A vertical 1/4 wave antenna receives signals:

Equally from all horizontal directions.

The 1/4 and 1/2 antennas are the most popular, as the dipole are the most common. The propagation pattern of the dipole looks like a horizontal doughnut.

Please see USNA Edu site for the pdf presentation of EE302 Lesson 13: Antenna Fundamentals

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The resonant frequency of a Hertz antenna can be lowered by:

Placing an inductance in series with the antenna.

The Hertz antenna is simply the dipole antenna.

For more information, please see Wikipedia's article on Dipole antenna

See the Oregonrfid.com site for the basic information in Class-Handout-Antennas.pdf

Also, please see The Electronics Notes site for the article on Antenna Resonance & Bandwidth

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An excited 1/2 wavelength antenna produces:

Both electro-magnetic and electro-static fields.

These two fields at right angle to each other make up the radio wave.

For well-illustrated information, please see Brown Edu site for the pdf presentation 24. Antennas

Please see Interference Technology site for the article Antenna Fundamentals

And for more detail information, please see Wikipedia's article on Near and far field

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To increase the resonant frequency of a 1/4 wavelength antenna:

Add a capacitor in series.

For more information, please see All About Circuits site for the article Resonant Filters.

Also, see Electronics Tutorials site for the article Series Resonance Circuit

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What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?

It is decreased.

Bandwidth is inversely proportional to quality factor Q, and \[Q = \frac{\text{reactance}}{\text{resistance}}\] Thus, adding reactance reduces (decreases) the bandwidth.

For more information, please see the All About Circuit site for the article Q Factor and Bandwidth of a Resonant Circuit

Also, please see the Electronics Notes site for the article on Quality Factor / Q Factor; formulas and equations

And, see Wikipedia's article on Q factor.

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To lengthen an antenna electrically, add a:

Coil.

Antenna has to be 1/4 to be resonant at a frequency. To have a shorter antenna still resonate at that frequency, a coil is inserted either at the bottom or in the middle of the antenna. It is really a resistance to the transmission line, which helps reflection of sent signal.

The coil presents an inductive reactance of same value as the capacitive reactance, with those two cancelling each other.

For more information, please see Wikipedia's article Loading Coil, especially the Radio Antenna section.

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