path a dead spot directly overhead is quite unimportant.
 

whiskeytango

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wouldnt they use diff ant systems depending on where the aircraft is? if that airplane is 200 miles away it wont be communicating with the airport its landing at
 
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N_Jay

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They are all similar systems.

Also, remember, the base antennas are not very high (by LMR standards) because the mobiles are.

So you don't hear a base as far on the ground as you might have otherwise expected.
 
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In some instances the actual ground control base will be located on the field; while approach/departure and other ground to air bases may be located at a remote facility located serveral miles away, usually with other FAA radios such as ATC, enroute etc. There may be two sites one receive only and the other with transmitters only.
 

VK2GEL

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Pagers and FM broadcasts can also interfere with airband AM and reduce your range.
 

JStemann

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I'm not up on airport transmitter power but its fairly low, maybe an airport radio guy can fill us in. I think the airport reception discrepancy has a lot to do with the listeners surroundings and how high the airport antennas are mounted.
prcguy
Most VHF tx will run 10 watts carrier and UHF runs 10 or up to 50 with a power amp. This is at the transmitter so figure on about 5-7 watts at the antenna.(with a 10w output). This would be at a typical airport or rcag. The little municipal airports could have just about anything.

jeff
 
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