Antenna with most gain

I'll try to keep this short but hope I can give you some keywords if you want to search for more info.

Gain antennas work because they do not radiate in all directions. For broadcast you generally, but not always, want omnidirectional radiation. Since listeners are neither in the air above nor the ground below the antennas are designed not to radiate up and down. That otherwise wasted power is now available to radiate horizontally and increases the field strength in this direction which has the same effect as increasing the tx. power. TV and FM stations many times use antennas which produce a pattern, which if you could see it, would look like a giant pancake perched on the tower going out to the horizon.

For directional use a YAGI or BEAM antenna or a mast mounted vertical dipole are used. The spacing between the dipole and the metal mast determines the pattern and gain.

For omnidirectional patterns an experimenter can usually build a 5/8 ground plane whip with a loading coil.

The highest gain antenna I have built for VHF is a COAXIAL COLINEAR. The more elements one adds the more gain. This does not require a ground plane but it gets pretty long. It is also within the means of a hobbyist's abilities and budget.

Another choice to consider is a STACKED DIPOLE. The are hard to build and usually are "store bought" for ham and commercial applications.

Almost any gain antenna on part 15 FM can lead to problems with the FCC due to the field strength so keep that in mind.

Neil

I always wondered what what the results would be if one places a Part 15 transmitter and antenna (together on a mast-like a lot of us do).

And THEN strategically puts up a high metal fence or a flagpole or two behind the Part 15 transmitter and antenna.

Would this act as a quasi directional antenna of sorts, intended or unintended?

it would be illegal if the radiated field over a 3m path exceeded 250uV

Thank You,

Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org

Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree

Non directional antennas can exhibit gain. An example is a circularly polarized antenna commonly used in FM and TV broadcast. A single antenna is negative gain as the polarizations compete with each other. Two stacked antennas of this type are close to unity. Four or more begins to increase the ERP

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

kyradio,

Although the geometry of the configuration you described is a lot different from the "whip and mast" antenna, it is actually equivalent to the whip and mast. This is because the radiating conductor that supplies most of the radiated power is concealed, and not counted toward the antenna length.

When a large resonant structure, like, for example, a quarter wavelength AM broadcast tower, is placed near a Part 15 antenna, RF energy is capacitively coupled to to the resonator. Both the Part 15 antenna and the resonator radiate, but the resonator radiates a lot more. This is because, since the resonator is a lot bigger than the Part 15 antenna, it has a much larger radiation resistance. This arrangement is not really directional, since practically all of the radiation comes from resonator, and not the Part 15 antenna. This is similar to the whip and mast antenna where nearly all of the radiation comes from the "mast."

Of course, the resonator does not have to be as large as a quarter wavelength. A 50-foot antenna with a loading coil to make it resonant at the operating frequency of the Part 15 transmitter will radiate very effectively.

I discussed this previously in the EH antenna thread. I explained that the EH antenna, and its relatives, like the CFA, are all whip and mast antennas. The CFA is an apparently short antenna intended to replace tall AM broadcast towers. In a couple of their demonstations, The CFA promoters have placed their small antenna near large resonant structures in order to make their antenna seem to have better performance than it really has.

I have never heard of this trick being used in Part 15 AM. I don't think it would fool the FCC. Using their equipment, they can easily find where the "real" antenna is located.