[rgr331]

How come as I drive away from my home town and get further away, I can no longer receive my favorite radio station in my car?

[Pete Svarrior]

Primarily because of the law of conservation of energy. As the waves spread from the transmitter, they carry the same amount of signal over a wider area. Add to that the many sources of RF interference and you've got yourself a recipe for a very limited useable range.

If you want to listen to stuff from the other end of the Earth, stick to low frequencies, like those Russian number stations do.

[rgr331]

Primarily because of the law of conservation of energy. As the waves spread from the transmitter, they carry the same amount of signal over a wider area. Add to that the many sources of RF interference and you've got yourself a recipe for a very limited useable range.

Is it because the earth is round and the distance the horizon is from my eye is directly proportional to my altitude above the ground?

If you want to listen to stuff from the other end of the Earth, stick to low frequencies, like those Russian number stations do.

Okay, then how come in my aircraft I can pick up the stations from much much further away.  For example, when I’m over Houston, I can pick up stations from Oklahoma City, but when I’m in a car in Dallas, I can not pick up signals from Oklahoma City.  It’s still the same signal strength carried over a much much larger area and there is still the same amount of RF interference.

Is it because the earth is round and the distance the horizon is from my eye is directly proportional to my altitude above the ground? 

Higher altitude - Horizon further away - therefor not blocking the signal.

Lower altitude - horizon closet - therefor blocking the signal.

[KevAmiga]

To add, look up HF atmospheric bounce.

Im a HAM radio op, at certain times I can speak to people many many miles away...then, the just fade away.

Dependancy on the ionosphere being charged by the sun. I don't know the total science behind it, but it happens.

For example, using something around 27mhz, its generally quiet. I live in Wales... all of a sudden boom, ill get newcastle operators coming in as if they are sitting next to me.

All about that wavelength VS trop conditions.

[KevAmiga]

Primarily because of the law of conservation of energy. As the waves spread from the transmitter, they carry the same amount of signal over a wider area. Add to that the many sources of RF interference and you've got yourself a recipe for a very limited useable range.

If you want to listen to stuff from the other end of the Earth, stick to low frequencies, like those Russian number stations do.

Okay, then how come in my aircraft I can pick up the stations from much much further away.  For example, when I’m over Houston, I can pick up stations from Oklahoma City, but when I’m in a car in Dallas, I can not pick up signals from Oklahoma City.  It’s still the same signal strength carried over a much much larger and there is still the same amount of RF interference.

In your aircraft there are no obstructions to the tower / transmitter. Or at least, far far less.

Aircraft use UHF, thats around 130mhz Amplitude modulation. This frequency does rely more on line of sight (higher the frequency, the shorter the wavelength, so it cannot "pass" around hills, buildings, etc.

Lower you go, further it goes, with less power.

[rgr331]

Primarily because of the law of conservation of energy. As the waves spread from the transmitter, they carry the same amount of signal over a wider area. Add to that the many sources of RF interference and you've got yourself a recipe for a very limited useable range.

If you want to listen to stuff from the other end of the Earth, stick to low frequencies, like those Russian number stations do.

Okay, then how come in my aircraft I can pick up the stations from much much further away.  For example, when I’m over Houston, I can pick up stations from Oklahoma City, but when I’m in a car in Dallas, I can not pick up signals from Oklahoma City.  It’s still the same signal strength carried over a much much larger and there is still the same amount of RF interference.

In your aircraft there are no obstructions to the tower / transmitter. Or at least, far far less.

Aircraft use UHF, thats around 130mhz Amplitude modulation. This frequency does rely more on line of sight (higher the frequency, the shorter the wavelength, so it cannot "pass" around hills, buildings, etc.

Lower you go, further it goes, with less power.

Well the obstruction of the Horizon is certainly much further away and the most significant. Buildings however are almost completely insignificant and make almost zero difference unless I am right next to the building. Think about how your car radio works on the AM band. Are you able to receive the local signals while driving through a downtown area.  Yes you are.

My aircraft DOES NOT use UFH, it uses VHF, most military aircraft use UHF. But those are both frequency modulated (FM) bands that would not be able to receive the signal from an AM (amplitude modulated) station.

Also, my aircraft can not dictate what type of signal the radio station sends.

I also have an ADF which can receive AM signals.  When I am over Houston, I can receive signals from OKC, yet when I am on the ground in Dallas, I can not receive the same signals from the same station at the same aircraft using the same equipment.

[KevAmiga]

Ah sorry, yes I meant vhf not uhf, (frequency will be close though).

So, AM isn’t a band. AM is a modulation type. Your AM radio is probs using MF band, so less than 1mhz. 100 or so times bigger wavelength than your aircraft radio.

Don’t confuse band with modulation type.  That’s why it travels so far, and isn’t affected by buildings and walls. Trust me, do the same with a 70cm signal and literally a wall can wipe it out. PMR radios are a good example.

[KevAmiga]

Also, my aircraft can not dictate what type of signal the radio station sends.

Yes it can. All radio receivers have a detection stage. Your radio in the aircraft prob doesn’t have a switchable detection phase so will be AM only. This is why we have band plans. Pilots prob wouldn’t appreciate someone transmitting using SSB on their frequencies. Could be a disaster.

[rgr331]

Also, my aircraft can not dictate what type of signal the radio station sends.

Yes it can. All radio receivers have a detection stage. Your radio in the aircraft prob doesn’t have a switchable detection phase so will be AM only. This is why we have band plans. Pilots prob wouldn’t appreciate someone transmitting using SSB on their frequencies. Could be a disaster.

dictate, not detect

If the AM station sends an AM signal only, then there is no way for me to receive it with VHF equipment.

So, for starters, let’s exclude cloudy days, because AM signals reflect of the ground and clouds on cloudy days.  in the manner, the can travel around the earth. 

But on a clear day:
The same signal sent from a station in Denver to the East with no buildings interfering with it will be lost at ground level once  the receiver is 100-150 miles away.  However, from 37,000 feet the very same signal from the very same station, with no building interference could be received from a position over Kansas City on a clear day.

This is because on the ground, the receiver’s line of sight is blocked by the horizon at a short distance. In the air, the receiver also gets its line of sight blocked eventually, just at a much much larger distance.

this is because the earth is round.  An AM signal on a flat earth on a clear day with no obstructions could reach both the ground level receivers and airborne receivers no matter what distance they are from the station.

Why do you think stations put these transmitters at the top of tall towers?

[KevAmiga]

Ok so :

If the AM station sends an AM signal only, then there is no way for me to receive it with VHF equipment.

False.

Also your statement is riddled with errors, you need to read up a bit more about modulation types vs frequency.

I have no experience with radio comms from aircraft, but sure listening from ground based stations is just the reverse.

If you like, I’ll post a video of my uhf / vhf radio receiving in AM modulation mode, if that helps.

[rgr331]

Ok so :

If the AM station sends an AM signal only, then there is no way for me to receive it with VHF equipment.

False.

Also your statement is riddled with errors, you need to read up a bit more about modulation types vs frequency.

I have no experience with radio comms from aircraft, but sure listening from ground based stations is just the reverse.

If you like, I’ll post a video of my uhf / vhf radio receiving in AM modulation mode, if that helps.

Since you have no experience with radio comms in aircraft, then I guess you didn’t know that our equipment CAN NOT be switched from AM to FM or vice versa.

We have a radio that we talk on

How come as I drive away from my home town and get further away, I can no longer receive my favorite radio station in my car?

- operates on VHF

Some have radios that operate on UHF

We have ILS/VOR receivers - they operate on VHF

We have ADFs (automatic Direction Finder). They can receive a voice transmission that is broadcast from an AM radio station like 1310AM.  ADFs can not interpret VHF signals.

None of this has anything to do with why the signals are blocked at much closer distances from the transmitter while on the ground vs in the air.

The answer is, the earth blocks the signal because the earth is round.

[KevAmiga]

Im glad we agree on one thing in that statement haha

Quote : Since you have no experience with radio comms in aircraft, then I guess you didn’t know that our equipment CAN NOT be switched from AM to FM or vice versa.

I never said that, you said : If the AM station sends an AM signal only, then there is no way for me to receive it with VHF equipment.

You can get VHF equipment that does AM, FM, USB, LSB, CW modes. Not in aircraft, no, im aware of the AM VHF Airband allocation.

Quote "None of this has anything to do with why the signals are blocked at much closer distances from the transmitter while on the ground vs in the air. "

It does, but i get what you mean.

Im glad we both agree we live on a round ball.