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Author Topic: "Best" Antenna Length for New Mobile Installation  (Read 10905 times)
KD4LLA
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Posts: 450




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« Reply #15 on: October 20, 2013, 03:18:23 PM »

VHF is primarily "line-of-sight".  I have found that a (center) roof mounted quarter-wave works as well, or better, than an ill-mounted 5/8 wave antenna.

Mike
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AA4HA
Member

Posts: 1377




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« Reply #16 on: October 20, 2013, 04:37:38 PM »

A quarter wave antenna (simple whip) has a radiation pattern that is shaped like a doughnut so you have gain (unity) in places where  you don't necessarily need it. Depending upon the terrain you are moving through, the band and how you have the antenna mounted on your vehicle you can "squash" that doughnut radiation pattern and have a positive gain (above unity) when using a 5/8 type antenna.

That is all that gain is on an antenna. It is nothing magical, you do not get something for nothing on an antenna. You compress the radiation pattern from isotropic and try to put those lobes where they do you the most good. On VHF/UHF that is probably to the horizon, HF it is either as a "cloud burner" for NVIS or a low takeoff angle for DX.

Depending upon those conditions (mounting, the angle from your vehicle up or down to the other station) this can be beneficial or harmful.

I try to start out with all other things being equal; a good antenna mounting position (smack dab in the top center of the roof) and then to do a comparative analysis of the antenna on its own merit. You will see a few dB of gain, hopefully in the direction that does you the most good.

Many years ago I did a wholesale replacement of the quarter wave antennas on a fleet of emergency vehicles. It did make a difference on fringe radio performance (15-20 miles out). In no case did it make things worse. These were those black, conical 3 dB gain Antenna Specialists mobile antennas with a whip. This was all VHF and UHF voice communications and made more of a difference where a moving vehicle would not be dropping in and out of squelch.

Picket fencing is a completely different effect than the bending of an antenna while the vehicle is in motion. It is moving though bands of diffraction on marginal signal conditions where reflected and refracted signals become additive or subtractive as the vehicle moves through these bands (think of ripples on a lake at a distance).

It could be argued that the larger antenna of a 5/8 wave antenna also has a larger effective aperture (capture area) and has a slight efficiency advantage as well.

Sure, a bigger, longer antenna can get wiped off the top of a vehicle if you drive under a tree branch, under a low garage door or car wash. For car washes I take off an NMO mount antenna, it inconveniences me by 20 seconds to do that.

When we compare things lets try to be apples to apples or oranges to oranges and not mix things up with different radios, bands, mounting spots, etc.. look at an antenna on the merits of the antenna and nothing else.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
K0BG
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Posts: 9833


WWW

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« Reply #17 on: October 21, 2013, 06:07:12 AM »

Danny Richardson, K6MHE, wrote this article for CQ some time ago. I have a copy on my web site, and here is the URL:

http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf
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KQ6Q
Member

Posts: 962




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« Reply #18 on: October 22, 2013, 08:27:56 PM »

If you're working repeaters, you don't need gain. As has been mentioned, and HT ducky will work. the mobile equivalent is a Hood lip mount and a short dual band whip. You'll never have to worry about the clearance in a parking garage.
kq6q
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