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Author Topic: Which antenna?  (Read 2126 times)
KA4NMA
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« on: August 22, 2014, 06:07:03 PM »

Which setup would be better on a 2005 Ford Escape xlt?
1 tarheel model 100 bumper mounted
2 Little tarheel II mounted on a side panel?
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K0BG
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« Reply #1 on: August 23, 2014, 07:28:38 AM »

Actually, you'd be better off with the 200A. There are ways to mount it on the side, with no more issues than mounting a Lil Tarheel. You might want to visit the Photo Gallery on my web site.
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KA4NMA
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« Reply #2 on: August 23, 2014, 03:14:47 PM »

I already have these antenna's. I currently have the 100 ground mounted outside my apartment (with permission).
Randy
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K0BG
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« Reply #3 on: August 24, 2014, 06:04:02 AM »

Since radiation resistance, hence efficiency, is a square function of the electrical length, and how current flows over that length, ask yourself which antenna would be better, all else being equal?
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KA4NMA
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« Reply #4 on: August 24, 2014, 06:37:52 AM »

The model 100 would be more efficient, but what is the effects of mounting on the bumper?
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WX7G
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« Reply #5 on: August 24, 2014, 08:30:30 AM »

Let's see what a comparative NEC-2 simulation shows at 7 MHz. by comparative I mean it is suitable from determining if antenna A is superior to antenna B. It does not predict the gain for the actual vehicle and antenna over lossy ground.

The vehicle is modeled as a rectangular box having 2.5' grids.

The radiation resistance with the Little Tarheel II is 0.5 ohms. The Tarheel is mounted 2.5' forward of the rear of the vehicle and halfway between the bottom and the top of the vehicle. Assuming a loading coil Q of 100 and using the standard 32" whip the free space gain is -16 dBi.

Using a 72" whip the radiation resistance is 1.0 ohms and the free space gain is -10 dBi.

The radiation resistance with the Tarheel 100A bumper mounted is 2.4 ohms. Assuming a loading coil Q of 200 the free space gain is -7 dBi. Change the whip from 6' to 7' (making the installation as tall as the 200A with a 6' whip) and the gain is -6 dBi. Using a 9' whip the gain is -2 dBi.

The radiation resistance with a Tarheel 200A bumper mounted is 2.9 ohms. Assuming a loading coil Q of 200 the free space gain is -6 dBi.

One variable that is important is the antenna spacing from the vehicle. The side mounted Little Tarheel II is 3" from the vehicle. Moving it 12" away increases the gain by 3 dB. The bumper mounted Tarheel antennas are mounted 12" from the vehicle.
« Last Edit: August 24, 2014, 09:10:34 AM by WX7G » Logged
M6GOM
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« Reply #6 on: August 24, 2014, 09:38:19 AM »

Thank you for those figures. I didn't ask the question but I've been pondering if there would be a massive difference between my Little Tarheel II with a 72" whip mounted on the roof of my Ford Mondeo and a larger screwdriver antenna mounted lower down. It looks like I'm looking at a difference of maybe 3dBi or so on 40m and probably less difference on 20m and up which doesn't justify the cost and the aggravation mounting a larger one.
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WX7G
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« Reply #7 on: August 24, 2014, 09:49:20 AM »

14 MHz:

Little Tarheel II with 32" whip mounted on the side of the vehicle: -7 dBi.
Little Tarheel II with 72" whip mounted on the side of the vehicle: -2 dBi.
Tarheel 100A bumper mounted with 72" whip: 1 dBi.
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KA4NMA
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« Reply #8 on: August 24, 2014, 06:13:23 PM »

Wow! Thanks guys.

Anybody want to trade my Little Tar Heel II for a model 100 or 200:  Grin

Randy Ka4nma

PS can anybody help me to learn antenna modeling?
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KA4NMA
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« Reply #9 on: August 24, 2014, 11:01:27 PM »

How do the other Tarheel Antenna's and other screwdriver antenna's also model?
Randy
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WX7G
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« Reply #10 on: August 25, 2014, 04:18:34 AM »

To model a screwdriver antenna the manufacturer's dimensions are used for the length and the loading coil location. Assigning inductor Q is an estimate or sometimes a guess. Closeup loading coil photos help. I have a Tarheel 200A and a 40A that has helped me define these antennas a bit better.  

The NEC-2 models can be used to compare antenna A to B, to compare mounting location A to B, and to explore antenna spacing from the vehicle. However, the performance in an actual installation will be different than modeled. Due to real-world ground loss, which NEC-2 appears to not model well - there may be less difference between antennas than predicted. This is because ground loss degrades the performance of an efficient antenna more than a less efficient antenna.
 
« Last Edit: August 25, 2014, 04:22:20 AM by WX7G » Logged
K0BG
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« Reply #11 on: August 25, 2014, 10:29:54 AM »

Quote
This is because ground loss degrades the performance of an efficient antenna more than a less efficient antenna.


While there is truth in the statement, it isn't exactly correct. All antenna resistive losses, except stray coupling losses, are in series with one another. If you increase the current flow in the superstructure of the antenna, you also increase it in the resistive ground losses as well. But saying that ground loss affects more efficient antennas, more than lessor efficient ones, is incorrect.
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WX7G
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« Reply #12 on: Yesterday at 08:07:54 AM »

Here's a numerical example comparing the ground loss effect on antenna A and antenna B:

Antenna A has a radiation resistance of 1 ohm, a base-referred loading inductor loss of 10 ohms and the base-referred ground loss is 5 ohms. The radiation efficiency is 6.3%. Now change the ground loss to 10 ohms and the radiation efficiency is 4.7%. The change in ground loss reduced the antenna gain by 1.2 dB.

Antenna B has a radiation resistance of 1 ohm, a base-referred loading inductor loss of 20 ohms and the base-referred ground loss is 5 ohms. The radiation efficiency is 3.8%. Now change the ground loss to 10 ohms and the radiation efficiency is 3.2%. The change in ground loss reduced the antenna gain by 0.8 dB.

The less efficient antenna is less sensitive to ground loss.



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K0BG
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« Reply #13 on: Yesterday at 01:04:19 PM »

This is similar to Pa Kettle math. If you just look at some static numbers, you're correct. But remember, the resistances are in series. So ask yourself this; what happens to the radiation resistance when you change the ground loss?
« Last Edit: Yesterday at 01:06:44 PM by K0BG » Logged

KA4NMA
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« Reply #14 on: Today at 02:54:54 PM »

I see that the little tar heel II antenna is not the best.  Would a 8 ft whip feed with high voltage, large gauge wire to a SGC tuner work better?

Randy ka4nma
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