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Author Topic: 1/4 or 1/2 wave on vehicle  (Read 545 times)
K0PRP
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Posts: 25




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« on: May 01, 2006, 04:15:47 PM »

I am installing a dual band (2M/70cm) antenna on my Trailblazer. Due to some height restriction problems, I have two choices:

1) 1/4 wave unit, installed near the top of the windows - Roofline is about 3" higher but only shadows the base of the antenna

or

2) 1/2 wave unit, installed at the beltline (bottom of windows), but about 1/2 of the antenna will be shadowed by the roof of the vehicle. The antenna would be mounted near the A or C pillar, which slopes away from the antenna about 30 degrees but the upper half of the antenna would be in "free air"

Suggestions on which will give me the better performance? How do you estimate the affect of nearby metal? I am certain it would distort the pattern to some amount.

Thanks

Paul P
K0PRP
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N8EMR
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Posts: 235




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« Reply #1 on: May 01, 2006, 05:40:21 PM »

Why not go with a full 5/8 and put a automatic antenna raising motor on it.
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K0BG
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« Reply #2 on: May 01, 2006, 08:08:14 PM »

The most popular dual band antenna is the Larsen NMO2/70BK. Over all, it is about 3 feet tall, and sturdy enough that even low garage doors aren't going to be a problem.

Forget about playing the Gain Game. The Larsen NMO2/70BK will out perform any import antenna, regardless of the advertised gain. Since it is a TRUE NMO mount, if you install it correctly, it will not leak, EVER.

Alan, KØBG
www.k0bg.com
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K5LXP
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« Reply #3 on: May 01, 2006, 08:51:07 PM »

If clearance is a problem, you could mount a 2/70 on the fender.  

If 'performance' is your ultimate goal, either the 2/70 on the top of the cab, or separate quarter waves and a diplexer would be 'best'.  Quarter waves will take a beating from minimal clearance garage doors better, the 2/70 is a one-hole solution if you've got the clearance.

I use 'quotes' because any pattern distortion or gain differences are pretty much inconsequential in a practical installation.  In one sentence you are concerned about pattern distortion, at the same time proposing using some kind of compromise side mount.  If you want performance, perforate the roof.  If you want aesthetics, put performance out of your mind and pick whatever looks nice.

I personally don't think the cost and physical size of 'gain' antennas for 2M is worth it, the only reason I would entertain the idea of a 2/70 is it's dual band coverage.  All of these goofy Maldol, Comet, Diamond, etc 'gain' multiband antennas are complex, fragile and have questionable gain figures, not to mention expensive for what you get.  Think about the mounts too, an NMO with coax and connector is like $15.  Hard to beat.

Mark K5LXP
Albuquerque, NM
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K8KAS
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Posts: 569




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« Reply #4 on: May 04, 2006, 09:36:36 AM »

A true 1/4 wl on the roof is a very very hard antenna to beat. The small gains some of the other antennas mentioned are not worth the cost and the fact they can be  antennas that are fragile as well. 5/8 wl and 1/2 wl antennas mounted on the lower body parts/windows,etc will not out perform the simple 1/4 wl on the roof. As an RF product development type in the automotive world I have spent many hours on an antenna range confirming these facts. When range testing we always referance with a 1/4 wl antenna on the roof and the test antennas almost never beat the 1/4 wl referance antenna.
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KL7IPV
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Posts: 984




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« Reply #5 on: May 05, 2006, 10:20:35 PM »

As luck would have it, I just changed my 2 meter antennas on my Dodge pickup this week. I originally had a Hustler 5/8 wave on the roof and switched the new Maldol HF/6/2 antenna I just installed. The 2 meter seemed weak and the bars on the 706 were down from the 5/8 wave. So I removed the 5/8 whip from the Hustler mag mount and put in a 11" whip to see the result. The result was a real surprise to me. The bars on the 706 on receive from the 34/94 repeater were almost double of either one of the other antennas. The SWR on my 259 analyzer was down to a flat 1:1 from 140 to 148 MHz. The 706 showed one bar on transmit too. So for me the 1/4 wave stinger is going to be the antenna of choice from now on. The actual stinger above the 2 meter coil is 10" and the total antenna is 12" above the coil.
73,
Frank
KL7IPV
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