On Sun, 4 Oct 2015 09:05:57 -0500, John S wrote:
How less efficient is a short vertical than a 1/4 vertical?
Are you talking about radiation efficiency or total efficiency? Are
you including the losses in the matching system (loading coil or
antenna tuna losses) needed to match a shortened antenna?
http://www.antennex.com/w4rnl/col0504/amod75.html
http://www.antenna-theory.com/basics/gain.php
I would like to see some numbers.
http://802.11junk.com/jeffl/antennas/Monopole/index.html
This is a study of what various monopoles, over a perfect ground, look
like in terms of gain, impedance, efficiency etc. I should probably
make a summary table, but I'm busy today.
Note that the shortest antenna (0.050 wavelengths) still has 100%
efficiency:
http://802.11junk.com/jeffl/antennas/Monopole/monopole_0_050/slides/monopole_0_050.html
That's because there are no dissipative components anywhere in the
antenna system. If you shove 100 watts of RF into this ideal antenna,
it will either radiate or reflect 100 watts, with no losses anywhere.
Well, an antenna with a 6594:1 VSWR isn't terribly useful, but if you
could find a suitable ideal matching network, it would work as well as
the ideal 1/4 wave monopole. Notice that I said "ideal" as in a loss
less matching network. That's not going to happen. The reason short
monopoles are a problem (such as an HF antenna on a vehicle) is that
the matching losses are ummm... lossy. As the antenna becomes longer,
the mismatch is less, the matching network less critical, and the
overall losses are less.
If you look again at the various results, you'll notice that the
shorter antennas have far more current going through them than the
longer antennas. If there are resistances in the elements (such as in
a loop antenna), the higher currents will result in higher losses for
shortened antennas. This may be a consideration for your less
efficient shortened vertical.
It is a ground plane with 4 radials
(typical). Free space.
Assume a source at the base. The type of source is your choice. EZNEC
defaults to one amp, but can be changed to a constant power of your choice.
I'm sure I've left out additional requirements, but maybe this will be
a healthy discussion even so. Suggestions are welcome.
Well, I do have a suggestion. Monopole antennas are very sensitive to
changes in the counterpoise, earth ground characteristics, number of
radials, elevation, etc. There is no univesal monopole model that
works for all frequencis, all conditions, and all applications.
Usefulness of the antenna also depends on the radiation angle and
ability to be matched to 50 ohms. I don't think a discussion that
bounces around all these parameters is going to be very useful.
Perhaps if you could provide a more specific set of conditions, we
might be able to analyze the situation in realistic terms, and
possibly provide recommendations and alternatives.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558