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[aprssig] Re: Ants for Bikes WAS: Broken digis (Ohio to Oklahoma and everywhere inbetween

w2pi at optonline.net w2pi at optonline.net
Thu Aug 2 00:01:42 UTC 2007


I have had the same experience when bicycle mobile - 1/2 wave works much better than 1/4 wave.
 
Paul, W2PI

----- Original Message -----
From: "Stephen H. Smith" 
Date: Wednesday, August 1, 2007 7:21 pm
Subject: [aprssig] Re: Ants for Bikes WAS: Broken digis (Ohio to Oklahoma and everywhere inbetween
To: TAPR APRS Mailing List 

> 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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> 
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> 
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> 
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> 
> 
> 
> 
> 
> 




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