Owen Duffy wrote:
On Sun, 01 Oct 2006 14:22:40 -0400, Walter Maxwell
wrote:


On Sun, 01 Oct 2006 01:12:02 -0400, jawod wrote:


Hi Owen--great job! But you said "comments welcome", so please don't be offended
by what I have to say.


Walt, the intention was to seek constructive comment. Of course, there
is always the risk of personal comment... but that is part of the cost
of technical review as you know.


I have always been curious about all the hype, excitement, and marketing
popularity of the G5RV, So I included a section on this antenna in both editions
of Reflections, ed 1 in 1990, and ed 2 in 2001, in an attempt to educate the
newcomers to its realities. So I invite you to read the pertinent section from
the book below:


...


The following is a quote from Chapter 20 in "Reflections-Transmission Lines and
Antennas," authored by W2DU.

"Sec 20.2.4 The G5RV Antenna

With this background on random-length dipoles behind us, it seems
appropriate to make a critical examination of a particular 102-foot dipole that
is enjoying a great deal of popularity--Louis Varney's G5RV dipole. In spite of
its popularity, its operation is not well understood among many amateurs, so
I'll shed a little light on the G5RV. First of all, the reason for the 102-foot
length for the G5RV is no secret, but it is not well known. Being unaware of
certain antenna principles, many amateurs have come to believe that there is
some sort of magic in the 102-foot length, and that their all-band success with
this antenna is dependent on this specific length. Nothing could be further from
the truth, because, except for 20 meters (as I'll soon explain), any random
length of at least 3 lambda/2 long at the lowest operating frequency will
perform equally well.


I am not sure if "3 lambda/2 long at the lowest operating frequency"
is what you really meant... perhaps I am misunderstanding it.

Copying a quote from one of my articles, "my experience is that where
a centre fed dipole less than about 0.35 wavelengths in length, it is
difficult to achieve acceptable feed system efficiency in practical
configurations". In fractional terms, I would state that minimum
length as about 3/8 wavelength.


What is the significance of the 102-foot length? Unbeknown to many amateurs


...

I heard this story from an amateur friend (now deceased) who knew
Louis in the old country, and he used chuckle at the newfound role of
the G5RV as an efficient all band antenna (which it isn't).

I think there is a significant proportion of the Ham population that
refers to just about *any* wire antenna as a G5RV. I've heard about half
wave G5RV's, Coax fed, ladder line fed, (as in doublet) lots of
variations! ;^)


It is unfortunate that many amateurs believe that the balun should be omitted.
These people have been misled, because failure to include a balun between the
balanced open wire and the unbalanced coax results in RF radiation in the shack

from current flow on the outer surface of the coax shield.

Agreed. Unfortunately Varney confused the issue with his later article
that rescinded his advice to use a balun.

I think the words in may latest article "Varney originally described
the G5RV with a balun at the coax to parallel line transition, and
changed his mind in a later article due to uncertainty about the balun
design. More has been learnt of baluns and antennas in the meantime,
and there is no doubt that inclusion of an effective choke balun at
that transition will assist in minimising feedline contribution to
radiation, and conversely, feedline pickup." are valid.



Owen, I didn't include this to detract from your excellent work--you've done a
great job. But my position is that since an antenna tuner is necessary anyway
for the antenna to be multibanded, why insert any coax at all?


Agreed Walt, and no offence taken. Discussion of the issues is what
our hobby is (or was) about.

Our dear departed friend Reg would hop into me when I wrote about
G5RVs suggesting that I sold the things, that my perspective was that
of a salesman and that I was not detached. He was wrong, I have
erected a G5RV for the purpose of experiencing the thing, of learning
about it, but the trade-offs involved in a G5RV haven't suited my
interests for more than that few hours.

The graphs in my two G5RV articles are based on thousands of model
outcomes, starting from NEC models of the radiator in a typical
configuration, and then exact transmission line models and tuner
models.

The thing that I have learned is that most of the loss in typical
configurations is in the coax, the loss in the coax is driven by what
it happening on the radiator and open line section, and that reduction
of coax loss is the key to efficiency. An obvious way to reduce coax
loss is to reduce (even eliminate) the coax (though a systems
perspective shows that you don't just deduct the coax loss, ATU loss
will increase marginally at some frequencies). That is the subtlety in
my words "This article deals with optimising a typical G5RV (as
distinct from an optimal G5RV)" The theme being that if you have a
typical G5RV, here is how to go about getting the best out of what you
have, if you want to improve it, read the other article linked at the
bottom the page.

Perhaps getting the best out of it is to lower it to about 4 feet, and
use it as a cattle fence? As noted above, we might as well eliminate the
coax altogether.

- 73 de Mike KB3EIA -