FM Antenna - Which Polarization to Choose?
The question was presented to me if you have an FM radio receiving antenna which polarization should you use? I will defend my answer in more detail below.
Polarization is an age old problem with only one simple solution for reception of FM Radio Broadcast, which is horizontal (mobile applications differ).
General thoughts FM polarization:
Almost all systems that use elliptical polarization also use a preferred plane polarization also. Take FM for the example. The preferred plane polarization is horizontal. The only exception to this I am about are a few stations on 89.9 that are required to transmit vertical to reduce interference to TV Channel 6.
One will find on FM if a station broadcasts elliptically, normally it's in an equal x and y planes called circular polarization. Some use dual plane polarization. But you almost never find FM stations using elliptical where the ellipse is shaped so the vertical is stronger. I am not even sure if that is legal. Trip knows FCC rules better.
Is there a preferred handedness to elliptical polarization? I have never looked at in FCC records exhaustively, but I believe I have read elsewhere, no. Even if there is, once a right handed signal bounces off a building for example, the reflected wave is then left handed. Now if the receive antenna is right handed, because their favorite stations are right, yet possibly unknown to the receiver there are strong reflections, stronger than the main signal, then the entire system becomes 20 db down.
If I transmit right handed and you receive left handed circular, you loose 20 db of my signal. Just like if I transmit plane vertical and you receive plane horizontal, you loose 20 db of my signal. As you can see it's rather easy for all the trouble to build a circularly polarized antenna just went out the window. To build an antenna that will switch polarization is possible but seldom done except in OSCAR (Orbiting Satellites Carrying Amateur Radio) ground stations.
So why do FM stations transmit elliptically if it's all this trouble?
First it's not that much trouble. Slightly larger antenna as each bay has to be a little bigger. The difference in cost to the station is very little in light of erecting a tower, feedline and erecting or renting towers.
Ok, then what advantage is it to the consumer or receiver for the signal to be elliptically polarized? After all if you put up the wrong handedness it's a waste of time. Plus where do you buy elliptical antennas anyway? Well the advantage is two fold. Partly for fixed stations and a lot to do with mobile reception.
Fixed Reception: If a station transmits just horizontal and is clear line of site with no obstructions or reflects, then there is no advantage, but that is not the real world. It's very likely the reception, in particular indoors, from a horizontally transmitted signal will remain horizontal. If this were not true, then we would all orientate our rabbit ears purely horizontal, always. The signal will twist polarization depending on the type and incidence of reflection and passing through walls with any metal in them at all. In other words, a horizontally transmitted signal may not be horizontal at all when it arrives at your antenna. Any degree from purely horizontal if you antenna is horizontal results in a diminished signal. Horizontal waves also can reverse phase by 180 degrees bouncing off the earth in front of the antenna, leaving you with a stronger vertical signal.
The best answer the broadcasters could come up with was to use elliptical polarization, most commonly with equal x and y axis components also known as circular polarization.
A plane polarized receiving antenna in the presence of an circular signal will always find part of that signal, normally if circular transmission to plane reception, at 3 db below the broadcast power.
The same is true if a plane incidence wave is 45 degrees to the received plane polarized antenna, there is a loss of 3 db. However between 45 and 90 degrees there is greater than 3 db loss from a plane polarized system. But with circular transmitted and plane polarized system the loss remains at 3db.
Now to make up for this, the FCC measures an FM stations power in the horizontal plane. If they are licensed for 100KW, then using circular polarized antenna lowers their horizontal power by 3 db to 50KW. What the heck is the advantage to that? Well then they can boost their overall ERP to 200KW and if their antenna is purely circular then only 100KW ends up in the horizontal plane.
Does this give them more range? Well first, only if received with a circular polarized antenna and even then not much, maybe 5% more range.
But person at the receiving end with their plane polarized antenna can now orient it in any plane and receive the effective 100KW (speaking purely free space reception if anyone wants to nit pick). Now lets say the signal hits a building on the way there and reverses handedness. Your plane polarized receive antenna could care less. It still gets a full signal minus 3 db.
And of course, most impulse noise is vertically polarized in it's electrical field and since most of us use dipole type antennas that receive the E-field, being horizontal reduces noise reception by many db. The only thing better is a loop or quad driven element or antenna (not a folded dipole as that is still an electrical antenna). Since the quad loop intesects the magnetic part of the EM, ElectroMagnetic Wave there is even less noise reception.
Mobile Reception: This should now be obvious to the advantage of circular transmission to a mobile antenna. Reflections to a mobile happen frequently and rapidly. A mobile whip antenna will intersect the circular wave and pull out 3db below the total power, even as the received wave twists and turns down the street.
Now something only a few car manufactures do is slant the mobile antenna. This helps if a station is either purely horizontal or dual polarization (but not elliptical). By slanting the antenna you can pickup different reflections which are more common on the highway from other cars, trucks, signs, building, etc etc than a home installation.
I think a slant from purely vertical is best for mobile.
Maybe the T2FD needs to be resurrected for FM RX?
I am looking to make a custom FM and possibly TV antenna out in the country, over the horizon a few degrees, based on your excellent description of how FM TX works nowadays: do you think that a Navy style T2FD is the best? the tilt about 30 degrees from Horizontal plane. think about intercepting (more) mag field, rejecting Vert E fields, terminating to wave impedance, rejecting Horiz E fields (from over the horizon twisting).... man I'm thinking you could beat a TERK monster with the T2FD out in the country.