Re:

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From: Jim Kortge, NU8N (jokortge@sun.lisp.com)
Date: Sun Jun 18 1995 - 09:37:17 EDT


Doug Faunt wrote...
>Could you tell me more about this antenna you use on your bike?
>73, doug
>
Sure can Doug. It is a modified ASA "hamstick" like antenna. By 'hamstick'
I mean helically wound fiberglass base with a detachable whip on top. The
original antenna was for 17 meters. I have added about 38 uH of inductance
in the form of a piece of B&W inductor stock at the top to also make it
resonant on 40 meters. If fact, there is a tap two turns up from the bottom
of that piece of coil stock so I can cover the whole 40 meter band. Taps
could also be added for 30 and 20 meters, but mine isn't since I don't
operate those bands from the bike.

I chose the ASA mobile whip over an actual Hamstick because the whip
attaches with a ferrule base which unscrews. Since the fiberglass base is
hollow, I can invert the whip when I don't want to operate, and store the
whip upside down in the base. Makes carrying the whole thing nicer. If I
remember, Transel Technologies sells a mobile antenna using the same sort of
design. A Hamstick would also work if storing the whip in the base isn't
important.

The mobile antenna is attached to the bike using an extension plate off the
rear rack aluminum carrier. The plate is 1/8 thick aluminum and about 2 1/2
X 8 inches. It extends past the rear edge of the rack by about 2 or so
inches. It flexes a bit, but that's probably good and helps to mechanically
decouple the antenna from the bike. When riding, I really can't fell it
back there. It swings around quite a bit during normal pedalling though.

One of the things I have observed while experimenting with hf bicycle mobile
antennas is that they usually have a feedpoint impedance of 50 ohms, so no
matching is required. I suspect it's because the bike itself is providing
too little of a counterpoise for the antenna, compared for instance with a
car. Anyway, this is the fourth antenna design I have run on the bike and
they all acted the same way! Makes matching really easy!! :-)

Hope all this isn't a waste of bandwidth.

72/73....Jim

 
Jim Kortge, NU8N | Bicycle Mobile Hams
jokortge@lisp.com | __o of America
Fenton, MI |_`\<, Mizuho 17m/40m QRP SSB
... .. .. . . . .. (*)/(*) . . . . . ..


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