Yup ... that all makes sense. The counterpoise as described by that
link is at the antenna where it should be, and it compensates for the
fact that the network can't produce an infinitely high impedance feed.
I stand corrected.
Using the coax for a "counterpoise" is a really bad idea, though. There
would essentially be no defined counterpoise. There would be no
control at all over what kind of balance the coax provides to the
matching network at the end of the EFHW. Length of the coax would have
an effect, and not just the requirement for some minimum length ... the
transmission line effect of the length would change its characteristics
as seen by the antenna. Proximity of the coax to nearby structures
would also affect what the antenna actually saw.
73,
Dave AB7E
On 7/14/2016 4:56 PM, Don Wilhelm wrote:
Dave,
All that is theoretical does not translate directly to the physical
world.
Take a look at http://www.aa5tb.com/efha.html for more information.
The counterpoise does not need to be very long, but it does need to be
present.
If the coupling between the high impedance side of the matching device
is connected at the "ground side" to the shield of the coax, then the
coax shield can act as the counterpoise (with the attendant risk of
RF-in-the-shack), but does require some minimum length of coax to be
effective - the PAR EndFedZ is one example.
73,
Don W3FPR
On 7/14/2016 5:40 PM, David Gilbert wrote:
I'm confused why an EFHW should need a counterpoise. If it needs a
counterpoise it isn't actually acting like an EFHW. If it needs a
counterpoise that means there isn't enough choking impedance at the
feedpoint, and it means that the feedline is radiating with the
counterpoise acting as ... well, a counterpoise.
Manufacturers state a minimum length feedline simply to have the
feedline losses help swamp out SWR variations along the line, and of
course to marginally lower the SWR.
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