[aprssig] Re: Ants for Bikes WAS: Broken digis (Ohio to Oklahoma and everywhere inbetween
Stephen H. Smith
wa8lmf2 at aol.com
Wed Aug 1 18:20:19 CDT 2007
William McKeehan wrote:
> 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.
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.......
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.
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.
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.]
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.
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.
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.
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. ]
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.
Stephen H. Smith wa8lmf (at) aol.com
EchoLink Node: 14400 [Think bottom of the 2M band]
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