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Author Topic: bicycle mobile help!  (Read 2133 times)
M3MFZ
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Posts: 12




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« on: March 02, 2010, 12:11:57 PM »

Hi
Im trying to put a set up on my bike. My idea, and what ive done so far is mount a metal plate on my seat rails. Ive got a snail mount attached near the back and a 1/4 wave of 2m goes ontop of that.
Coax then runs to my bars where my handheld will sit.
Ive done all this, but i just cant get the SWR down. For the 4watts that i put out, the SWR is between 2.1 and 3 on the meter.
Ive tried a different mount that goes on the car. This brings it down more. It sits at 1.5 on the SWR meter.

How do i need to bring the SWR down? Ive adjusted the antenna. The coax is 3*1/4 wave on my mount, a set length on the one for the car(was built with the mount) but no mater what i do this SWR isnt coming down.
Can anyone offer any advice?
I do not want to do a big J pole and put that on the bike so that idea is out the window.
My bike is Alu frame with Carbon forks.
Any suggestions? Its driving me nuts!!!!!!!
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WX7G
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« Reply #1 on: March 02, 2010, 12:41:48 PM »

I don't think a handheld cares that the VSWR is 3:1. The match into a rubber duck can be much worse. I'd use it as is and have fun.
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M3MFZ
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Posts: 12




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« Reply #2 on: March 02, 2010, 12:44:11 PM »

It does make a big difference to it. I cant spark up a repeater with it with the quater wave on the bike, but if i put the rubber duck on i can open it up.
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KC7YRA
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Posts: 256




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« Reply #3 on: March 02, 2010, 08:03:15 PM »

I think you need more groundplane.  1/4 wave antennas rely heavily on having a decent groundplane.

Try to increase the metal that the antenna is mounted to.  If you cannot, try mounting a 1/2 wave antenna as it will not need a groudplane.  

Many companies make them.  I know Maxrad's part number off the top of my head (MHB5802).  But I would personally prefer Larsen's version as I think they make the toughest mobile antennas around.

Good luck,
Brad
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WX7G
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« Reply #4 on: March 03, 2010, 05:36:10 AM »

M3MFZ, so you're saying that when you put the rubber duck on the bike (same mount?) in place of the 1/4 wavelength whip it works much better? If so the GND plane (bike) is good and the whip is defective. Wrong length perhaps? It should be 0.49 meters.
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K0BG
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« Reply #5 on: March 03, 2010, 06:35:39 AM »

Dave is correct when he says the handheld doesn't really care what the SWR is. Rubber duckies are not resonant, and in most cases you can't even call them an approximation. Further, inexpensive VHF wattmeters and SWR bridges are notoriously inaccurate, especially when we're dealing with and antenna which presents a load with common mode as the main radiation means.

Probably the best place to mount an antenna on a bicycle is on the real luggage carrier if it has one. In most cases, and good old 1/4 is more than adequate, especially if the handheld doesn't have a build in receive attenuator for use with gain antennas.

If you really want a challenge, try HF bicycle mobile. It is strictly QRPp!
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M3MFZ
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Posts: 12




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« Reply #6 on: March 03, 2010, 09:13:29 AM »

Hello,
Right, im now moving the mount to the rack mounts(sorry, no mount going on this bike, not yet anyway!) Just got to do a few more cuts and abit of finishing off and i can give it a go here.
Will any luggage rack thats made of aluminium be ok? Take it it will be.
Seen this one http://www.evanscycles.com/products/tortec/tour-rack-ec005641 and this one both made from aluminium.

Cheers

Joe
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K9WI
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Posts: 27




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« Reply #7 on: March 03, 2010, 10:40:20 AM »

URLS to check

http://75.60.24.163/BMHA/
http://www.arrl.org/tis/info/pdf/9303052.pdf
http://www.gobahams.org/how_to_wire_a_bike.pdf
http://www.lafetra.com/bmha/Newsletter/images/V14N01p05.jpg
http://www.stealthtdi.com/RidingTheAirwaves.html

and many more just a google away.
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WB6BYU
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Posts: 13288




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« Reply #8 on: March 03, 2010, 06:42:54 PM »

What sort of a mount are you using?  Something that clamps to the aluminium
should be fine, but of course a magnetic mount won't work.  Do make sure that
all the connections to the frame are free from paint or other insulation.  Actually
a bicycle frame isn't all that good of a ground plane on 2m because it tends to
be more vertical than horizontal, and it isn't solid, allowing various resonant
currents to flow around the frame.

There are some problems using a 1/4 wave whip on the back rack of a bicycle,
especially if it is too close to the rider.  The seat post (and everything on it)
rises up above the ground plane and has more capacitance to the whip, besides
blocking signals to the front.  I presume you are adjusting the whip length and
checking the SWR with the rider in place and in the normal riding position?

Actually, if you can hit the repeater with the HT but not with the mounted whip
(presuming they are at the same level) then it sounds like there is more to your
problem than just SWR.  I'd check the coax and mount for shorts and opens.
One problem I've had with bicycle mobile is that the rubber duck vibrates and
wears down the center pin of the BNC connector, eventually becoming intermittent
while moving but reliable when stopped.  (Yes, isn't that fun to debug!)

Actually, I HAVE operated bicycle mobile with a J-pole built on the side of a
full-length CB whip (2.75m) mounted to my rear hub.  It worked great, though
it made it difficult to swing my leg over when getting on.  You may find that
a half or 5/8 wave whip gets the radiating portion up higher, thought they
also tend to whip around more.  I've made some good 2m half wave antennas
using the 1m tall base-loaded CB whips as a starting point.

Good luck!
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M3MFZ
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Posts: 12




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« Reply #9 on: March 04, 2010, 10:14:26 AM »

How did you turn the CB antenna into a 2m whip? Ive got an old mobile CB antenna thats not being used. Did you just put it on and SWR it or what? Id love to know more, it would be brilliant if i could turn the CB into a 2m whip for the bike.

Im putting a rack on when i get it to see how it goes. Im also going to put some metal plate on the top of the rack. Ill see how that goes.

Thanks very much for the help.

73
Joe
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M3MFZ
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Posts: 12




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« Reply #10 on: March 04, 2010, 10:18:06 AM »

Oh, the mount was a metal plate cable tied to the seat rails.
Is there a minimum amount of horizontal plate that i would need to get it to work, or is it just trial and error?
One other idea i had was to put metal plate ontop of the rails on the rack, then another bit underneath. Depending on which rack i go for i could either have 2 or 3 plates of metal. But, the problem is they are quite close together, which i guess wouldnt be so good?

Cheers
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KE3WD
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Posts: 5689




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« Reply #11 on: March 04, 2010, 04:41:35 PM »

The metal frame of the bike should make for plenty good enough ground plane.  

Sounds like that small metal plate is not strapped to the frame with a low resistance connection.  Try a piece of copper braid from plate to one of the bolts on the bike frame, as short as you can make it and still get the job done with a bit of leeway for flexing, etc.  



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WX7G
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Posts: 6080




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« Reply #12 on: March 04, 2010, 07:28:37 PM »

The quarter wavelength whip will work best when worked against a counterpoise that is 1/4 wavelength from antenna to end. This is 0.49 meters. You might attach a single 0.49 meter radial at the antenna base. This is in parallel with the existing rack counterpoise. If the radial improves the match it tells you that the counterpoise you have is too large or two small.
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M3MFZ
Member

Posts: 12




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« Reply #13 on: March 05, 2010, 11:13:23 AM »

WX7G, you have confused me abit with counterpoise. I guess you mean to attach a metal rod to the earth side angled down to see what the SWR would be like?
Ive just put a rack on today. Got to get some more metal plate to go on the top. Its going to go across the top of the rack with a small bit off the back for the antenna mount.
Hopefully this will work. Ive got 4 contacts that go from rack to the mounts.
Ill also put some thick copper wire on from one mount to the bike in the hope that makes a sure contact.

Cheers
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WB6BYU
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Posts: 13288




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« Reply #14 on: March 05, 2010, 12:52:45 PM »

Quote from: M3MFZ
How did you turn the CB antenna into a 2m whip? Ive got an old mobile CB antenna thats not being used. Did you just put it on and SWR it or what? Id love to know more, it would be brilliant if i could turn the CB into a 2m whip for the bike.

Many of the base-loaded CB antennas are about 1 metre long (often a 36" radiator
on top of a housing with a base loading coil inside.)  This makes an excellent half
wave antenna for 2m.  Conversion depends on the exact details of the antenna
base - it works best to have some extra room to install components.

The loading coil is often 20 to 30 turns of wire between the antenna and ground,
with the coax feed tapped up perhaps 5 turns of the coil above ground.  I removed
the coil from the housing and rewound it to have only 4 or 5 turns total, with the
coax tapped about 1/2 or 3/4 turn up from ground.  Then connect a trimmer
capacitor across the coil and tune it for minimum SWR.  If it isn't low enough,
move the coax tap by a fraction of a turn and try again.  If there isn't room inside
the housing at the antenna base to mount the variable capacitor, you may need
to use a different housing with the whip portion bolted to it.  The exact details
are highly dependent on the specific antenna you have to start with, but if the
whip is a bit shorter than 1 metre you should be able to use it as a half wave
antenna with a suitable matching network at the base.
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