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Author Topic: Antenna performance.  (Read 1274 times)
KD0AFK
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Posts: 245




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« on: July 28, 2007, 07:51:24 PM »

To start with, I know that the mirror is the worse place to mount a tarheel antenna, so nobody has to tell me in their replies to this post although I get 5/9 reports from all of the stations I DX with. With that said, I would like to know how the performance of the antenna will improve or degrade if I mount the tarheel on the step or behind the sleeper. If I mount it on the step, it will be about 2 feet from the ground and it will have the aluminum step under it; plenty of ground plane image but it will be mounted right next to the truck but open on the direct front, rear and left. How much of the signal will be blocked by the aluminum body of the truck? Same goes for behind the sleeper but there it will also have a 53' trailer blocking it to the rear but it will have the massive steel frame of the truck for a ground plane. I asked this question in my last post about the tarheel but nobody answered the question, all they said was "mirror, bad". If someone did answer it and I just missed it then I am blind and I am sorry. Thank you in advance.
73
kd0afk
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KE3WD
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« Reply #1 on: July 28, 2007, 09:45:01 PM »

Both solutions have drawbacks.  Between the two I'd try the step mount over putting the vertical between the tractor and trailer behind the sleeper.  

But how about trying this first?

Try cutting some stranded insulated wire to make radial wires and attach to base of antenna right there on the mirror mount.  Two or even one wire, cut to 1/4 wavelength of the lowest frequency you will be on.  Okay to wrap them on the tractor body any way you can, affix them with wireties, whatever doesn't intrude and looks good.  Maybe one counterpoise wire on the side the mirror is on and the other going up an over the hood at the windshield bottom or the like.  

I'd bet two 1/4 wave counterpoises would improve the situation.  


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K0BG
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« Reply #2 on: July 29, 2007, 06:42:54 AM »

It is the mass UNDER the antenna that counts. Finding a good place to mount an HF antenna is very difficult on most OTR tractors. If you have a fuel tank rail, that is about as good as you're going to get. Even though the body is mostly fiberglass, you still need to keep the antenna as far away form the cab as you can.

This location will also allow you to use a longer whip which aids efficiency. Remember, the radiation resistance increases by the square of the length increase. In other words, a 12 foot antenna will have twice the radiation resistance of an 8 foot one.

Behind the sleeper is not ideal, as any trailer you pull will greatly effect the tuning.

Alan, KØBG
www.k0bg.com
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KD0AFK
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« Reply #3 on: July 29, 2007, 07:51:31 AM »

The body is aluminum. The hood is fiberglass and the sleeper from the top of the door on up is fiberglass. Even the sleeper extension panels are aluminum (those are the panels that extend back past the sleeper to add aerodynaminicity. (cool word huh?)).
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K0BG
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« Reply #4 on: July 29, 2007, 11:44:51 AM »

As I said, it is the mass UNDER the antenna that counts, not the mass along side. When you mount an antenna close to any surface, some coupling between the antenna and that surface will occur. Obviously, the better a conductor of RF it is, the more coupling there will be.

A nominal HF mobile antenna will exhibit about 20 to about 45 pF, depending on its length, diameter, and the frequency of operation. What ever coupling there is, directly shorts some of the RF to ground. This reduction (loss) shows up as an increase in input impedance.

Since the body is aluminum, most likely the only decent place is on the cowl. Again, to put the mass under the antenna, not along side. The problem with this is, aluminum is not ideal to mount an antenna to. You almost have to fashion your own mount. The only OTR driver I know who has done this (I'm sure there are others), is Chris Brand, KD4EUQ. He didn't actually make it, but he designed it and had it made.

All of this just points out the fact, there is not free lunch.

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




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« Reply #5 on: July 29, 2007, 12:06:02 PM »

So mounting the antenna low to the ground and along side the cab of the aluminum truck will improve the performance in all direction, even to the right of the antenna, if what you say is true, quoting you:"As I said, it is the mass UNDER the antenna that counts, not the mass along side." So I should just ignore the metal of the truck and mount the antenna to the step to increase the ground plane as well as the reception/transmition in ALL directions. right?
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KE3WD
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« Reply #6 on: July 29, 2007, 07:32:52 PM »

Um, that big piece of metal alongside the antenna has got to reflect some signal back at the antenna and further...


Base of antenna must be free and in the clear, located above the mass of metal that is groundplane to get an omnidirectional pattern.  


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KD0AFK
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Posts: 245




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« Reply #7 on: July 29, 2007, 08:22:29 PM »

That's what I am thinking but every time I ask if the metal in the truck is going to effect the transmitting quality of the antenna, K0BG says that the metal along side of the antenna is inconsequential and it is only the metal under the antenna that matters. But whatever, its not like I ask questions to get answers or anything. I mean, I haven't read the whole antenna book so maybe it will say something in it about mobile mounting and tarheel antennas or at least something about the transmiting properties of aluminum.
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KC2WI
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« Reply #8 on: July 29, 2007, 08:39:27 PM »

The mirror might not be all that bad a place, if you can run a few good ground straps to metal parts of the truck. At least the antenna not right next to the truck body and it is fairly high.

I have a bracket that sticks out through the spare tire on the back of my Geo Tracker on chich I mount my High Sierra HS-1800. Although it is metal, I also added a few ground braid straps to ensure a good connection to the vehicle body. It works great. Granted this is a lot different than a truck and since the Tracker is has a soft top, there is not a lot of metal on the same plane as the antenna. I have the antenna mounted fairly high so all of the whip and some of the coil is above the roofline anyway.

Everything I have read about antenna mounting recommends that you not mount on the side of a vehicle where it will be next to the body. It is not so much a question of signal being blocked, more like absorbed or wasted due to ground losses. I don't think the step is a good idea because the antenna is going to be very low with most or all of it too close to the truck body. There will be a lot of loss.

Behind the sleeper would be good if it is higher than the mirror mount and the antenna is at least as far away from other metal next to it, and if you can tie the mount/coax shield feed point to a good metal 'ground.' If not it still might work well if you run a few pieces of wide braid to different metal portions of the truck.

Make the runs different lengths so that even if one of the lengths is close to 1/4 wave (where it looks like a high impedance - probably only an issue on the higher frequency bands) the others will still be effective. Also get the widest braid you can because this will diminish resonance effects and impedance so you have a good tie-in to the metal of the truck.
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KD0AFK
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« Reply #9 on: July 29, 2007, 11:07:01 PM »

This is what I am talking about with the bumper bracket. It would place the coil and whip above the fiberglass hood and the bottom of the antenna would be VERY close to the ground so that it would have the same ground plane image as a tarheel mounted to the trailer hitch of a car or truck. I follow close at all to any cars or trucks so chances of the thing getting damaged on the road is slim.  Also, the whip would have a Hustler quick disconnect and the Pivot Bracket would have a cable connecting it to the  Bumper Bracket(not pictured)to allow it to pivot 90 degrees to be out of the way of the hood when it is opened. If anyone would like to make this for me or has any improvements to it, bring it on. I would love to be the prototype tester. Might make a bit of cabbage for yourself. I have even seen a shock absorber style mirror mount on another install that could be incorporated into the design.

http://img175.imageshack.us/my.php?image=bumperbrackettt5.jpg
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