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Old 03-29-2011, 11:45 AM
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Inukshuk Inukshuk is offline
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Join Date: Aug 2005
Location: Denver, CO
Posts: 3,604

Originally Posted by DaveInDenver View Post
Being ground plane independent is a misnomer, an end fed 1/2λ antenna still needs a return path. Being 1/2λ just means the antenna is self resonant and does not need the counterpoise to radiate RF.

The feedpoint impedance will be VERY high and can develop a pretty sizable voltage (at 50W it can be 250 to 500V pretty easily, the theoretical feedpoint impedance is infinite and practically ends up >1800 ohms). This is a particular problem if you don't give the antenna a balanced feed, i.e. when you use coax and mount it with zero ground plane. It is critical that the antenna mount be grounded. This is why Larsen recommends an NMO mounted in the roof of your car or artificial ground radials. Trust me, you can get a pretty good tickle from outside of an uninsulated floating 1/2λ feedpoint...

BTW, this is not common mode current per say, but a byproduct of the actual differential counterpoise currents (consider it the 3rd path on the outside of the shield, you will have current on the center conductor and the inside of the outside conductor). So choking the shield is a band-aid for antenna inefficiency. OTOH, without a choke the coax will be radiating, which is never good.

This is the problem with unbalanced feedlines, resonant antenna currents are always differential and there has to be a conversion somewhere if you feed with unbalanced cable (coax). If you don't provide it, RF will figure it out for you (it finds the assumed capacitively coupled return) but it might not be ideal. Antenna manufacturers know this limitation and build in LC tanks to couple the feedpoint (transform impedances) to the unbalanced feedline, but that circuit is assumed to be referenced to RF ground. It should be mentioned that it's almost impossible to have a 100% balanced system, so chokes are absolutely acceptable to minimize common mode currents on coax. Just key to understand that choked currents are lost to heat and are not doing any work for you.

Thus, this is why people drill NMO holes or bolt directly to the body of the truck. IOW, if you are mounting the antenna electrical isolated from your RF ground the advice to use braid to ground the outer ring of the mount is very good indeed.
Great advice. I just don't have the brainpower to understand it.....
Daniel Markofsky - KFZJ
Denver, Colorado, TLCA # 9972,
1993 FZJ80 "enhanced", re-built Engine, and prototype GOBI Stealth roof rack
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