On Mon, 17 Oct 2005 16:05:54 GMT, Cecil Moore wrote:

Owen Duffy wrote:
More importantly, if you found any faults with my analysis, let me
know?

Just found one and replied to your posting. You and Cebik
seem to disagree about 12m where you say it won't work on
any WARC band and he says the impedance at the twinlead/coax
junction is "Resistive (90-100 Ohms)", i.e. SWR=2:1

No, I said:

"Contrary to oft stated views, the Classic G5RV is not an all band
antenna, it:
* does not have acceptable feed performance on 1.8MHz;
* does not have acceptable feed performance on any WARC bands;
* does not have acceptable feed performance on 28MHz."

Two main factors were in my mind in determining whether performance
was "acceptable", they we
- the magnitude of the losses on this particular model in-band for the
band in question; and
- whether the losses changed rapidly on adjacent frequencies, such
that installations with small differences (eg height, earth parameters
etc) were not assured of acceptable losses.

For example, it is possible to tweak a G5RV a little to minimise the
feed losses on 10.1MHz, and they can be low enough, but it is a very
sharp notch and likely to be not realisable with seasonal changes in
soil moisture etc.

I have reviewed the specific case you mention about 25MHz. Two of the
feed arrangments are marginally under the criteria that I set, but
they are under it and for consistency they should have been included
in the "acceptable" category. Omitting them was an error on my part.

I have ammended the article accordingly Cecil, thanks for the
feedback.

It is important to keep in mind that the article is analysis of a set
of common feed configurations of a dipole at a specific height over
specific ground, and that the results may vary in other
configurations.

Owen
--

On Mon, 17 Oct 2005 05:27:08 -0700, Wes Stewart
wrote:


A bit of mutual citation, bit obvious isn't it when there isn't some
indirection like a few intermediate authors / articles!

....

Clearly you have missed the humo(u)r that I intended.


Wes, I did take it as light hearted, and hence my response above.

I am not an academic, nor do I have the experience of that university
environment, but I think there is probably more than a little humour
to be had from examples of self-citation, mutual-citation,
self-citation indirectly by n levels (more subtle than mutual
citation)... etc.

Thing that had my head spinning was trying to follow the logic (I use
the term loosely of course) of some recent threads.

Owen
--

Owen Duffy wrote:
I have ammended the article accordingly Cecil, thanks for the
feedback.

So one more myth is busted. You had me thinking that I was the
only one capable of making a mistake.
--
73, Cecil http://www.qsl.net/w5dxp

You are proposing a slightly shorter than 1/2 wave dipole for 75.
This should be no problem as it will have a little capacitive
reactance and a tuner should handle it.

Thank you, Steve. This was my main concern.


The G5RV is pretty much the same thing, except it has some feed line
gymnastics to get a "fairly good" (50 ohm) match on many bands. This
is nothing more than an "antenna tuner in feed line" which doesn't
require a knob rather than one in a box which does. (:-)

I have pretty much decided to just feed ladder line with a balanced
tuner rather than make a G5RV. This should give me more flexibility.


Ed

That's a very good length for 40m, 17m, & 10m. You can analyze the
antenna yourself by downloading the free demo version of EZNEC
from www.eznec.com. If you want, I'll send you a model of your
antenna so all you have to do is click the mouse.


Thanks for the offer, Cecil, but at this time I will decline. I plan
on just putting up as much wire as I can effectively mount between the
two trees, about 100 feet, and just feed it at the center with ladder
line. A new balanced tuner will tune the whole thing. I expect this to
give me a bit more flexibility than the G5RV would.


Ed K7AAT

Ed wrote:
Thanks for the offer, Cecil, but at this time I will decline. I plan
on just putting up as much wire as I can effectively mount between the
two trees, about 100 feet, and just feed it at the center with ladder
line.

That's the antenna I was offering to model for you. You will,
no doubt, run into some pitfalls, for instance an impedance
of 8 ohms or 4000 ohms that your tuner may not be able to handle.
EZNEC can predict such problems before they bite you.

As a data point, the MFJ-974 gives you a matching range of 12-2000
Ohms.

EZNEC can also predict your radiation patterns which are important
on the high bands if you want to hit certain areas of the world.
Rotating my dipole in AZ by 15 degrees made a lot of difference.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 18 Oct 2005 03:18:39 GMT, Cecil Moore wrote:

Ed wrote:
Thanks for the offer, Cecil, but at this time I will decline. I plan
on just putting up as much wire as I can effectively mount between the
two trees, about 100 feet, and just feed it at the center with ladder
line.

That's the antenna I was offering to model for you. You will,
no doubt, run into some pitfalls, for instance an impedance
of 8 ohms or 4000 ohms that your tuner may not be able to handle.
EZNEC can predict such problems before they bite you.

As a data point, the MFJ-974 gives you a matching range of 12-2000
Ohms.

EZNEC can also predict your radiation patterns which are important
on the high bands if you want to hit certain areas of the world.
Rotating my dipole in AZ by 15 degrees made a lot of difference.

For any given frequency, would that 8 ohms or 4000 ohms be decided by
the length of the dipole or the length of the ladderline, or both?

bob
k5qwg

Bob Miller wrote:
For any given frequency, would that 8 ohms or 4000 ohms be decided by
the length of the dipole or the length of the ladderline, or both?

Both. For instance the resonant impedance of a G5RV on the lower
part of 80m is usually about 8 ohms. If the feedline for a one
wavelength dipole is 1/2WL, the impedance will be about 4000 ohms.
The feedline impedance seen by your tuner is somewhat unpredictable
and EZNEC can tell you what ballpark impedance to expect. EZNEC
has saved me an enormous amount of time - 10 minutes of simulation
Vs 10 hours of antenna erection time.
--
73, Cecil http://www.qsl.net/w5dxp

The loss in a switched transmission line tuner is somewhat greater
than in a conventional tuner which can be adjusted for an exact,
low-loss, impedance match.
---
Reg.

That's the antenna I was offering to model for you. You will,
no doubt, run into some pitfalls, for instance an impedance
of 8 ohms or 4000 ohms that your tuner may not be able to handle.
EZNEC can predict such problems before they bite you.

OK. I'll take you up on the offer, afterall. Figure 100' of #14
copperweld, centerfed with 450ohm ladderline or with 600 ohm open wire
line (2"). Height should be about 45' at ends and 35 feet center.
Length of feedline is projected at about 45 feet.

I'll go with the 600 ohm wireline feed if I can find it, otherwise,
the cheap 450 ohm plastic stuff for now and build wireline next summer.



EZNEC can also predict your radiation patterns which are important
on the high bands if you want to hit certain areas of the world.
Rotating my dipole in AZ by 15 degrees made a lot of difference.

Not real concerned with the patterns of anything above 40M. I'm
mainly interested in 80/75/60/40M use. I'll take what I get and see
how it goes..... might be nice to see 20M and 10M patterns, though.
Thanks.


Ed