[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]
Home Page:          http://wa8lmf.com  --OR--   http://wa8lmf.net

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