Dave,
Most of the commercial end fed halfwave antennas are designed for low
power (100 watts and under). So the use of the coax shield as the
counterpoise element is not a big problem in most cases.
Add to that, the EFHW is an easy solution for portable operation - an
easy up effective antenna.
At the home station where more than QRP power is common, that is where
the difficulties with EFHW antennas become more problematic. In other
words, RF-in-the-shack at QRP levels may not be noticable, but at higher
power levels, it can cause problems.
That being said, I can run my LNR/PAR EndFedZ for 40/20 meters at 100
watts with no apparent problems. I would not want to push it much higher.
73,
Don W3FPR
On 7/14/2016 8:26 PM, David Gilbert wrote:
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.
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to xda...@cis-broadband.com
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to donw...@embarqmail.com
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
