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William McKeehan wrote:
<blockquote cite="mid:3944.192.168.1.1.1186005395.squirrel@mckeehan.homeip.net" type="cite">
<pre wrap="">My results with a setup on a motorcycle were less than expected with a 1/4
wave antenna even with 5 and 50 watt setups.
Changing the antenna to a 1/2 wave made a HUGE improvement for me.
</pre>
</blockquote>
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Because a 1/4-wave requires a HORIZONTAL metallic surface (i.e.
groundplane) underneath it to make up for the missing part of a
half-wave radiator. You can get the metallic mass of the bike's body
to counterpoise the 1/4-wave radiator for purposes of establishing
resonance and getting a low SWR, but the radiation angle is mostly up
in the air; i.e. you're doing a "moonshot" instead of putting out your
power close to the horizon where the digipeaters are. Great for
satellite operation but lousy for terrestrial operation. REMEMBER:
A low SWR doesn't prove you are radiating (or radiating in the right
place). Consider a dummy load.......<br>
<br>
A 1/4-wave on a car's roof (i.e. big chunk of HORIZONTAL sheet metal)
is somewhat better with the major radiation occurring at about 15-25
degrees above the horizon. <br>
<br>
A 1/2-wave radiator, whether end-fed like a J or center-fed like a
dipole, will radiate almost straight out toward the horizon WITHOUT
additional ground plane mass under it. <br>
<br>
The "secret weapon" for serious 2M DX radiation from a bicycle,
fiberglass body boat or car, or motorcycle is the Hygain Model 270 or
285. These 8-foot long fiberglass whips, unfortunately no longer in
production, are derived from a VHF marine antennna. They are a PAIR of
2-meter 5/8-wave radiators stacked and center-fed so they don't need a
ground plane. The base is a 3/8-20 screw stud, like an HF antenna or
CB whip, but is for mechanical support only - it is not an electrical
connection. The attached RG-58 feedline comes out of a grommeted hole
in the side of the mast about 1 inch above the mount stud. You mount
this thing on a bumper mount, split-ball, mirror clamp or monster
mag-mount as you would a 108" CB whip. [ Years ago, I ran one of
these mounted on the back of a Vespa moped, hooked to a Standard 826 2M
hand-held, while I was in college at Sault Ste Marie, MI for several
years with great results! I mounted a classic split-ball mount on the
lid of the cargo box behind the seat to support it.]<br>
<br>
They are great for portable fixed antennas also. You tie a nylon
string to the tip of the whip and just hoist it up into a tree,
2nd-floor balcony, curtain rod or whatever else is handy. Or tie-wrap
the first foot or so of one to a portable PVC pipe or fiberglass mast.
You don't have to worry about radials, ground planes, etc. I've used
them many times for ham club demos of APRS, packet and SSTV, and for
field day, etc. <br>
<br>
Or hoist one up into a tree HORIZONTALLY by tying strings to BOTH ends
to form a 3dBd gain broadside horizontal array for 2M SSB. <br>
<br>
<br>
The 270 was a stiff white fiberglass sleeve for it's entire 8-foot
length. The 285 was a black fiberglass sleeve for the first half of
it's length and a thin flexible stainless steel whip, similar to the
usual 5/8-wave whip for the other half. These antennas are well worth
looking for at swapmeets, if you need to run mobile installations on
vehicles without horizontal sheet metal. <br>
<br>
<br>
<br>
A much smaller alternative, currently available that works very well,
is the Diamond NR-770 which is a no-ground-plane 2M/70cm dual-bander
with an open (corkscrew) coil at it's center-point. This antenna is
about 38-40 inches tall (i.e. 2M half-wave). Available in both NMO and
UHF base versions in either chrome or matte black, it's perfect for
screwing into a UHF bulkhead ("barrel") connector mounted through an
L-bracket on no-ground-plane vehicles. It's also ideal with all-in-one
ammo-box trackers. Just place a barrel plug through a 5/8th-inch hole
in the top of the case and screw it in. It works VASTLY better than
simple 1/4-wave whips when the box is too small to be an effective
ground plane, with vastly less RFI into the GPS or Tinytrack. [ You
need a surface of at least 19" radius to be an effective ground plane
on 2M. Further, even when properly ground-planed, a quarter-wave will
NEVER have as low an angle of radiation as a half-wave. ] <br>
<br>
I have used a 770 taped to a fiberglass pushup mast as a temporary
base-station antenna. Just splice the male UHF connector in it's base
to a run of coax with a barrel plug and hoist it up. No radials or
ground plane needed. Or pull it up into a tree by tying a piece of
nylon string or 20-lb-test mono-filament fish line to it's tip. <br>
<br>
<br>
<br>
<br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
EchoLink Node: 14400 [Think bottom of the 2M band]<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.com">http://wa8lmf.com</a> --OR-- <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
NEW! World Digipeater Map<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/APRSmaps">http://wa8lmf.net/APRSmaps</a><br>
<br>
JavAPRS Filter Port 14580 Guide <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs/JAVaprsFilters.htm">http://wa8lmf.net/aprs/JAVaprsFilters.htm</a><br>
<br>
"APRS 101" Explanation of APRS Path Selection & Digipeating <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/DigiPaths">http://wa8lmf.net/DigiPaths</a> <br>
<br>
Updated "Rev H" APRS <a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs">http://wa8lmf.net/aprs</a><br>
Symbols Set for UI-View, <br>
UIpoint and APRSplus:<br>
<br>
<br>
<br>
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