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Author Topic: Antenna Height  (Read 1509 times)
WA5UHK
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Posts: 131




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« on: September 23, 2005, 08:30:35 AM »

After a few months using my new mobile install, I'm upgrading my Hamstick to a bug catcher to get more metal in the air and a higher radiation resistance.  I've made a mock-up with wooden dowels and an old 102 inch whip to design the mast support and get a feel for the height. I think I want an overall antenna of 11.5 feet (measured from road surface to tip of whip).

The tallest antenna I've used in my ham-past was a little less than 10 feet. I never had any real problems with it outside of paying for a lamp in a convenience store once. That experience pre-dated SUV's and parking structures seem to have gotten taller since then perhaps because of these big cars.

When I was in college I worked for a power company in the summers. I remember their requirement was to maintain a 22 foot clearance across major highways. I did measure some clearances as low as 12 feet across small roads and I've seen communication cables that could have been lower than that. Also, I don't see many overpasses lower than 12 feet. That's how I arrived at the 11.5 foot max height.

Before I commit to a total mast/whip length I want to get some thoughts from the eHam community. Does anyone have any "road" experience with an 11.5 foot (or taller) antenna? My main concern is road-speed collisions with overhead structures.
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KE4SKY
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« Reply #1 on: September 23, 2005, 10:01:30 AM »

Residential feeder lines are "supposed" to be 12 ft., but many aren't.  More of a problem will be "tree bashing" on country roads.
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K7VO
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« Reply #2 on: September 23, 2005, 10:53:18 AM »

This is probably the major argument for antennas which fold over (i.e.: a Hustler mast or any larger Diamond antenna) because you *ARE* going to have impacts on the antenna.  Some places clearance is low particularly on state and secondary roads.  Your concerns are legitimate.

73,
Caity
K7VO
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N3ZKP
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Posts: 2008




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« Reply #3 on: September 23, 2005, 11:31:16 AM »

<< This is probably the major argument for antennas which fold over >>

Kinda hard to operate in motion with the antenna folded over. Smiley


What will take a beating is the tip of the whip, as long as there is a spring in there somewhere to take the shock off the rest of the antenna. That said, the tip of my Hi Sierra is 11 feet above the ground at the bottom of 75/80m. On 20m, where it normally stays, it's about 9.5 feet. The only things I have ever had a problem with, other than the occasional tree limb, are bank and fast food overhangs and one low bridge on a country road I am frequently on. In neither of these cases am I making anything other than crawling speed so no problem - the tip simply drags. I have never hit anything on the open highway.

Lon
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N3ZKP
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« Reply #4 on: September 23, 2005, 11:34:13 AM »

One other place I have a problem - low parking garages. But for that I DO have a foldover on the whip, but then again, I'm probably not on the air on HF inside a parking garage. Smiley

Lon
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K0BG
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« Reply #5 on: September 23, 2005, 01:19:22 PM »

It isn't the length of the antenna, it the height above the pavement that counts, quite obviously. My mobile antenna is 13.5 feet in length, and is mounted 30 inches up on the left quarter panel. The tip is just under 16 feet. Except for an occasional tree, it doesn't hit anything about these parts. I've traveled to Denver, and not had any problems. Traveling in Kansas City, is a problem especially in the older parts of town. I live in the desert SW (Roswell, NM), not in NYC. In the east, I understand it is difficult with antenna heights of only 11 feet. So it depends on you location.

Back in the 70s and 80s, I used a full-length 20 meter antenna mounted on top of the quarter panel of a Fury III. This placed the tip at just over 21 feet. There was a spring between the mast and 8 foot whip, which allowed the whip to lay over at speed. I replaced about 15 or 20 whips over those years, but that's the price of admission.

Length does matter, and you should strive to place the base as high as you can (top of the quarter panel is better than the bumper), and the length as long as possible without banging wires in your area. I don't worry much about the trees, as CB whips are cheap ($9.95 at the local truck stop).

Alan, KØBG
www.k0bg.com
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K0BG
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« Reply #6 on: September 23, 2005, 01:31:35 PM »

Lon, I should have read your reply more carefully before posting.

I have hit several "things" over the years. I used to drive under the Perry Street under pass a lot when I lived in Denver. There are enough pigeons under it that it was more common to hit one than miss one. I even had a guy in a Corvette really get PO'd because I knocked in his lap.

Up in Wyoming I hit a rather large Canadian goose, and cut his head clean off. I stopped, and the guy behind me picked it up before I could and thanked me for his Thanksgiving dinner. True story. It even hit Douglas Wyoming newspaper.

I hit a tree once, wrapped the antenna around it, coil and all. It pulled the ballmount right out of the quarter panel. The only thing thing that saved the mount and mast was the coax. It cost $300 to fix the car. That would probably be a good $1,500 or more today.

But height, matters. Right?

Alan, KØBG
www.k0bg.com
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WA5UHK
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« Reply #7 on: September 23, 2005, 02:05:00 PM »

This is rather encouraging.  As long as I don't hit a 13KV line, I'd be happy.  I have rattled around in parking garages at the airport and ground the tip off my 5/8th 2m antenna but I've never dispatched a bird.

I'm thinking I may be aiming low at a 11.5 foot overall length.  I want the coil to be midway up at the shortest length of the antenna.  I can use more than one whip length depending on how lucky I feel and where I'm mobile.  Here in Dallas I think short is better but when I travel west sounds like I could get away with 13+ feet.

Mobile operation is definitely "combat" hamming.
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N3ZKP
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« Reply #8 on: September 23, 2005, 02:47:14 PM »

Alan,

I can't begin to top any of those! About the only airborne things I have hit are bugs, although I was riding in a vehicle that nailed an owl with the hood ornament - impaled the sucker in an almost upright position!

Boy, did it have a surprised look on its face!!!

Lon
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AC2RC
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« Reply #9 on: September 24, 2005, 05:27:41 AM »

Interstate highway overpasses are 13.5' above the roadway, so you must be at least less than that. My 8.5' whip is mounted on the rail of my truck so it's total is 12.5'.On some back roads I do hit trees but nothing serious and on those roads I'm not going full speed anyway.
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