First off an antenna does not have to be resonant to work. There are many
designs using non resonant antennas.
An example is a random wire using a tuner.
See URL:
http://www.qsl.net/ve3mcf/elecraft_r...n-Resonant.txt
For 1/4 to 5/8 wave antennas
Going from 1/4 wave to a half wave to a 5/8 wave vertical results (in each
case) in a lower angle of radiation, thus 'gain"
See URL for a pictorial of this
http://www.wingsandwheels.com/page14.htm
Another at URL:
http://web.wt.net/~nm5k/acompari.htm
Be aware of what antenna gain means
Antenna gain is somewhat a misnomer, as "Ya can't get out more than ya put
in". In reality no antenna has gain, only losses due to inefficiencies So
what is meant by antenna "gain"? The gain spoken of has to do with shaping
the antenna radiation pattern so that more energy is radiated along a given
plane as compared to a reference antenna. In effect the radiation pattern is
squished into a narrower directional beam. Squeeze a round toy balloon to
visualize the effect.
Several reference antennas can be used, but the ones most often used are the
dipole and an isotropic antenna. When using the dipole as a reference, the
subjective value of zero dB gain (dBd) is assigned (except NMEA, National
Marine Electronics Association, which assigns it a subjective value of 3 dB
gain). The isotropic antenna is by theory an infinitely small sphere that
radiates equally in all directions. By comparing a real antenna to this
standard, a figure of merit can be expressed in dB, usually stated as dBi.
That is, the gain of the real antenna over the isotropic reference. Since
the real antenna does not radiate equally in all directions, some plane of
directivity must be specified.
Every antenna, other than an isotropic will have gain in some direction at
the expense of that in others. This gain of course, comes from not radiating
power in all the other directions. And the beam width is measured at the
minus 3 dB points (half power points) so the antenna "hears" and "talks"
well beyond the beam width specification.
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A Half wave vertical will have a lower radiation angle than a 1/4 wave and
presents a very high impedance feed point. The feedpoint is a voltage
maximum, has high impedance, and thus must use a critical matching network
to be fed with conventional coaxial cable. Such a network is often no more
than an autotransformer with about a 2000 Ohm secondary and a tap at the 50
Ohm point. In any case, since the natural impedance of the end-fed half-wave
is so high, it is not influenced by the presence or absence of ground.
A 5/8-wave antenna adds 1/8-wave to the length of the 1/2 wave radiator,
which shifts the voltage-current relationship by 45 degrees and creates a
low-impedance antenna. This is also easier to match to coaxial cable, since
the match ratio is much smaller and the network used will be more efficient.
5/8 wavelength antennas are not resonant. The purpose of such an antenna
design is to raise the current maximum higher up the antenna, thereby
yielding a lower angle of radiation.
Normally a matching method is used, since the characteristic impedance of
such an antenna is capacitive and the resistive component is greater than 50
ohms
A tapped coil is used to feed the antenna with coax, thereby canceling the
capacitance and providing an impedance transformation to 50 ohms.
http://www.qsl.net/w4sat/five8th.htm
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As you extend the wavelength above 5/8 (actually 3/4 lambda as I recall) the
antenna begins to exhibit split lobes with some at relatively high radiation
angles. So 5/8 is about the maximum practical wavelength for a vertical
antenna.
For a quarter wave antenna, the feedpoint impedance might be around 32 Ohms
(various factors enter into this), but many designs use drooping radials to
achieve a 50 Ohm match.
The ARRL Antenna Handbook will elaborate on these designs.
--
Hope this helps
Caveat Lector
"bbnn" wrote in message ...
I need help understanding these different types of verticals.
What is the feedpoint Z of each of these?
I can't understand how a 5/8 can even work. It seems like it has to be
multiples of 1/4 wave for any antenna to work.
Why not a 7/8 wave antenna?
Why does a 5/8 wave antenna need a loading coil and a 1/4 wave antenna
doesn't?
Is there such a thing as a 1/2 wave whip ant. fed at the bottom?
Are there other sizes of whip antennas?