Roof mounted HF antenna

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stevetull

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Mar 9, 2010
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Paige, Bastrop County, Texas
Hello all.
Every once in a while I get less busy at work and I renew my interest in operating on HF. My one-and-only radio, a Kenwood TS-480HX sometimes finds itself on the desk inside and sometimes in the car. I'm about to take a rather long road trip so the rig is back in the car.
I'm upgrading the installation by ditching the 3 mag antenna mount and I'll be drilling for and installing a 3" puck mount, and upgrading the feed line.
The antenna is an Outbacker 80 through 6 meter. I like that antenna a lot, but I think in the past my installation hasn't done it justice. I hope to rectify the situation.
The car is a Toyota 4Runner. I understand that a roof mounted antenna would be the most efficient as close to the center of the roof for the best possible signal propagation. That puts the antenna pretty high in the air. If I drive stupid and find myself going under a low-hanging limb at least with the mag-mount, the antenna has a chance to lay over and then pop back up, (at low speeds of course). However once I permanently mount the thing, (I will have a QD fitting), low-hanging impediments will likely destroy the antenna. Would it be appropriate to add an antenna spring or are springs for sissies?
Edited: I can't seem to find the important information needed in order to order an appropriate spring online, so if any of you may have some insight into this I would sure appreciate any help. I did see a black Larson spring that would go well with the black antenna, but I don't know how stiff of a spring I would need and the screw fittings seem to be a secret.
Thanks.

-Steve
 
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krokus

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Premium Subscriber
Joined
Jun 9, 2006
Messages
5,964
Location
Southeastern Michigan
Hello all.
Every once in a while I get less busy at work and I renew my interest in operating on HF. My one-and-only radio, a Kenwood TS-480HX sometimes finds itself on the desk inside and sometimes in the car. I'm about to take a rather long road trip so the rig is back in the car.
I'm upgrading the installation by ditching the 3 mag antenna mount and I'll be drilling for and installing a 3" puck mount, and upgrading the feed line.
The antenna is an Outbacker 80 through 6 meter. I like that antenna a lot, but I think in the past my installation hasn't done it justice. I hope to rectify the situation.
The car is a Toyota 4Runner. I understand that a roof mounted antenna would be the most efficient as close to the center of the roof for the best possible signal propagation. That puts the antenna pretty high in the air. If I drive stupid and find myself going under a low-hanging limb at least with the mag-mount, the antenna has a chance to lay over and then pop back up, (at low speeds of course). However once I permanently mount the thing, (I will have a QD fitting), low-hanging impediments will likely destroy the antenna. Would it be appropriate to add an antenna spring or are springs for sissies?
Edited: I can't seem to find the important information needed in order to order an appropriate spring online, so if any of you may have some insight into this I would sure appreciate any help. I did see a black Larson spring that would go well with the black antenna, but I don't know how stiff of a spring I would need and the screw fittings seem to be a secret.
Thanks.

-Steve
http://www.outbacker.biz/OUTBACKER/MOBILE-MOUNTS/mobile-mounts.html


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Hey Steve :)
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You did the right thing about a spring base. All my radio existence I have cringed when I see mobile antennas without some sort of stress reliever- ie: "springs." If you think in terms of vector forces, there are tremendous pressures placed at the base of the mobile whip- and of course the longer and stouter the antenna, the greater these forces. With time its no wonder they fail from the constant flexing - to say nothing of what this can do, over time, to the thin sheet metal of the vehicle frame. Only short Motorola NMO mounted quarter wave V/UHF antennas get a pass from me here....
And like you said, what about low branches or garages ?.. Even bent over slowly and carefully, they don't like it much or for long.
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Getting the base of a HF mobile whip as high up as possible (and esthetically pleasing as any such thing can be- for nothing is worth turning your Toyota into some freaky geeky monster :) ) -- as high up as possible is*very* important. Its called Ground Return losses. You want the return fields to be back to the base thru good conductors- the lower the base, the greater that return will be thru lossy soil and less thru the vehicle body. You are looking at maybe a 10-15 % efficient antenna at 7 Mhz for a modest whip- at 75 metre's it will maybe be all of 5-6%. Not great, but a 12% efficient antenna is only ~2 "S" units less less than a full quarter wave against a proper ground. But every percentage point matters here.
That car body is not much of a ground plane at HF frequencies, but the you'd be really surprised at how much difference there is between a bumper mount and one near the roof top- as much as 3 Db's..... half your power lost just becuz the base placement (why, for various other reasons too, never use a bumper mount.)
And don't neglect how you feed the base of the antenna- think of the passage of the feedline thru the vehicle body as a thru a large coaxial capacitor. You want the least coupling to the fame as possible- that translates to a *Large* hole, big, low loss insulating base, and solid connections to the the antenna. The greater the Mickey Mouse factor here, the more pathetic will be the results.
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I have used HF sideband for many years. Installed in our government "Utes," these radios have always worked well.... on 8-10 Mhz primarily- the daytime NVIS ranges, mobile to mobile, is 500-800 miles- consistently. On the higher frequencies ?--> the World !
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Good luck- I hope that adds to your mix of ideas :)
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Lauri :)
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