[Antennas] Only dumb question is the one not asked

[fkamp@attbi.com]

RF current flowing on the outside of a coaxial feedline is
not a normal situation.  It is caused by an abnormal
condition.  You get that by connecting a balanced antenna
(dipole) to an unbalanced feed line (coax).  Yes, you can
reduce and even eliminate the unwanted current (and
radiation) by using a choke at the antenna connection.
However, a better solution is to install a balun at the
antenna and connect the feedline to the balun.  This
eliminates the cause of the common mode current and results
in a non-radiating feedline as well as a more efficient
transfer of power to the antenna.

Is this realy needed?  Well, if you are concerned about
common mode currents running down the coax and causing
radiation and hot spots in the shack, yes.  It is
important.  

I have found that it really only becomes important if your
power levels are over 300 watts (output) or in situations
where every fraction of a percentage point in efficiency
makes a difference.

It does not matter much at power levels around 100 watts or
so.  At that level, I have not seen much difference in
connecting the coax directly, using the decoupling choke
trick or going with a balun.

Regards,
Frank Kamp
K5DKZ
 

Harvey&Bessie wrote:
> 
> The impedance right at the feed point is (when SWR is low) close to 50
> ohms. the coiled co-ax near the feed point forms an RF-choke only for
> the outer conductor (the field of the inner conductor is entirely
> contained within the outer conductor acting as a shield) so, the RF
> "seeing" a higher impedance (several hundred ohms) in the path down the
> outer conductor, as compared to the 50-ohm impedance of the inner
> conductor, does not travel down the outside of the outer conductor.
> As for connecting a "ground" directly to the outer conductor at the feed
> point -- how would you propose doing that? The antenna, hopefully, is
> elevated some distance from earth -- the source of any true ground
> connection -- any wire lead you connect there has to have considerable
> length to get to "ground." It is still ground for DC, but not for RF. In
> fact if it is exactly (or nearly) one-quarter wavelength long it is no
> ground at all but an open circuit at RF!
> Harvey/W4TG
> 
> - - -
> 
> Your moderator for this list is:
> Larry Wilson KE1HZ antennas-owner@mailman.qth.net
> _______________________________________________
> Antennas mailing list
> Antennas@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/antennas