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Author Topic: Installing a Screwdriver Antenna with a truck cap.  (Read 4396 times)
AB4D
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« on: May 13, 2011, 10:20:10 AM »

Hello All,

I am in a long term project of outfitting my pickup (Toyota Tundra) with amateur gear.  Right now, I am focusing on antennas/mount options.  I've already drilled the roof of the truck, installed an OEM Motorola NMO mount, a Japanese UHF mount, and a custom 3/8 mount.  I installed these different mounts, because one of the things I do enjoy is, experimenting with various antennas.

I am considering adding a fourth mount for the installation of a large screwdriver antenna, such as the Scorpion SA-680 or the GS-3 from GS Manufacturing.  However, as an obstacle, I also have a Truck Cap which I want to keep on the truck.

I've read a ton of information from various sources, antennas shoot out results, and looked at Alan's, K0BG's site.  From what I've read, using a trailer hitch or other low mount is less than optimum for most antennas. So I have a dilemma of where to mount an antenna of this size and still try to maximize performance.

I've thought about using an aluminum truck box in the bed, mounting the antenna to that, drilling a large hole in the cap and installing a pipe boot on the cap to allow the movable portion of the antenna and the whip to extend above the cap, while still keeping the weather out.  To me, that plan is on the extreme side, but something I would consider without better options.

I also noted on the GS site, that some have installed the base part of the screwdriver antenna on the underside of the cap with the whip mounted on the roof of the cap, this is achievable with the GS-3 because it has no external movable parts, but I am unsure of how it would affect performance.

Thoughts/Suggestions?

Thanks

73

Jim AB4D     
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KC7YRA
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« Reply #1 on: May 14, 2011, 06:04:20 AM »

This is a bit of a tough install.  And it is really going to depend on how extreme you want to go.

I took interest in this as I just (3 days ago) installed my screwdriver on my 09' Tundra.  I do not have a topper so it was a little easier.  I built a stake pocket mount, an external shunt coil, and found that an entire radio fits nicely in the center console.  And surprisingly, the truck is FAIRLY quiet on how much RF it produces.

With a topper, I see 3 options.  The first is the hitch mount.  I don't want minimal performance and it is not worth it to me.  So I would not use this option.

The 2nd would be to put a backing plate on the truck topper and mount the antenna on top.  This would be a potent performer and would be ugly as sin!!!  It may be a little tall and is kind of the extreme end of what I would do.  It would not damage the truck at all (a PLUS).

The 3rd is an option I recently saw on a tundra forum I am a member on.  I asked how other hams are doing their installs.  This feller sent a few pics.  He used a Breedlove mount on the side of the bed, and a bracket up on the topper.  It looks better than a hitch mount, but worse than a topper mount.  And it would require a hole in the truck itself.  Hopefully you can see the pic without being a member

http://www.tundrasolutions.com/gallery/files/5/0/1/1/7/img_0164_copy.jpg


As for your idea of a hole in the topper and letting your coil rest inside, I would not IMO.  I think you are really going to struggle keeping it weatherproof.  I also worry about shifting of the topper, which would damage the antenna.

Brad
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K0BG
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« Reply #2 on: May 14, 2011, 06:44:44 AM »

The Scorpion weighs about triple the GS (≈18 pounds), and requires a very substantial mount. If you go to the photo gallery on my web site, bring up the other installs album, and do a search for K7KKP. Note the aluminum plate under the base of the antenna. It has since been replaced with a thicker piece, to minimize sway. His e-mail in on QRZ in case you need to correspond with him.
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AB4D
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« Reply #3 on: May 14, 2011, 06:54:50 AM »

Brad,

Thanks for your insight.  I agree the trailer hitch mount is a no go, IMO too much trade off in performance.  However, I am not too sure about installing a ball mount from Breedlove on the side of the truck. Since you own an 09', mine is an 08', you're probably aware of how thin the sheet metal is on the side of the bed. Even with a support bracket on the cap, I would still be worried about sheet metal flexing/metal fatigue over time, eventually causing stress cracks in the sheet metal around the bolt holes.

My idea about the hole in the roof of the cap would ensure that the antenna is mounted high enough on the truck box so the coil is outside.  I am fairly certain I can weatherproof the installation with a boot and a stainless steel sealing plates, as there is no movement of the cap on the truck.   Thanks again for the information.

Alan,  thank you for the information about the antenna weight.  With this information in mind, it seems that my original idea would not only have to include a good bottom mount, but one on the top as well, probably as part of the boot assembly.  What is your opinion about the GS-3 mounted horizontally under the cap with the whip mounted outside?  For obvious reasons, it would be a performance trade off, but how much?

  
 73
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W5DXP
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« Reply #4 on: May 14, 2011, 10:09:20 AM »

Is the cap (camper shell?) made from fiberglass? If so, you don't need to worry about it interfering with RF. A friend of mine mounted a ball mount on the tail gate and didn't worry about his fiberglass camper shell.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AB4D
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« Reply #5 on: May 14, 2011, 11:46:34 AM »

Cecil,

Yes, the shell is fiberglass.
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W5DXP
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« Reply #6 on: May 15, 2011, 03:24:34 PM »

Yes, the shell is fiberglass.

In that case, except for any wires or metal supports, it is virtually invisible to RF. Looks like the tailgate is your major problem for installing the antenna in the trailer hitch. Here's how I solved the tailgate problem.Smiley

http://www.w5dxp.com/shootout.htm
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5LZ
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« Reply #7 on: May 24, 2011, 05:13:32 AM »

Nothing 'new' here, just a little emphasis on the mounting of that Skorpian antenna.  Think huge for it's mount!  I didn't to start with and found out the hard way that 'huge' is better than not.  There's a lot of momentum built up with that antenna when it moves when that truck moves.  It was enough to tear the mounting bolts of a tool box loose from the bed.  That box was mounted well enough when it was only the box.  Adding that Skorpian antenna, and it's "lever arm", definitely changed that 'well enough' thingy.
There are some nice mounting ideas on the Skorpian web site.  Don't think a bed-cap was mixed in them, but still some interesting ideas.
As a last resort, that 'hitch mount' is better than nothing, just involves a different set of problems.  That's what I'm looking at now (sold the truck, 'hitch mount' about the only practical way for the new car...rats!).
Good luck.
Paul
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W8NYY
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« Reply #8 on: May 25, 2011, 04:51:28 AM »

I fortunately drive the same truck as K0BG so guess what kind of mount I'm doing.  Exactlywhat K0BG designed.  Took about a square foot of inch and a quater Dupont Delrin which is interesting stuff to work with but I figure I can't go wrong going with a proven design.  BTW, I had my Ridgeline before Alan bought his.  Smiley
I take delivery today on the Black Widow version of the SA-680 and the mount is all fabricated except for the final sizing cuts.  Alan once again has been a fantastic help not only with the information he has on his website but in his personal answers to my questions.
By the way, looking at pictures of other SA-680 installs, how could anyone install the SA-680 without a properly wound choke on the control line?  I see so many that just put three wraps in a mix 31 choke and call it done.  Having three years experience now of running high power mobile with a Tarheel 100 A/HP I can attest to what Alan has been preaching and few listen to and that is the control line is ripe for RF egress into the vehicle.  If he says get 18 awg stranded wire and wrap it 12-13 times without overlapping the wraps, then that's what needs to be done.  He knows his stuff.  I've got a choke on each end of the control line just to be sure.  The wire is cheap as are the chokes (5 with .75 id from DX Engineering for 29.95).  Either pay me now or pay me later with RF issues.
Good luck and 73.
W8NYY
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W7WX
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« Reply #9 on: May 25, 2011, 11:32:11 PM »

I use the Scorpion SA680S just behind the cab of my pick up and a steel mount on the bed rail which is sandwiched between the fiberglass canopy and bolted inside the bed.  The bracket didn't flex a bit but the bed rail did just a tad.  I fabricated a simple angled mild steel secondary support bracket that bolts though my fiberglass canopy.  Being the canopy moves with the bed anyway it's been quite stable.  I just insulated the mount from the antenna and used a large hose clamp to secure it to the bracket.  The bracket doesn't have to be big, just not flex.  My first attempt at a secondary support bracket I fabbed up a heavy piece of HDPE which worked fine but I didn't like how it looked and don't think it was as strong as what I have now.  Both work fine though.  The steel bracket turned out allot cleaner looking and didn't affect performance whatsoever that I can tell.

Best of luck.
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KG6ORW
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« Reply #10 on: June 08, 2011, 01:48:11 AM »

Dear AB4B:
Check out the "Hi-q" website for large HF mounted antennas.  They show very large antennas mounted in many different locations on trucks and cars alike.  Good luck.
KG6ORW
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