[Elecraft] KX2

[Guy Olinger K2AV]

We need to distinguish between an end fed wire (EFW) and an end fed
HALF WAVE wire (EFHW). If one has an actual half wave wire being
called by the name EFHW, several things are true. One, the feed Z at
the end is ridiculously high, with HV and painful burns in play even
for 100W. Many (most?) antenna tuners do NOT handle Z that high, while
most tuners DO handle 600 ohm Z from an EFW well, particularly with
common 4:1 reduction devices.

One of the advantages of an EFHW fed at the ground, is that voltage
being very high means that current is very low, and even otherwise
pathetic grounds are efficient. You CAN feed an EFHW against a ground
rod. In a portable situation something like one of those six inch
galvanized nails driven mostly into the ground and soaked with a
little water, will settle the counterpoise issue nicely.

On 80 meters an end fed half wave L (EFHWL) fed against ground is
probably the best all around performing single wire 80m antenna
available. It's lack of popularity is due to not being able to
successfully feed it directly from coax and lack of tuner boxes off
the shelf Needs a matching network at the feedpoint, and at QRO, has
to deal with very high RF voltages. It has dual polarization, working
for both DX and local, with no holes in the pattern, and the current
maximum is up around the bend in the L, reducing vertical polarization
loss to both ground and nearby clutter.

On the other hand an end fed wire (EFW) that stays well away from even
quarter wave multiples on bands in use, particularly with a 4:1
reduction device, will tune easily. The disadvantage with EFW is that
pathetic ground no longer functions well as a counterpoise. The tuner
needs to isolate ground and the case from the antenna side tuner
connections, one of which needs to be a planned counterpoise, and the
isolation needs to be efficient (not lossy) with high reactance on the
antenna side.

You can get RF through lesser arrangements and make contacts, but you
are paying for the "lesser" with loss.

I agree that EFHW's are "tweaky". IMHO more than made up for by
performance. EFHW "tweaky" *can* be tamed to considerable degree, but
that's another subject.

73, Guy K2AV


On Thu, Jul 14, 2016 at 8:26 PM, David Gilbert <xdavid at cis-broadband.com> 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