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Author Topic: Radiation Pattern  (Read 459 times)
KC8AXJ
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Posts: 303




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« on: December 21, 2003, 12:45:45 PM »

Anybody know of a site that shows the radiation pattern of a HF mobile antenna. A "screw driver" type or 102 inch whip with a tapped coil at the feed point would be interesting to see. I would be most interested of one on the rear of a vehicle as this seems to be the common mount area.

Thanks !
Steve  KC8AXJ@arrl.com

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K0BG
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Posts: 9863


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« Reply #1 on: December 21, 2003, 09:47:22 PM »

You can download the demo version of EZNEC and plot your particular case, and come close. There are just too many varibles to say with any certainty what yours will look like.

On average, however, the stronger signal is to the area of larger mass. In other words, if the antenna is mounted on the left quarter panel, the strongest signal will be to the right fender. This said, it depends on the antenna. Really lossy antennas tend to radiate up, not out. Further, if you mount the antenna correctly, bond the vehicle together, or increase its size the efficiency goes up. Use poor mounting techniques, lossy antenna coils, poor bonding, etc., the efficiency goes down.

Alan, KØBG
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AA4PB
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Posts: 12840




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« Reply #2 on: December 23, 2003, 03:25:06 PM »

I believe it is the latest ARRL Handbook that has some modeling work done on a mobile HF whip. Basically it was pretty much omni-directional, even in the direction somewhat blocked by the vehicle.

When you get to the higher bands (10M perhaps and certainly VHF and UHF) the vehicle will act as a ground plane and the best signal will be in the direction of the most metal provided the antenna is high enough that it is not being blocked by the metal. On lower bands the vehicle is so small (in terms of wavelength) that it has little effect as a ground plane. In that case the vehicle acts more as a capacitor, coupling into the ground it is setting on - or so goes the theory that I've read.

In practice I don't notice any directional characteristics of my mobile HF antenna but I haven't put it on a turn-table and taken any measurements. It would be an interesting exercise. Some years ago I actually did put a car on a turn table on an antenna test range and take measurements of various VHF and UHF antennas.
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WA9PWP
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Posts: 55




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« Reply #3 on: May 10, 2004, 10:34:28 AM »

Yes,
  With the HF mount in the center of the front edge of the trunk lid on my Saturn, I notice a "lobe" toward the front of the car on 20 and 15 meters. Not noticeable on 40 meters.

           73, Paul WA9PWP
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