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Author Topic: Temporary platform for contesting in truck bed  (Read 1360 times)
KB1ITX
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Posts: 4




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« on: April 11, 2005, 08:41:27 PM »

I have a brand new Dodge Ram 2500 pickup on order, and a contest (roving) to particpate in 2 weeks after I take delivery.  What I would like to do is create some sort of platform that sits in the stake pockets in the bed, and can be easily removed when its not needed.  The main reason for this is to have something to attach mag mount antennas to without scratching the paint on the roof.  The other use for the platform would be to mount a 3'x5' dish on a mast with a rotor.

The idea I had was to use a 4'x4' sheet of 3/4" plywood on four "legs" that fit into the stake pockets.  The legs would be bolted in to the pockets so the platform won't fly off while driving.  I was going to cover the top of the plywood with some sort of metal sheeting (suggestions??) for a ground plane for the mag mounts.  Finally, I need to figure out some way to mount a rotor, so that when I stop I can easily drop the mast in, and attach the dish.

Wire management is not an issue, I'll just stick them through the rear window (this is only temporary).

Finally, I'm guessing that it would be optimal to have this platform at the level of the cab roof, but I was worried that this might be too "jittery", even with considerable crossbracing of the legs.

Any suggestions, or "this'll never work" comments will be happily accepted.  Thanks.

-Steve
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WA4PTZ
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Posts: 527




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« Reply #1 on: April 12, 2005, 05:27:24 AM »

Sounds like a great idea. I'm not sure that any
metal sheeting would be necessary as long as ground
lugs/strips are provided. I believe an earth ground
will be much better than a simple ground plane.
A little advance scouting can get you a good location
for a ground rod or ground source. Lakes,streams and
ponds can be very good grounds.
The most important factor will be the weight,
especially if you have to manage this platform by
yourself.
Good Luck and 73,
Tim
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K0BG
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« Reply #2 on: April 12, 2005, 09:55:33 AM »

A ground rod, placed even in salt water, is NOT a groundplane. It might suffice as a DC ground, but little else. In essence, a vertical is one half of a dipole, and the groundplane acts as the missing half. Without it you have half an antenna.

Using plywood perhaps is the least expensive platform material, but you will need to provide some sort of metallic surface to act as a minimal groundplane for VHF/UHF. It is immaterial whether this metallic surface is DC grounded or not.

One of the guys you should pose this question to is Steve Katz, WB2WIK/6. He has more rover experience than anyone I know. Perhaps he will see this post and respond.

Alan, KØBG
www.k0bg.com
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KB1ITX
Member

Posts: 4




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« Reply #3 on: April 12, 2005, 10:57:14 AM »

Tim and Alan, thanks for your quick responses.  I agree with Tim, that I should probably think about grounding the entire assembly.
Alan, is there any recommended size that you can think of for the metal sheeting?  I've heard that 28"x28" is a good start for VHF/UHF.  Obviously, I'd only be transmitting from one antenna at a time.

Another thought since last night...Would it be more benefical to have a separate "sheet" for each antenna, or put them all on the same sheet (like a normal roof would be)?

Thanks,
Steve
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AA4PB
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Posts: 15065




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« Reply #4 on: April 12, 2005, 01:22:51 PM »

A ground plane is a must, especially for VHF/UHF. An earth gound will add nothing. The wire between your antenna base and the earth would probably be several wavelengths long on 2M and higher so it wouldn't provide an RF ground at all.

The best size for the ground plane is 1/2 wavelength or more across on the lowest frequency you will be operating. Since it needs to be magnetic for the mag mount antennas, I'd suggest a thin piece of steel bolted and glued to the plywood. You might also want to consider marine grade plywood to avoid moisture problems unless you don't plan on using it very often.

You could also use aluminum and then bold pieces of steel to it (make a good electrical connections) just large enough to hold your mag mounts.
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Bob  AA4PB
Garrisonville, VA
K0BG
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Posts: 10248


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« Reply #5 on: April 13, 2005, 06:20:32 AM »

If I were doing this, I'd go to my local Ace Hardware and pick up some DIY sheet steel. They sell a 4 foot square chunk for about $10 and it appears to be 16 ga. That should be adequate for 2 meters and up. A little Rustolium paint to keep it from rusting is also in order.

Bob's suggestion about marine plywood is a good one too. Be prepared, however, as it isn't cheap. a 3/4"x4'x8' is about $40.

Alan, KØBG
www.k0bg.com
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KB1ITX
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« Reply #6 on: April 13, 2005, 08:03:16 AM »

I stopped at the local Home Depot myself last night to do a little pricing.  I couldn't believe how much the cost of plywood has gone up in the past year or so.  I did find some steel plates that I could use, and also long rolls of aluminum flashing (obviously not to put the magnets directly on).  Overall, it looks like I should be able to accomplish this entire project around $150, which doesn't seem to bad to me.  Thanks for all the input.  I'm still looking around, and probably won't get to building the thing until late May/early June.

Steve
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KL7IPV
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Posts: 984




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« Reply #7 on: April 19, 2005, 10:24:36 PM »

For $150 you can buy a metal rail mounted tool box to put on then take off when you done. Mount it in the middle of the bed instead of near the rear window. It will give you a surface for the dish and the mag mounts. Good luck.
Frank
KL7IPV
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VE6BSK
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Posts: 1




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« Reply #8 on: April 27, 2005, 08:20:32 PM »

It has been so many years since I've been active that I've forgotten a lot of obvious stuff.

I have a new Dodge Dakota and want to see if my old Icom 2meter rig will still work. I want to put my old mag mount on the roof without scratching the new paint.

Do I need a metal-to-metal contact on the roof? Or can I put a thin piece of rubber or plastic to protect the paint job underneath the mag mount?  Or will that insulation ruin the ground plane effect?

Thanks.

John (VE6BSK)
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KB1ITX
Member

Posts: 4




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« Reply #9 on: May 04, 2005, 11:33:28 AM »

I've got another quick, although slightly unrelated question.  Are the stake pockets on full size pickups (i.e. NOT Dakotas, Frontiers) set a standard distance apart, both across the bed, and along the bed?  In other words, if I build this platform for this truck, what is the chance I will be able to drop it in another truck?
Thanks again.
Steve
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