RF ground vs Electrical ground

[K8POS]

I was trying to explain my mag mount antenna to a co-worker who asked what is the difference in RF ground vs Electrical ground?
Now my mobile rigs chassis is metal and grounded to the body of my car.
The shield of the coax is at electrical ground or negative.
I was trying to explain that although the antenna is electrically grounded it was not RF grounded due to NO connection at the antenna mount.

Can some one give me a LAYMEN explanation to better explain it.
 

[AD4U]

In a mag mount there is significant capacitance between the 4 inch or so diameter magnet base and the car body.  At RF a capacitor acts ALMOST like (almost as good as) a direct hard connection to the auto body (in most cases).

If given a choice I would always opt for a permanent mount VS a magnetic mount.

Dick AD4U

[WB2WIK]

In a mag mount there is significant capacitance between the 4 inch or so diameter magnet base and the car body.  At RF a capacitor acts ALMOST like (almost as good as) a direct hard connection to the auto body (in most cases).

If given a choice I would always opt for a permanent mount VS a magnetic mount.

Dick AD4U

I agree.  What your antenna has with a mag mount actually is an RF ground, although probably not a real electrical ground at the antenna end.  Doesn't really matter, as "good" mag mounts work well at VHF and UHF.

On HF, you could have a pretty lousy RF ground because the magnet's not nearly big enough to provide enough coupling to the metal below it.  Some of the really BIG 3-magnet and 4-magnet mounts work better at HF, but still far from perfectly.

[WA3SKN]

Inside that mag mount antenna is some foil connected to the shield of the coax.  This is one plate of a capacitor.  The other plate is the metallic car body.  So your coax shield is capacity connected to ground... RF connected but not DC connected.  This capacitor is large enough for VHF and UHF, but is really too small for HF use.  This is why mag mounts work OK at VHF/UHF, but do not work well for HF.
73s.

-Mike.

[KE3WD]

And, once that magnet foil capacitor is mounted on the car and the RF couples to the car sheet metal, the body of the car is the RF "ground" -- or more exactly, the "counterpoise" for the antenna. 


73

[K5LXP]

or more exactly, the "counterpoise" for the antenna. 

Technically not a counterpoise, but a groundplane.  They are very different.  All of these conflicting definitions only compound the confusion ("ground" is bad enough...).


Mark K5LXP
Albuquerque, NM

[KE3WD]

A counterpoise is a type of electrical ground that is not connected to earth. It is used in radio antenna systems when a normal earth ground cannot be used because of high soil resistance[1] It consists of a network of wires or cables (or a metal screen) parallel to the ground, suspended from a few centimetres to several metres above the ground, under the antenna. The counterpoise functions as one plate of a large capacitor, with the conductive layers of the earth acting as the other.

source:  http://en.wikipedia.org/wiki/Counterpoise_%28ground_system%29

Unless you are confusing the term, "couterpoise wire" - a single wire cut to a resonant freq. - with "counterpoise"...


73

[K8POS]

I think " Capacitive Counterpoise " was the term I was searching for.
I understand how and why it works.  Try to explain it to a non electronic person is another thing.

Thanks for the replys

[N6AJR]

electrical ground is a return path for dc current. so a ground from the radio to the chassis of a car will be a electrical ground, also usually acts as the return for the dc 12 volts.  RF ground is a different beast all together.  it is used against an rf ground ( think ground plane ) and can be an antenna with the antenna grounded electrically on a DC ground, but also isolated from RF ground at the same time.  RF ground changes too with the differences in frequency, so it is dependent on what you are transmitting. A mag mount can actually create a capacitive link to the ground plane of the car and have no electrical ground, but have an adequate rf ground.

[WB2WIK]

And, once that magnet foil capacitor is mounted on the car and the RF couples to the car sheet metal, the body of the car is the RF "ground" -- or more exactly, the "counterpoise" for the antenna. 


73

Good ones are made that way, but I've seen many mag mounts that unfortunately aren't.

A local ham friend brought over a "Firestick" CB antenna mag-mount with a very powerful magnet and coax connected to it.  It worked terribly.  I measured continuity between the shell of the coax connector and the base of the mag mount, and there wasn't any at all -- complete open circuit.

So, I took the base apart.  Inside, the coax outer conductor was bolted directly to the magnet, but there was no foil or anything anywhere (factory stock), and between the magnet and the outside world was a thin layer of plastic (anti-scratch), but no foil in between.

The magnet was 100% non-conductive (like millions of Ohms) and the magnet itself evidently was intended to be the "capacitor."  It didn't do a good job of that.

Replacing the mount with one from Larsen took care of the whole problem.

[N8BOA]

XC=1/(2piFC)