[K3PZN-List] Re: J-pole question

[Scott, KB3JQQ]

An antenna radiates EM waves. These are created when magnetic field
lines move. The current moving in the radiating portion must have
somewhere to go. This is going to be the groundplane, the other half
of the dipole, the short end of the Zepp or j-pole. In the worst case,
it flows on the outside of the coax shield. Think of antennas as air-gap
transformers with lots of space between the windings. If you want to get
out you must flow some current in your winding!




--- "ke3fl at juno.com" <ke3fl at juno.com> wrote:

> The J-pole antenna is fed between the two connections which form a J,
> ANYTHING below the J is NOT in the equation, period. That is what makes
> the J-pole such a wonderful antenna for mounting, you can mount it on
> ANYTHING, wood etc or metal.
> 
> Why is that?  Well, here's a very simple idea, (Gedanken Experiment)
> remember how to figure the current flow in a DC circuit? If you add a
> piece of random wire to the circuit but it doesn't go anywhere, does
> current flow on it? No.
> 
> In sort of the same way, an antenna has current on the part which is
> "fed" between the two points in this case & extending (in this case)
> up, anything below the center wire connection is ignored because the
> current can see & go to the shield connection, there is no "reason" for
> current to go anywhere else in that direction.
> 
> Sorry, this is very simple & leaves much out & even probably has things
> "wrong" but it's not a bad way to look at it.
> 
> Hope that helps.
> 
> Oh, if you mount a ground-plane antenna on a metal pole does the metal
> pole act as part of the antenna? No, for similar reasons.
> 
> 73 de ke3fl
> Phil K



 
____________________________________________________________________________________
Sponsored Link

Mortgage rates near 39yr lows. 
$510k for $1,698/mo. Calculate new payment! 
www.LowerMyBills.com/lre