Vertical & Horizontal polarized antenna questi

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Allen Turner:
I have two questions.  What is the difference in dB's if one station is using a vertical antenna and the other station is using a horizontal antenna in the VHF and UHF range VS if both were vertical or horizontal?  As I understand it radio and TV stations transmit using vertical polarized antennas.  Why is it then that we install our TV antennas horizontally?  

Thanks, Al, n7ioh

Dan:
"What is the difference in dB's if one station is using a vertical antenna and the other station is using a horizontal antenna in the VHF and UHF range VS if both were vertical or horizontal?"

I think we're talking easily 20dB, possibly 30dB+.  Depends on a lot of things.

"As I understand it radio and TV stations transmit using vertical polarized antennas. "

Radio stations use CIRCULAR polarization as far as I know.  The penalty for going either handedness of circular polarization to any linear polarization  is only 3dB.  

You'll note your car radio antenna is vertically polarized... easier to do an omni vertical whip there.

TV stations either use horizontal or circular, I think... I'd have to look it up.  I'd expect a broadcast expert or two to show up soon  if not while I'm typing ;-)
 
Horizontal polarization gives a little more gain over the horizon and you get some ground gain from horizontal polarization that you don't from vertical, so fringe radio and TV reception will be generally better with horizontal antennas if the transmitting station is using circular.



73,
Dan

Mike Brenza:
At VHF and UHF, 20-30 dB is possible!
Most TV and FM stations broadcast with both horizontal and vertical polarization.(there is a story to this)
AM broadcast stations broadcast using vertical polarization.
Amateurs use horizontal polarization with "weak signal" VHF/UHF work, along with narrow bandwidth antennas.  FM is vertical because it is used mostly in a mobile/portable environment.
73s.

-Mike.

David E. Greer:
Not much that I can add to N3OX's explanation. Cross-polarization on VHF/UHF can decrease signal strength by 20-30 db -- a very significant amount. I was very active on 2-meter SSB/CW for more than 20 years. It was sometimes difficult to get new operators -- guys experienced on HF but new to 2-meter SSB -- to understand that this was a different situation than HF where cross-polarization on HF skywave makes little or no difference. They'd try 2-meter SSB using their 2-meter FM  ground plane vertical and not understand why they weren't hearing signals.

As for FM broadcasters, they use circular polarization since it has components of both horizontal and vertical polarization which should minimize fading.

Early ham VHFers settled on horizontal polarization because it was thought to be a little more quiet than vertical polarization. Some think the cross-polarization issue prevented 2-meter SSB from ever really taking off in the U.S. and becoming as popular as it might have.

73, N4KZ

Tom Rauch:
Al,

Most FM stations are either circular polarization or DUAL polarization. There are some exceptions that are horizontal only. In my life I've only seen two or three out of dozens that were strictly horizontal. I've seen several dual poalization but most are circular or claim to be circular. The reason is most home antennas are horizontal, and a large number of portable radios (like cars) are vertical. They have to split power to cover both. This also means for direct wave the antenna can be almost any tilt except the same circular sense or a tilted angle that cancels both. A right hand circular transmit has to have a left hand receive when each is circular, but when the signal is scattered you would not want to have circular (it could increase fading) so no long distance circular antennas are ever used. They are all horizontal.

Nearly all TV stations are straight horizontal polarization. I have never even heard of one using circular. The reason is circular polarization would increase ghosting. There are no TV antennas made for circular so any mix would reduce signal to noise by 6 dB, that's why TV antennas are installed horizontal.

AM radio is all vertically polarized because only vertical polarization will work for groundwave. This is NOT a problem for FM or TV, since they work at VHF and higher by direct or scattered waves in which horizontal will work just fine.

Any wave scattered or through reflection from the ionosphere either rotates polarization constantly or has a tilted or has random polarization. This is why polarization sense at each end only matters on direct wave (like VHF or UHF TV or FM) or ground wave (where only vertical makes it any distance).

On direct wave you could easily have 30 dB or much more attenuation between cross polarizations. As a matter of fact if the receive antenna is one polarization and the transmit antenna exactly 90 degrees off, the response would be zero. There would be no signal at all. In the real world that doesn't often happen because of skewing or very sight polarization tilt.

Here's an interesting fact. I have a vertical on 160 meters. If I want minimum possible signal level of my own transmitter in an antenna 2500 feet away, I use a dipole for that second antenna. Minimum signal occurs when the dipole is BROADSIDE to the vertical, NOT when the dipole end is pointed at it. As a matter of fact the signal is almost maximum when the dipole end is pointed at the vertical.

Do you know why that is?? It is because a perfect horizontal dipole is only perfectly horizontal polarization exactly broadside to the dipole.

Polarization is more complex than most people ever realize.

73 Tom


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