On Fri, 07 May 2010 16:31:48 +0000, James186282
wrote:
The
first part that puzzles me is the "ground" side of an antenna. Or if I
can use a Dipole as an example one side of the dipole. Or if we use a
groundplane the radials. With an HT are we just "doing without"?
The case of the HT and your hand are acting as the counterpoise (other
half of the antenna). There have been some efforts to improve on the
HT antenna by adding a wire counterpoise:
http://www.hamuniverse.com/htantennamod.html
It goes under the trade name of "Tiger Tail". There are some boring
You Tube videos describing the construction.
My second question is in asking how effective is the typical rubber duck
antenna versus say a truly resonant antenna or even a 5/8th wave which
tries (if I understand it) to squish the pattern down from a sphere to a
doughnut so its hears and talks better out then "up and down"
If you start with a monopole, shortening the antenna in half with a
loading coil will result in half the gain or -3dB loss. 1/4th the
length is 1/4th the gain or -6dB loss. Etc. Shortening the antenna
doesn't change the pattern much. You still end up with roughly a
torus (donut) pattern. The torus is far from perfect and probably
resembles more of a sphere. Therefore, you don't have to worry much
about the loss of signal in the axial directions.
A 5/8 wave antenna is the longest length (and therefore highest gain)
antenna that can be built before the antenna pattern starts to
resemble a cloverleaf (with deep nulls). The gain is about 1.5dB more
than a dipole. That's a fabulous 19% increase, which is kinda
marginal considering the added length.
All this assumes that some effort has been made to match the proposed
antenna to 50 ohms.
In my mind I would simply like an antenna that was effective but
flexable.
Flexible as in like rubber? Or flexible as in versatile?
Length is not "critical" as I plan to hang it up high on an
LB harness. The idea is a rough and tumble antenna that can work in the
woods on a Search and Rescue mission but will give me the very best
range out (not so much up/dn)
Large antennas in the brush is a bad idea. It will tangle with the
brush and the brush always wins. Your requirements are probably the
same as any emergency responder. It has to be unobtrusive (not get in
the way) but still effective. Smashing through the brush with a 5/8
wave 2m antenna is not my idea of unobtrusive.
My suggestion is to have two antennas. One is the traditional 6"
rubber ducky antenna that comes with most HT's. It's not great, but
it's small enough not to get in the way. In your back pack, carry a
length of RG-58/u with the last 19" of braid pulled back over the
outer jacket. This forms a "coaxial antenna", where the exposed 19"
of center conductor is the driven element, and the folded back braid
is the counterpoise. The impedance is officially 70 ohms, but the
mismatch loss to 50 ohms is minimal. The trick is that this antenna
is attached with a long length of coax cable, that can be thrown into
the trees, or hoisted on a pole, to get added elevation.
If you have a harness, it might be possible to sew an antenna into the
webbing. While not the best for gain and non-directionality, it is
very convenient not to have an antenna sticking you in the armpit.
Has anyone done any serious work with HT antennas?
Ummm... the cell phone and public safety radio sector has done
considerable work on optimizing the antenna. The cell phone horde has
decided that non-projecting antennas are more important than
convenience. They've also decided that it's acceptable to place the
antenna at the BASE of the cell phone (as in the Motorola RAZR).
Convenience is more important than performance. The public safety and
land mobile sector has correctly decided to solve the problem at the
other end of the link. They're leaving the antennas at about 3-6" in
length, and concentrating on improving performance by adding sites,
better base antennas, voting systems, etc. Why mess with 1,000 HT's
when you can get better results by improving one mountaintop site?
There are also external speaker microphones for HT's that have
antennas in the microphone (thus insuring that your brain gets the
maximum RF exposure).
http://www.dhgate.com/kmc-26-microphone-speaker-mic-for-kenwood/p-ff808081220be3b101220d87650c1504.html
Is there anyone
clued into how to match these properly?
Sure. The 5/8 wave uses a transformer or tapped inductor to step up
the 50 ohms to about 1000 ohms. Shorter than 1/4 wave antennas use a
loading coil (series inductor). 70 ohm coax antennas and 35 ohm
monopoles are close enough to 50 ohms for VSWR not to be an issue. The
added bulk and RF losses of the matching system, has to be balanced
against the antenna mismatch loss. It's often better to tolerate the
mismatch loss.
And has anyone looking into
using spring steel (Such as Measuring tape) for the antenna as the
Military uses in many of their radios?
http://theleggios.net/wb2hol/projects/rdf/tape_bm.htm
http://www.open-circuit.co.uk/tape.php
The problem is that spring steel (as found on a tape measure) is not
exactly a good RF conductor. The military HF dipole antennas I played
with were stainless steel, which has better surface conductivity.
http://www.qsl.net/n2ckh/HA4000.htm
The problem with tape measure antennas at VHF and UHF is accuracy. The
cut length of the antenna needs to be fairly accurate to work at UHF.
For example, the difference in length of a UHF 1/4 wave radiator is:
440 MHz = 170.5 mm
450 MHz = 166.7 mm
To get it somewhere in the band, you need to adjust the tape measure
to within plus or minus 1.9mm. That's not easy (or practical).
There's also no antenna tuner to compensate for errors. That's why
field adjustable antennas are not very popular at UHF frequencies.
Instead of floundering around with generalities, could you kindly
disclose what problem you're trying to solve and what equipment you
have to work with? Perhaps the answers will be more specific.
Thanks for any thoughts on this de Jay W0VNE
Hint: Making a better HT antenna is easy. Making one that is better
but also smaller, more efficient, and ergonomic, is not so easy.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558