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Author Topic: ground plane on a fiberglass canopy  (Read 4063 times)
MARKHWEBSTER
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Posts: 12




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« on: April 11, 2011, 12:48:23 PM »

Hi folks,
New ham here. My 2004Toyota Tacoma pickup (regular cab) has a 20 meter hamstick on the roof, mounted to a huge 3 magnet magmount. It works ok there, I've talked across the country.

Trouble is, that magmount is ugly. I'd prefer to mount the hamstick on the roof of my fiberglass canopy without the magmount. Tried it there, but the swr is too high, due to the missing groundplane, I assume.

I tried putting a 3 foot square piece of aluminum on my Yakima rack on the fiberglass canopy, and mounting the hamstick on the edge of that....no dice. High swr.

If I got a big sheet of tin roofing, say, the size of the canopy, and screwed that down to a big sheet of plywood, and mounted the ham stick in the middle of that, would that give me a decent groundplane, or does the bottom half of the dipole require the car body to be directly attached to the metal roof of the cab?

Last question, is an antenna analyzer worth the money? I like to make things, and the MFJ-259B is looking better all the time, despite the bad reviews:

http://www.eham.net/reviews/detail/13


Mark
KF7NYH
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AJ3O
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Posts: 124




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« Reply #1 on: April 11, 2011, 03:03:41 PM »

The aluminum may still work. Going with your assumption is the only way that I think that it would work. Solder braided grounding straps at the corners of the aluminum back to the body or frame if you are able. That should help it a lot. The plate on its own isn't going to do much as it looks as if it is just floating in space to the antenna.

But.... check with Alan or one of the other guru's here.
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MARKHWEBSTER
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Posts: 12




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« Reply #2 on: April 11, 2011, 04:00:05 PM »

It is strange how a car can substitute for the bottom half of a dipole.

I would consider drilling into the roof of the cab, but all that sheet metal flexes, and I can't see it supporting that big hamstick, or whatever I end up putting up there eventually, like a screwdriver, or homemade bug catcher.

Is there an illustrated "how to" site somewhere with pictures of drilling through a car or truck roof, and the gaskets and hardware that make the antenna mount water tight and strong? I suppose the dealer might be able to tell me where to drill, since they know the car roof, and where any inner ribs are located.

Be simpler if I could get the canopy to work somehow.
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KI4SDY
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Posts: 1452




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« Reply #3 on: April 11, 2011, 08:20:22 PM »

For 20 meters, your probably going to need all of the metal car body grounded to the antenna for decent performance in addition to whatever metal you add to the fiberglass canopy. That type of thing has never worked well for me and it always seems like more effort than it is worth. You might try a well grounded bumper mount and rod extender to get the antenna higher up for better performance. Smiley
« Last Edit: April 11, 2011, 08:26:33 PM by KI4SDY » Logged
K0BG
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Posts: 9879


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« Reply #4 on: April 12, 2011, 08:26:58 AM »

If you're going to stay with Hamsticks, just purchase a hood seam mount. Isn't the best, but it'll sure beat a mag mount.
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MARKHWEBSTER
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Posts: 12




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« Reply #5 on: April 12, 2011, 09:05:42 AM »

I do have the hood seam mt, I'll try that. There seems to be no way to avoid big ugly things on my car if I want to be a mobile ham.

I've only been a ham for a couple months, but I've had so much trouble with antennas, I'm leaning towards the MFJ antenna analyzer. Is it worth it?

I do enjoy building stuff from scratch...even when it doesn't work. Gets me off the computer.
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MAGNUM257
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Posts: 159




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« Reply #6 on: April 12, 2011, 01:43:24 PM »


If I got a big sheet of tin roofing, say, the size of the canopy, and screwed that down to a big sheet of plywood, and mounted the ham stick in the middle of that, would that give me a decent groundplane, or does the bottom half of the dipole require the car body to be directly attached to the metal roof of the cab?

Mark
KF7NYH

Wouldn't that be uglier than the huge 3-way mag mount?
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KE3WD
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Posts: 5689




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« Reply #7 on: April 12, 2011, 03:54:18 PM »

I have had success using contact adhesive and copper sheeting or flashing on the underside of fiberglass canopies such as this one. 

The entire underside of the top is covered with copper sheet, which is then soldered together at points along the seams such that it is electrically one sheet. 

Strips of flashing down the corners to where they can be electrically fastened to the steel of the truck body. 

Mounting the antenna on top of the fiberglass makes not much difference, the antenna will still "see" the ground plane of the Cu foil underneath. 

I then spray painted the copper with that dark gray flocking paint, for the sake of good looks. 



73
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M6GOM
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Posts: 945




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« Reply #8 on: April 14, 2011, 01:18:11 AM »


I would consider drilling into the roof of the cab, but all that sheet metal flexes, and I can't see it supporting that big hamstick, or whatever I end up putting up there eventually, like a screwdriver, or homemade bug catcher.


The metal is thicker than that on my Ford Mondeo. It supports a Little Tarheel II fully extended going at 75MPH into a headwind just fine.
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MARKHWEBSTER
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Posts: 12




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« Reply #9 on: April 14, 2011, 10:30:39 AM »

I discovered nmo mounts recently, that looks awesome for small antennas. I will devote a Saturday to pulling down the roof liner and mounting a couple of those.

While I am in there, I'd like to somehow attach a support for a heavier antenna. Something like an Atas 120 or large homebrewed bugcatcher/hamstick.

I can fabricate or buy a stainless steel bracket, but how do you drill mounting holes for bolting the bracket (antenna support) to the roof, or side of the cab, and keep them waterproof?

The nmo mount has that all figured out with the rubber gasket.

Also, if I get the bracket mounted to the roof, or side pillar of my Tacoma, how do I get the coax through the sheet metal, watertight?
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K3GM
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Posts: 1816




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« Reply #10 on: April 14, 2011, 11:13:40 AM »

Nobody's addressed you secondary question concerning the antenna analyzer.  My 259B is probably one ot the most used pieces of equipment in the shack.  Although it's capable of doing a number of tasks, I find it most useful for sweeping new antennas, to plot their curves.  The box make a formerly painstaking process a piece of cake.  I personally wouldn't be without one.  That's the upside.  Downside is that it eats batteries like crazy.  I buy those bricks of 36 at HD, and it keeps me in business for a while.  Yeah, it's a little rinky dink in construction, but nowhere near as bad as Autek which I previously owned.  The shame of it is, if MFJ put another few dollars of quality parts, like better pots into it, they'd have a nice product (for a change).  I keep stored mine in a little Pelican case, and it travels well.  This is one MFJ product that I wouldn't hesitate recommending; it's MFJ, so adjust your expectations accordingly.
« Last Edit: April 15, 2011, 05:14:01 AM by K3GM » Logged
MARKHWEBSTER
Member

Posts: 12




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« Reply #11 on: April 14, 2011, 12:54:36 PM »

Thanks for the info on the 259b. I'm very tempted by that. It seems like it would speed up my antenna building projects.
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W3LK
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Posts: 5639




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« Reply #12 on: April 14, 2011, 02:52:50 PM »

I discovered nmo mounts recently, that looks awesome for small antennas. I will devote a Saturday to pulling down the roof liner and mounting a couple of those.

Just be aware that not all NMO mounts follow the Motorola specifications. Anything not from Motorola, Larsen and one other company, who's name escapes me at the moment, are suspect. Do not buy any NMO mount from a Pacific Rim company, including Diamond and Comet. Not only do the threads not match, the center contact is often intermittent and the combination is often NOT watertight.
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
KE3WD
Member

Posts: 5689




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« Reply #13 on: April 14, 2011, 04:17:05 PM »

I discovered nmo mounts recently, that looks awesome for small antennas. I will devote a Saturday to pulling down the roof liner and mounting a couple of those.

While I am in there, I'd like to somehow attach a support for a heavier antenna. Something like an Atas 120 or large homebrewed bugcatcher/hamstick.

I can fabricate or buy a stainless steel bracket, but how do you drill mounting holes for bolting the bracket (antenna support) to the roof, or side of the cab, and keep them waterproof?

The nmo mount has that all figured out with the rubber gasket.

Also, if I get the bracket mounted to the roof, or side pillar of my Tacoma, how do I get the coax through the sheet metal, watertight?

Well, you don't have to drill the roof in order to bolster the ability to support an antenna. 

Consider using adhesive such as epoxy to mount an Aluminum or Steel plate on the underside of the roof, then after the adhesive has fully set, drill for the NMO and other antennas through both roof and plate.  The larger you can make the dimensions of that plate, the sturdier the mount would be. 

BTW old fashioned copper (not the newer plastics, of course) window screening can make a very good ground plane when placed under the fiberglass, might be another way to go if you don't like the copper flashing. 

73
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K3GM
Member

Posts: 1816




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« Reply #14 on: April 14, 2011, 04:45:45 PM »

Thanks for the info on the 259b. I'm very tempted by that. It seems like it would speed up my antenna building projects.
Just remember, if you buy fromMFJ you will pay suggested retail.  If you buy from one of their vendors, you will get it at a discounted price.
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