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Author Topic: unintentionally directional mobile antenna?  (Read 2184 times)
KD0NFY
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Posts: 75




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« on: January 06, 2011, 08:36:17 AM »

I had an NMO mount installed the other day for a dual band mobile.  It's in the center of the roof of my car, a couple feet in front of the stock broadcast receive antenna.  I'm thinking I might eventually add another mount in between the two antennas for (don't laugh) an 11 meter antenna when I take road trips.  So that would result in three antennas in line on the roof of the car.  Is that a bad idea because I'd end up with a weird radiation pattern? 

What would an arrangement like that be called?  Hog periodic? 

Thanks!
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K5LXP
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Posts: 5292


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« Reply #1 on: January 06, 2011, 11:53:39 AM »

In order for a passive conductor to influence the pattern of a nearby antenna, the spacing and length in relation to frequency is fairly critical.  In other words, unless you took particular pains to install specific length whips at critical distances, you won't distort the patterns in any huge way.  It's impossible to maintain the "optimum" non-interference spacing of antennas on the rooftop of a vehicle anyway, so just separate them as best you can.  In particular what I try to do is make sure the whips won't touch each other when going down the road or getting bounced around.  In the grand scheme of things during typical day to day operating, you wouldn't be able to tell any difference in performance.


Mark K5LXP
Albuquerque, NM


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K7KBN
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Posts: 3473




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« Reply #2 on: January 06, 2011, 09:04:01 PM »

Several years ago, a good friend of mine was installing a dual band VHF/UHF rig.  He mounted the antenna in the center of the roof and started the wiring.  Every so often he'd take a break and turn on the car's "entertainment" radio.  And when he got back to working, he found the SWR had changed pretty dramatically.  After an hour or so he gave me a call and asked me to take a look.

His car's "entertainment antenna" was normally recessed near the right rear wheel well.  When he turned the radio on while testing the ham rig, the emerging antenna did a number on the meter readings.

Answer to the problem:  don't listen to the one radio while you're transmitting on the other one.
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73
Pat K7KBN
CWO4 USNR Ret.
W7ETA
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Posts: 2527




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« Reply #3 on: January 07, 2011, 12:07:40 AM »

Only if you ride a HIG.
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K1CJS
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Posts: 6252




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« Reply #4 on: January 08, 2011, 05:31:12 AM »

You won't 'upset' the pattern of either antenna to any great degree.  What may happen is this--if you have one transceiver on while listening to the other, you may overload the front end of the one doing the receiving, giving you an earful of noise--and possibly damaging the receiver too.

The bottom line is that it is OK to put your antennas the way you state.  You can even have both rigs on while you're just monitoring the frequencies.  B U T --- If you're going to transmit, turn the other transceiver OFF!  You'll save your eardrums, nasty comments from the other passengers in your car--and maybe even your radios!
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