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Author Topic: Would curb feelers under a car help the ground plane?  (Read 5580 times)
AF8F
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Posts: 32




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« on: March 08, 2012, 07:04:24 AM »

Cars bodies act like a capacitor between the antenna and ground.  Would installing metal curb feelers under a car, with their ends near the ground, help HF antenna performance?

Thanks for your help.
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WX7G
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« Reply #1 on: March 08, 2012, 09:04:45 AM »

No, unless the ground is very low loss such as salt water.

An HF mobile antenna installation consists of the "antenna" and the automobile body. Both radiate and this can be considered to be a vertical dipole or a ground plane with elevated feed.

If we have a vertical dipole close to lossy ground we do not want to connect it to lossy ground.
If we have a ground plane close to lossy ground we do not want to connect it to lossy ground.


« Last Edit: March 08, 2012, 09:17:31 AM by WX7G » Logged
K1CJS
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« Reply #2 on: March 09, 2012, 08:02:34 AM »

Only if they were made to touch the ground ALL THE TIME.   Wink  But then, the static from the sparking would probably make reception impossible--not to mention the looks you would get from others!   Shocked  (Seriously--no, they wouldn't.)  73!
« Last Edit: March 11, 2012, 08:12:30 AM by K1CJS » Logged
MDNITERDER
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« Reply #3 on: March 09, 2012, 02:01:44 PM »

 I remember reading an old CB book they talked about using a chain to discharge static.
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K0BG
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« Reply #4 on: March 09, 2012, 03:05:01 PM »

Static drains are different animals, and don't do anything for the ground losses unless they're long enough to be radials! This is an obvious faux pas!

The reason there is ground loss to start with, is there are standing waves between the body of the vehicle, and the surface under it. And we're not talking about VSWR! While lots of folks model mobile antenna installations, they quite often miscalculate, or under estimate the magnitude of the ground loss, and why it is there. This leads to false impressions that the body of the vehicle radiates as much RF as the antenna, which it does not.

One important aspect to good radiation efficiency in a mobile, is to mount the antenna as high as possible. This forces more of the return current throughout the body of the vehicle, rather than the lossier surface under it. This said, if there was a way (make that an easy way) to reduce ground losses (curb feelers?, trailing wires?), I think it would have been discovered along ago. It has, sort of.

The ground plane under a vertical adds capacitance to the bottom at the antenna, much as cap hats add it to the top of the antenna. Both increase efficiency by changing the way current flows over the length of the antenna. And speaking of length, that's probably the most important attribute of all!

It should be obvious then (it isn't unfortunately) what direction we need to take to maximize the ERP of a mobile antenna. It isn't curb feelers, that's for sure.
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W4FID
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« Reply #5 on: March 12, 2012, 05:46:34 AM »

They could be of some help .................... if you aren't paying attention to what you're doing as you drive and they rub the curb the noise may alert pedestrians and give them a couple seconds to jump out of your way.

Electrically it's simple. The mass -- the size and area -- of the vehicle acts as one plate of a capacitor. The tires are the dielectric. That's how you get capacitive coupling to the earth. Small diameter short pieces of wire are a VERY small percentage of the vehicle's size/area and will have no measurable effect. That's why the passengers on the Titanic didn't bother to use tea cups to bail the ship out ............... no reason to think it would have any results.
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