Vertical vrs Horizontal antenna

It is very important to use vertical polarization for transmitting in the AM band because propagation is principally by a ground wave. Horizontal polarization does not produce a ground wave.

FM radio employs both vertical and horizontal polarization. Some stations do it with antennas that radiate both fields, other stations run separate transmitters and antennas at V and H polarity (example on radio-locator.com is WSIE Edwardsville, IL with 50 kw H and 41 kw V)

But Part15 FM always seems to be limited to vertical. Wouldn't it be important to improve the reception by having both polarities?

VHF has different propagation characteristics from MF. The ground wave is highly attenuated at FM frequencies, and FM propagation does not depend on the ground wave. For that reason, vertical polarization is not essential for FM, like it is for AM.

You can use circular polarization for FM if you wish, but you'll have to meet the FCC field strength limit. That would be difficult for most of us to verify with an outboard CP antenna. Also, it's important to point out that you would need to meet the limit in both the vertical and horizontal planes.

Most Part 15 FM applications involve transmission to portable radios, car radios, or table radios with wire antennas; all of which normally have a predominantly vertical pickup pattern. I have done some testing with a CP crossed dipole transmitting antenna and did not find any advantages for normal applications. The use of a CP antenna at the receiver can be quite helpful under certain circumstances; however they are pretty large for the FM broadcast frequencies.

Historically AM radio has used vertical fixed towers as transmitting antennas for nearly 100 years. However, recently the FCC approved a horizontal wire antenna to be certified and used by commercial broadcasters in the AM band. The antenna is called the KinStar. For AM receivers in cars; most are vertical accept the antennas in the windshield glass. Those are horizontal. At home, most receiver antennas are horizontally polarized. Vis a vi the iron core wire wound antennas in table top and portable radios.

On AM Part 15 stations, I would recommend staying with a vertical antenna because a horizontal antenna would create a huge amount of ground capacitance and be really difficult to keep tuned with out large components to tune out the reactance.

And finally, the answer to the original question: the difference in signal from vertical to horizontal or vice versa? The answer is 20 Db. Look on a VU meter and you will have some idea of the difference in signal.

Magnetic antennas, such as loopsticks, and slots formed by windshields, are vertically polarized when oriented in the horizontal direction. The polarization refers to the direction of the electric field. Thus, if the magnetic field is in the horizontal direction, the polarization is vertical.

The KinStar antenna is vertically polarized, and not horizontally polarized. The radials on top provide top-loading, but almost no radiation, to shorten the height of this vertical antenna.

True, the KinStar is a vertically polarized antenna. The vertical wires being 65 feet or so at 1400 KHz. The rest of the antenna is horizontal and acts as a capacitive top hat to load the very short vertical portion of the antenna. In practice, the horizontal wires are somewhat longer that the vertical wires. In amateur radio, for years, we have used the "mult-tee" antenna for low band operation. It, too, was a vertically polarized antenna with substantial length horizontal wires. Or how about the inverted "L" antenna made of twin-lead on 160 meters. These operate on a very similar set of design parameters. The original question in this thread was the difference in signal strength between horizontal and vertical polarization if transmit and receive antennas were in different orientations. The answer remains: 20 Db; all other factors being generally equal.

One last note: A horizontal MW AM antenna in part 15 use would operate at NO advantage over a legal length vertical antenna. In practice, the vertical will out-perform the horizontal every time.

The inverted "L" antenna, mentioned by kk7cw, is a good example of a vertically polarized antenna that appears like it is horizontally polarized, because it has a long horizontal element. Because the geometry of this antenna is simple, it is easy to explain its theory of operation. The invertrf "L" is a simplified form of the "T" and the "Star" antennas.

The horizontal element does not radiate much because, in the image of the horizontal element (produced by reflection from the ground) the current flows in the opposite direction of the current flow in the actual horizontal element. The fields produced by the opposite current flows approximately cancel each other out, and so there is very little radiation from the horizontal element. A similar thing happens in parallel transmission lines. Nearly all of the radiation is from the vertical element. The same thing happens when the top-loading is produced by two radials ("T" antenna), or multiple radials ("Star" antenna).

A variation of the inverted "L" antenna is the DDRR, where the horizontal element is a loop open-circuited at the end. For both of these antennas, the total conductor length for resonance is about a quarter wavelength, with the conductor for the DDRR being slightly longer. The DDRR can be made more compact by using two or more loops. The more the number of loops, the longer the conductor length at resonance will be compared to a quarter wavelengt. The limiting case of a multi-turn DDRR is inductive top-loading with a loading coil on top of the vertical antenna. The total conductor length at resonance with a loading coil on top is roughly a half wavelength.

While the conductor length at resonance of the inverted "L" antenna is about a quarter wavelength, no such simple relationship exists for the "T" and the "Star" antennas. In those cases, it is necessary to use an antenna analysis program, such as NEC or wipl-d.