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Author Topic: PU Bed Counterpoise  (Read 421 times)
KF7IS
Member

Posts: 18




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« on: December 18, 2002, 08:58:48 PM »

I have a screwdriver antenna and 706.  As might be expected, the swr on 40 and 80 is greater than the higher bands, not bad, just a bit higher than 1.4.:1.
So anyway, I was playing around with a 40 foot length of insulated wire and discovered it did not help matters when alligator clipped to the screwdriver whip.  So, in my quest to kill more time, I attached the 40 foot length of insulated wire to the bumper of the truck...... The best swr reading was even lower.  An idea began to germinate, ,e.g., what if I took a 6 conductor rotor cable cut to qtr bandlenths attached to the screwdriver base and coiled it about the pickup bed interior?  What if I ran said counterpoise under the bed?  So many questions, so little time.  
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WA4PTZ
Member

Posts: 528




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« Reply #1 on: December 19, 2002, 05:36:22 AM »

Good idea. You could also experiment with rf
chokes and transformers to see if these help. I
assume that you have the bed strapped to the
chassis and the cab. It sounds like a Chevy
S-10. They frequently have this problem.
73 - Tim
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WA4PTZ
Member

Posts: 528




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« Reply #2 on: December 19, 2002, 05:39:42 AM »

By the way.....anything less than 2.0 to one on a
mobile is considered fine. We don't live in a 1:1
world.
73 - Tim
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K0BG
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Posts: 9845


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« Reply #3 on: December 19, 2002, 08:39:07 AM »

One has to remember the vehicle acts as a capacitor to ground, and not as a groundplane (at least below 12 meters).

Adding a length of wire to the screwdriver whip will effectively lengthen the antenna requiring retuning, but if you are stationary, it very well might help some.

Adding a single radial may add some efficiency, but not much. It takes a great deal of such radials before any real change is noticed.

As for VSWR being "low", if it is, it's time to buy another antenna. Mobile antennas are short, highly capacitive, and require coils to cancel this reactance out. Depending upon the Q of the loading coil, and other factors, you see an VSWR of 2 or more to 1. If you have a very low one, someting is wrong.

You really can not set the resonant point of a mobile (or any) antenna with a VSWR bridge. Antennas have reactance so the voltage and current are not in phase, so setting it for the lowest VSWR is not the resonant point. Yes, your radio likes it, but your efficiency suffers.

If you want a better signal from your mobile, I suggest you search this forum for grounding and bonding information, an d other important mobile "do's".

Alan, KØBG
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KF7IS
Member

Posts: 18




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« Reply #4 on: December 19, 2002, 05:22:14 PM »

Thanks Alan and Tim, I am really quite happy with the existing setup.  Just thought I would play around with the counterpoise idea to see if the radio could be made happier on 40 and 80.  Specifically, whether or not the counterpoise could or could not be shieled frome the antenna when run under the bed as opposed to in the bed.  I do have additional grounding between the bed and the frame, as well as the cab and the frame.  And for what its worth, The tailgate was floater before grounding.  I thought the purpose of the counterpoise in this case would be to increase the RF ground.  Noise has not been a problem (Dodge diesel)that I have had to deal with in this vehicle.  I do have a shunt coil between the antenna feed point and immediate ground which has proved to raise the feedpoint impedance to something that can be worked with.  Also thought I would try the MFJ "mobile matcher" to see what it might do.  Anyhow, I appreciate the input and will post results about the counterpoise situation after Christmas holidays (providing the snow situation cooperates).  Happy Holidays to all and hope the jolly fellow brings lots of new toys.
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KF7IS
Member

Posts: 18




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« Reply #5 on: January 05, 2003, 07:47:26 PM »

Back from the holidays and found I did not have to play around with the counterpoise idea.  Had no problem checking-in to 40 mtr 1830 utc net on daily basis from Tucson to Salt lake City  (800 miles).  That was my main goal.  The screwdriver and 706 performed.  What else needs to be said.
Alan, very good article.
Jim
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