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Author Topic: HF mobile antenna mounting ideas  (Read 709 times)
N4HNO
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Posts: 10




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« on: September 18, 2009, 05:49:03 AM »

I have read that it's the amount of metal that's under the antenna and not beside it that matters in HF installations.  However, I have been looking at the Scorpion line of screwdriver antennas.  With 18 lbs or so and about 5'5" of height before the whip, I don't see many ways this can mount on a full-size pickup except a plate and tubular steel bar mounted to the frame and extending out below the rear fender.

I have read that shorter, stubbier screwdriver antennas are not as good a performer.  I wonder if there is some compromise.  If I were to get the shorter version and mount it on a platform just above the camper shell with a flat steel piece extending down to the bed to a mounting foot would this be almost as good?  I am probably going to run into excessive height issues with the whip with this mounting.  Any suggestions?
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K0BG
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« Reply #1 on: September 18, 2009, 11:39:54 AM »

Mount it in the bed. As long as you don't place the antenna right against the sides or at the rear of the cab, the amount of capacitive coupling will be low, and nearly immeasurable.

Low mounting (trailer hitch for example) does increase ground losses enough that you can measure them with an antenna analyzer. On 20 meters, the difference is usually 3 to 4 ohms, depending on how you go about is. Considering the calculated ground loss on 20 meters is about 4 ohms, it's easy to calculate the difference in efficiency.

Alan, KØBG
www.k0bg.com
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N4HNO
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« Reply #2 on: September 18, 2009, 12:32:17 PM »

When you say mount it in the bed are you referring to the method I described with a flat steel bar going up above the shell (between cab and shell) to a platform or mount to the inside of the bed?

I would like to keep the camper shell on the truck if possible.
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K0BG
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« Reply #3 on: September 19, 2009, 04:25:12 AM »

The real key is keeping the largest mass directly under the antenna. Ron has a photo he could send you, where in the owner mounted the antenna atop a post using a frame extension bracket. The antenna is mounted on a flat piece of diamond plate, affixed atop the post. This plate is then bolted to the bed rail under the bed cap (camper top). Perhaps not ideal, but it's a lot better than just the post sticking up.

When you mount antennas low on a vehicle, a goodly amount of the return current has to flow through the lossy surface under the vehicle. This fact increases ground losses. After all, the body is a much better conductor than the surface.

Heavy antennas like the Scorpion and HiQ, do require rather stout mounting. The advice is, don't scrimp on the materials used in mounting, and securely bolt things into place.

Alan, KØBG
www.k0bg.com
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K3GM
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Posts: 1776




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« Reply #4 on: September 19, 2009, 08:10:50 PM »

For my mobile HF installation, I initially persued a body mount on my 2007 Tahoe.  But I found there was an inner and outer skin on the rear quarter panel, and I didn't want to cut an access hatch in the inner skin, so I had to abandon that idea.  I was forced to use a hitch mount.  I properly bonded the antenna to the body and the chassis with wide copper foil, and bonded all the doors, hatch, hood, and exhaust. If my setup is a compromise, than an installation using an "ideal" mounting scheme must be one heck of a performer.
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K0BG
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« Reply #5 on: September 20, 2009, 06:55:56 AM »

Tom, the truth is, you can do a creditable job even with a setup with efficiencies no better than 5%, perhaps less as any Hamstick or ATAS operator can tell you. It is when the band appears dead, that a higher efficiency antenna system shows its stuff.

It is important to remember, even 1 dB is enough sometimes to make the difference between making the contact, and not.

Alan, KØBG
www.k0bg.com
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VE3XKD
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« Reply #6 on: September 22, 2009, 08:47:53 AM »

I have successfully mounted the HiQ 3.5 'short' to the roof of my Toyota Prius using the Breedlove mount and by adding in additional Aluminum backing plates. I found I was able to slide and wedge the additional backing plates between the roof sheet metal and the supporting ribs. This works well, and I can 'shake the car' if I grab a hold of the mount. One caution, some roofs are  contoured so the backing plates should have a slight curve, and the corners should be ground down to provide less of a point so that one can avoid 'digging in'to the roof sheet metal when wedging the backing plates into location.

The roof mount works well for me, although the whip is a bit shorter due to the antenna being further from the ground, I made up for some of the length reduction by using a capacity hat on the Hi-Q. The strength of the modified mount  more than compensates for the added windload of the capacity hat.
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K3GM
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« Reply #7 on: September 23, 2009, 04:50:38 AM »

.......I'm trying to picture my HS-1800 with 5' whip on the roof of my Tahoe! 8-)
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BASILIEN8NQU
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« Reply #8 on: September 27, 2009, 09:53:25 AM »

I am running the shorty II from Larry's antenna,www.kj7u.com.I mounted it on magnet mounts in the middle of the roof of my Geo Metro,the screwdriver antenna extends to 28 inches and I have a 84 inch whip above that.It works fantastic.Look my call up on qrz and you will see a picture of my work car.73.......Bill N8NQU
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