[Antennas] A Question on Open Wire Feeders

[Robert Lay - W9DMK]

On Wed, 28 Jul 2004 22:30:25 -0700
 "Kevin Nathan" <k7rx at comcast.net> wrote:
>  Recently, I changed out a coax fed, trap dipole for 80 and 40 meters
> in
> inverted vee configuration which I primarily use for local comms to a
> dipole
> with 60 feet per leg fed with 450 ohm ladder line, also put up in
> inverted
> vee configuration but oriented perhaps 45 degrees differently than
> the trap
> version.  I notice this antenna works much better on 30 and 17 meters
> and
> signals seem to be as strong locally on 80 and 40 as they were on the
> trap
> dipole.  What I do notice however is a significant increase in pickup
> of
> local noise sources.  I am using a current balun hooked to the wall
> outside
> my shack with coax coming through the wall to the tuner.
> 
> The question I have is why would this particular configuration pick
> up so
> much more local noise than the coax fed trap dipole?  I know this
> feed line
> is
> superior to what I had and it is a much better multiband antenna but
> I
> honestly don't know why this change should have taken place.  From
> what I
> remember of inverted vee patterns, it doesn't seem that a slight
> change in
> orientation could make this big a difference either.  I would
> appreciate any
> feedback on this one you all might see fit to give me.

Dear Kevin,

In my opinion, the dipole fed with open wire line or with ladder line
has great potential for acting as a vertical with a large capacitance
hat. That is, the dipole itself is just acting as the capacitance hat
while the feedline is acting as a short vertical. That entire system,
which looks like a vertical, is capacity coupled through your tuner to
the receiver. As you well know, verticals do pick up a much greater
level of noise that do Herzian antennae. A couple of postings have
mentioned such things as putting a twist in the line or re-orienting
the system to be further from the noise sources. In my opinion such
will be totally ineffective as most of the noise you are getting is
from greater distances and the twist will do no good at all. 

If there is a cure for this problem it would be to use a good parallel
tank tuner, such as the Johnson Matchbox tuners, and if necessary
install a faraday shield so as to eliminate the capacity coupled signal
into the receiver. 

Or, you can just ignore the whole thing, since it won't make that much
difference, unless you are working with very weak signals.

73 de W9DMK (Bob Lay in Dahlgren, VA)
http://www.qsl.net/w9dmk