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Author Topic: Got my Tarheel  (Read 1188 times)
KD0AFK
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Posts: 245




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« on: June 27, 2007, 11:09:53 AM »

Mounted it on my mirror. Plan on moving it somewhere else when I figure out where. In planning on a place for it, it was a toss up between directionality and a good ground plane. I have a good ground to the frame and a large steel plate under the antenna and it is out in the open and not hid behind the sleeper. The only problem is that I am getting pretty high impedance. I have seen other tarheels that have a coil between the antenna and the bracket. It came with a ferrite bead for the controller cable. Would adding the coil bring it more into balance?  
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AA4PB
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Posts: 12832




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« Reply #1 on: June 27, 2007, 01:14:16 PM »

Short antennas inherently have a very low impedance. If you are seeing a high impedance that means you have lots of loss, probably in the grounding system.

You can provide matching at the base in order to match the high impedance to the coax but it won't reduce the losses other than the relatively small amount caused by SWR in the coax.

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W3LK
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Posts: 5639




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« Reply #2 on: June 27, 2007, 02:50:26 PM »

With all due respect, I can't think of a poorer HF antenna installlation, except maybe a mag-mount.

A DC ground is not an RF ground, neither is a piece of metal iunder it an ground plane.

I suggest you go to www.k0bg.com and read Alan's web site. It will show you how to properly install the antennna for maximum performance.

Lon - W3LK
Naugatuck, Connecticut
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
KD0AFK
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Posts: 245




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« Reply #3 on: June 27, 2007, 05:12:30 PM »

Well, sometimes you have to mount it where you can. The bottom of the antenna is about 21" above the fuel tank and steps and 54" above the pavement. Here are the links to the photos of the install. I took another look at the analyzer readings and they aren't too bad. I'm using a 5' whip and getting lower than 1.9 VSWR on bands 10-30 and around 1.2-1.5 on 40-160. And yeah, I know it is close to me when I drive but I haven't lost my hair yet and no apparent side effects have presented themselves yet, mary had a little lamb.

http://img227.imageshack.us/my.php?image=picture003am2.jpg
http://img157.imageshack.us/my.php?image=picture002fd8.jpg
http://img149.imageshack.us/my.php?image=picture001ul7.jpg
I was kind of looking for the extensive information and photos pertaining to installing antennas on 18 wheelers. Got plenty of pictures of cars and SUVs but no big trucks.
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KO1D
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Posts: 387




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« Reply #4 on: June 27, 2007, 05:32:02 PM »

Check out K0BG.com and see his OTR section. Might help somewhat but yes, mounting on the big rigs is a challenge.

Dan S
KO1D
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N5DXL
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Posts: 43




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« Reply #5 on: June 27, 2007, 11:33:13 PM »

I took a look at how you have it mounted and remember 99%
of the hams have no idea about big trucks. Personal I use
a H/S Sidekick and it's mounted on my mirror, if you look
at my photo on QRZ you'll see it.
If it was my setup I'd add another ground going to just
inside of the door frame, there a very good ground and
you'll all so ground out to your tanks and steps.

73,
Tom - N5DXL
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K0BG
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« Reply #6 on: June 28, 2007, 06:15:54 AM »

The problem is Thomas, running a DC ground, or an RF ground, IS NOT a replacement for a Ground Plane! Mounting any antenna, even a 2 meter one, on a mirror introduces many ohms of ground loss. Whether or not you make contacts has no bearing on efficiency. Why this myth continues to propagate (pun intended) is beyond understanding.

Alan, KØBG
www.k0bg.com
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KD0AFK
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Posts: 245




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« Reply #7 on: June 28, 2007, 01:36:40 PM »

No offense intended K0BG but your site is very good.....for 4 wheelers. There really isn't a whole lot of practical information or even photos of big rigs. You keep saying how it is bad to mount an antenna here or there but all you do is link to your site. I drive an 18 wheeler and until you get some very good advise on that, I think I will just talk to other trucker hams who have perfectly good PRACTICAL set-ups instead of going on pure theory. Again, I don't want to offend you. I have been to your site and it is very good and extensive but 99.9% of it is for cars and SUVs. It really isn't a trucker site.
I think I am going to move the antenna up a bit. I know I am going to have a greater ground plane loss but the coil is staring me right in the face. I was talking to a fellow ham at field day who had 25 years of trucking and mobile ham experience and he said to mount the antenna as high as I can get it with a plate under it grounded to the frame and raise the coil all the way up past 160 meters then put a 90 degree bend in the whip pointing aft. I had seen this done on two other trucks and they said they have no appreciable loss because of it. Any loss they had was made up with the clearance of the coil over the cab.
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KE4DRN
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Posts: 3722




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« Reply #8 on: June 28, 2007, 03:02:11 PM »

hi shawn,

Here are photos of big rigs with a Tarheel

http://www.tarheelantennas.com/big_rig_photos

73 james
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KD0AFK
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Posts: 245




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« Reply #9 on: June 28, 2007, 09:54:09 PM »

I saw those photos before I bought mine. I don't have a grab bar so I thought the mirror would be the place.
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K0BG
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« Reply #10 on: June 29, 2007, 05:26:52 AM »

Shawn, I don't have any good picture of OTR installs, mainly because I've only seen one done correctly. Yes, a lot of them are done very well in terms of neatness, the aforementioned photos included. However, the issue isn't how nice it looks, it is how well it performs.  

The usual proofing remarks include the number of DX contacts made, or always getting a 59 report, etc. None of this means much with respect to antenna efficiency.

I have an upcoming article here on eham.net that covers it in a little more detail. However, the main thrust is simply that a mirror bracket does not provide the requisite image plane (ground plane). Ground straps are not a replacement for an image plane, although you'd think so by the comments made by most OTR drivers.

The truth is in the eating as the old adage goes, and unless you have operated a really-good mobile installation, you have no basis of comparison. As self serving as it may sound (after 35 years of operating mobile), I have.

It may be difficult for you to believe, but proper antenna mounting can make as much as 30 dB of difference in the received S+N/N ratio. If you doubt this premise, then drive through Roswell, NM, and I'll be happy to make a side by side comparison with yours.

Alan, KØBG
www.k0bg.com
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KD0AFK
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Posts: 245




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« Reply #11 on: June 29, 2007, 04:12:46 PM »

Like I said, I am not doubting your experience or knowlege. I put it on the mirror because I really have no other place to put it right now. When I make the bracket and can mount it on the bumper so it is closer to the ground, I will. I didn't want to put it on the step because it will limit the directivity to 180 degrees and the same with behind the cab. I would like to see the pictures of the good install. It wouldn't happen to be the flatbed driver in Virginia would it?
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K0BG
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« Reply #12 on: June 29, 2007, 04:29:55 PM »

Shawn, directivity in a mobile antenna is more myth than fact. In order to "hear" the difference, the change has to be more than 3 dB. In most cases, band conditions will be more than that from moment to moment.

The one installation I spoke about, was a late-model Freightliner. It was a 1998 model as I recall. Although the fuel tanks were covered with fairings, Tom had drilled  a hole about 5 inches in diameter through the fairing. He went so far as to have a flat u-shaped plate welded to the aluminum tank, replete with a raised portion to mount the antenna on. I was impressed, and Tom was (he's a silent key now) one of the few mobile operators with as many confirmed DX contacts as I have.

Alan, KØBG
www.k0bg.com
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KD0AFK
Member

Posts: 245




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« Reply #13 on: June 29, 2007, 05:01:32 PM »

Two questions.
Would the mounting system on the tank need to be welded or could a person use a steel band around the tank, sort of like the bands that hold the tank on to the truck? these bands usually have a rubber strip under the band to cut down on wearing the metal of the tank.
and
So the aluminum walls of the truck wouldn't block the signal in that direction? If not, then the best place would be right on the frame of the truck just behind the sleeper right?
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KD0AFK
Member

Posts: 245




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« Reply #14 on: June 29, 2007, 07:09:52 PM »

Also, if I mount the antenna close to the ground, what is going to be the hazard of RF radiation to my body?
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