Dave Platt wrote:
In article .com,
wrote:

Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you
stated,
the PVC will change the input Z of the radiator. Just about
anything
that is too close to the radiator will change things.

On second reading of that QST article, I admit to being a bit
curious.
The K4LPQ version (with a shorted inductive stub inside the radiator,
soldered to it) is clear enough. However, the W9WQ version using a
longer, open-circuited inductive stub wire isn't a straightforward
translation of this, because there's no soldered (or other DC)
connection between the radiator and anything else!

I infer that in the W9WQ version, the stub wire is performing two
functions at once - it's adding a series inductance, and it's also
coupling the RF out onto the radiator in a capacitive fashion. This
would imply that the stub needs to provide a bit more inductive
reactance than in the K4LPQ shorted-coax version, with some of this
reactance cancelling out the radiator's -j300 and the rest cancelling
out whatever amount of capacitive reactance exists between the stub
wire and the radiator.

Am I reading this right, or am I missing something?

[Regretfully it seems likely that both W9WQ and K4LPQ are now silent
keys, so I can't ask for advice from the horses' mouths.]

According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above
ground
is 80-j300. As such, with a single series inductor matching
section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.

I've seen a number of 5/8-wave antenna designs which deal with this
issue by using something other than a simple series coil. The
commonest approach seems to be to use a coil which is connected in
shunt between the radiator and the ground plane, with the "hot" side
of the coax being fed to a point tapped partway up on the coil. This
approach transforms the radiator impedance down to the 50 ohms needed
to match the coax, and also provides the series inductance needed to
cancel out the reactance. It also provides DC grounding for the
radiator.

http://www.fluxfm.nl/schema/5-8%20go...20radialen.PDF is one
such design. It requires some amount of tooling (e.g. to lathe down
the plastic parts to the specified configuration) but I suspect that
a
version could be homebrewed up using simpler materials and methods.
The photos show the way to build the tapped matching coil assembly,
and could probably be adapted to other coil construction methods. I
believe the ARRL Handbook article which adapts a Radio Shack CB
mobile
antenna uses a similar tapped coil.

Another approach might be to change the length of the radiator a bit,
to change the resistive part of the feedpoint impedance from 80 ohms
down closer to 50 ohms, and modify the coil to suit. I haven't run
any simulations to see how much change in the radiator length would
be
required, and what this change would do to the antenna's gain
pattern.

There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending
on
the application, is NOT the best performing radiator.

I'll have to look it up if I can find a copy of that edition to see
what they have to say. I agree, in some cases the horizon-directed
gain of a 5/8 isn't what you want. For in-city and in-the-hills
mobile use, a 1/4-wave might give more reliable performance,
precisely
because its RF energy is more broadly directed.

--
Dave Platt
AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Hi Dave, I'm sure you are reading it right, will look and see if I can
add anything later.
Gary N4AST