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?

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.

---------------------------


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

------------------------------------

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?

bbnn wrote:
Why not a 7/8 wave antenna?

OK for a horizontal antenna. Causes unwanted high-angle
radiation in a vertical.

Why does a 5/8 wave antenna need a loading coil and a 1/4 wave antenna
doesn't?

A 1/4WL vertical with a ground plane is resonant. A 5/8WL
vertical is not resonant and requires some sort of matching.

Is there such a thing as a 1/2 wave whip ant. fed at the bottom?

J-Pole is a good example. The bottom 1/4WL of a J-Pole is
the matching section.

Are there other sizes of whip antennas?

Any length of vertical whip up through 5/8WL will work well
if you can deliver the RF energy into the antenna. For
practical matching purposes, whips without internal loading
are usually limited to about 3/16 wavelength. I've used a
13 ft. whip, resonant on 17m, on 20m with an autotuner and
it worked well. On 40m, it was a good S-unit down from my
homemade bugcatcher.
--
73, Cecil http://www.qsl.net/w5dxp


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I need help understanding these different types of verticals.

What is the feedpoint Z of each of these?
================================

Feedpoint impedance is expressed in terms of R + jX. Both numbers are
functions of frequency or wavelength. For a simple wire vertical less than
3/8 waves, with sufficient ballpark accuracy -

R = Square( 24 * Height / Wavelength ) ohms,

X = - 550 / Tangent( Angle ) ohms, where -

Angle = 360 * Height / Wavelength ) degrees.

================================
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?
================================
Indeed, why not?

Any antenna WORKS quite satisfactorily REGARDLESS of how long it is. After
all it is only a length of wire. Why should it work better merely because
it is some special fraction of a wavelength?

================================
Why does a 5/8 wave antenna need a loading coil and a 1/4 wave antenna
doesn't?
================================

A 5/8ths antenna doesn't need a loading coil. It will work just as it is.
But a loading coil is useful to tune-out the reactance of the input
impedance and so provide a better match to a transmission line.

To assist in EXACT matching of a 1/4-wavelength antenna to a 50-ohm line a
loading coil plus capacitor is needed. But you can nearly always do without
either. With a 36-ohm line, if you can find one, there's a near-enough
perfect match. But then you have to match to 50-ohms at the transmitter end
so you get nowhere.

================================

Is there such a thing as a 1/2 wave whip ant. fed at the bottom?
================================

Yes. There is. Why not? It's quite a normal thing to do. The matching and
especially ground losses are exceptionally small.

================================
Are there other sizes of whip antennas?

================================

Yes, any size you like. As long as you can find room in your back yard. They
will ALL work quite efficiently. Unless, of course, they are extremely short
in terms of the free-space wavelength. Say less than 1/100th wavelengths.

Very short antennas result in losses in the matching or tuning arrangements
needed to match to the usual 50-ohm transmitter. Antennas are themselves
inherently very high efficiency devices.

For values of R + jX, and other information such as loading components,
download free programs from the following website. For clues, look under
each program name in the list for a brief one-line description of a
program's purpose.

Values of R + jX are usually of secondary importance. They are educational
in that they provide information about antenna behaviour. They are practical
only if it is the intention to design impedance matching networks or to
check the operating range of commercial tuners.

You could try program ENDFEED first. Just put the horizontal length of an
Inverted-L equal to zero and you are left with a vertical. But there are
several other programs dealing with verticals directly and which values of R
+ jX appear in the output data alongside values of L and C of the tuning
network.

Apart from the necessary nuts and bolts, you could dispense with half of the
ARRL Handbooks! All for free! ;o)

As for radiation patterns of verticals, from zero up to a height of about
0.6 wavelengths, for amateur practical purposes they are all the same.
Omni-directional. And in the vertical plane a couple of very broad adjacent
hemispheres with a vertical null.
----
Reg, G4FGQ
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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