On Wed, 11 Jan 2006 00:36:25 GMT, Cecil Moore wrote:


You already know what I am trying to say. I must not be saying it
very well. When a parallel cap is used on a 75m screwdriver antenna
to achieve 50 ohms, the screwdriver is tuned to 1/50 + j1/X, i.e.
slightly inductive. When a parallel coil is used, the screwdriver
is tuned to 1/50 - j1/X, i.e. slightly capacitive.

Negative susceptances are inductive. An inductive reactance of j5 is a
susceptance of 1/j5 or -j1/5.

I agree with your words, the sign of the admittances is wrong.

Owen
--

Owen Duffy wrote:

Cecil Moore wrote:
You already know what I am trying to say. I must not be saying it
very well. When a parallel cap is used on a 75m screwdriver antenna
to achieve 50 ohms, the screwdriver is tuned to 1/50 + j1/X, i.e.
^ should be -
slightly inductive. When a parallel coil is used, the screwdriver
is tuned to 1/50 - j1/X, i.e. slightly capacitive.
^ should be +
Negative susceptances are inductive. An inductive reactance of j5 is a
susceptance of 1/j5 or -j1/5.

I agree with your words, the sign of the admittances is wrong.

Yes, you are correct - sorry. But it now seems that you understand
what I was trying to say. If one takes an ordinary G5RV and
installs a parallel 1000pf capacitor at the coax/twinlead junction,
one will raise the resonant frequency and lower the SWR on the
coax for 75m operation. Very close to 50+j0 ohms can be achieved
on 75m through that simple act. When I lived in AZ, I switched that
cap in automatically using a relay and the frequency output signal
on my IC-745.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 11 Jan 2006 04:25:59 GMT, Cecil Moore wrote:


Yes, you are correct - sorry. But it now seems that you understand
what I was trying to say. If one takes an ordinary G5RV and
installs a parallel 1000pf capacitor at the coax/twinlead junction,
one will raise the resonant frequency and lower the SWR on the
coax for 75m operation. Very close to 50+j0 ohms can be achieved
on 75m through that simple act. When I lived in AZ, I switched that
cap in automatically using a relay and the frequency output signal
on my IC-745.

OK.

I played around a bit using the feedpoint impedances that I modelled
for my "Feeding the G5RV" article. With 31' of 554, I needed about
2000pF to "tune" it for low 50 ohm VSWR at 3.6MHz.

I plotted the impedance presented to the coax for a range of
frequencies from 3.5 to 3.8MHz, they are at
http://www.vk1od.net/temp/G5RV-W5DXP.GIF . The Smith chart is
normalised to 50 ohms. The solution seems fairly narrow band, the VSWR
at 3.55 was 6, at 3.6 it was 1.3, and at 3.65 it was 5.

Of course, implementations will have slight differences in actual
feedpoint impedances, and the outcome is very sensitive to slight
differences in feedpoint Z. This "no-tuner" matching scheme will
probably need significant customisation for each implementation.

Owen
--

"Owen Duffy" wrote
Of course, implementations will have slight differences in actual
feedpoint impedances, and the outcome is very sensitive to slight
differences in feedpoint Z. This "no-tuner" matching scheme will
probably need significant customisation for each implementation.


==========================================
The World-famous G5RV.
---------------------------------
What everyone appears to forget, is that Zo of the balanced twin-line
section, on all bands except at 14.15 MHz, has a considerable affect
on feedpoint impedances, swr, losses, etc.

When describing systems and performance nobody ever mentions what Zo
of the feedline actually is. Omission of Zo reduces any following
discussion to blythe, innocent nonsense.

R.L.Varney himself never gave a value to Zo. He didn't need to. He was
concerned mainly with 14.15 MHz. It would be unfair to accuse him of
not understanding the serious effects of Zo on other bands.
----
Reg.

Reg Edwards wrote:
R.L.Varney himself never gave a value to Zo. He didn't need to. He was
concerned mainly with 14.15 MHz. It would be unfair to accuse him of
not understanding the serious effects of Zo on other bands.

However, in The ARRL Antenna Compendium #1, he did describe the
matching section well enough to calculate his Z0. It is #14
copper open-wire separated by 1.75 inches. I'll bet that's
an improvement over 300 ohm twinlead.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
R.L.Varney himself never gave a value to Zo. He didn't need to. He
was
concerned mainly with 14.15 MHz. It would be unfair to accuse him
of
not understanding the serious effects of Zo on other bands.

However, in The ARRL Antenna Compendium #1, he did describe the
matching section well enough to calculate his Z0. It is #14
copper open-wire separated by 1.75 inches. I'll bet that's
an improvement over 300 ohm twinlead.
--
73, Cecil
==========================================

Cec, the facts are, G5RV never mentioned Zo, either because he never
attached any importance to it, or he didn't understand its relevance
to other bands.

When considering other than 14.15 MHz perhaps he should have done. In
any event, everybody else has followed suit.

Now we have the situation where dozens of people are busily seriously
comparing all-band versions, one with another, and unknown to each
other they are all using different value Zo transmission lines.

Many of those who purchased the antennas, or just copied G5RV's
construction, havn't the foggiest idea what their particular Zo is.
Yet, from an analysis point of view, which you are involved with, it
is a crucial matter.

Your own Zo is a non-standard 375 ohms because you measured it. Yet
you describe its performance to others as if everybody else's Zo is
the same, whereas there is no hope of anybody else reproducing your
particular results.

Its all a load of nonsense!

If anybody, at this very late stage in the art, should still wish to
acquaint themselves with the less than mediocre performance of a G5RV,
then download program DIPOLE3 from website below.

DIPOLE3 is a general purpose program which deals with a dipole of any
length, at any frequency, plus balanced-twin feedline of any length
and any Zo, plus balun, plus coax line of any length and Zo, plus
L-tuner. So it happens to include a G5RV.

All the data for the original G5RV can be inserted in the program by
depressing one key. Modifications to the system, such as changes in
Zo, can easily be done and changes in performance immediately seen.

The final important output figure is overall loss between transmitter
and radiated power. Individual losses in the antenna, in the two
transmission lines, and in the tuner, are reported seperately. Even
the tuner L and C settings and their circuit locations are predicted.

It is very easy to sweep over the HF frequency range to check when low
swr happens to fall into amateur bands. (Unfortunately it doesn't do
this very often.)

It is also easy to change the length of transmission line (as Cecil
recommends and advertises) to try to obtain an swr of less than 2:1.
But you can use any dipole length other than 102-feet to play with
this useful aspect.

Accuracy is better than needed for the intended purposes and is
generally as good as the accuracy of program input data. There is only
one known trivial bug which occurs when dipole length is extremely
short compared with wavelength. But clearly this is not of
consequence.

What more could you want from a G5RV? There's nothing to do but erect
it and then compare it with a dipole of any other length with an open
wire feedline of thick wire, of no particular length, all the way to
the shack. Plus a choke balun.

I'm still on Red, South African, Western Cape.

Download DIPOLE3. Its free to USA citizens.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg Edwards wrote:
Cec, the facts are, G5RV never mentioned Zo, ...

Now Reg, you know that most statements using the word
"never" are false. Here's a quote from G5RV, himself,
from "The G5RV Multiband Antenna ... Up-to-Date" in
The ARRL Antenna Compendium, Vol. 1.

"If you decide to use 300-ohm ribbon type feeder for
this (series) section, it is strongly recommended that
the type with 'windows' be used. ... Since the VF of
standard 300-ohm ribbon feeder is 0.82, the mechanical
length should be 28 ft. However, if 300-ohm ribbon with
windows is used, its VF will be almost that of open-wire
feeder, say 0.90, so its mechanical length should be
30.6 ft."
--
73, Cecil http://www.qsl.net/w5dxp

Owen Duffy wrote:
I played around a bit using the feedpoint impedances that I modelled
for my "Feeding the G5RV" article. With 31' of 554, I needed about
2000pF to "tune" it for low 50 ohm VSWR at 3.6MHz.

I'm just reporting what it took for my actual antenna under the
existing conditions at my QTH. The cap is actually 950 pf for
a minimum SWR of 1.3:1 on 3.9 MHz. The optimum value of the cap
would no doubt change at lower frequencies. With 22.5' of
Wireman #554 and a 950 pf cap, the 3:1 bandwidth is 145 kHz.
Adding sections of ladder-line lowers the resonant frequency.

Incidentally, this is a method for modifying the G5RV to work,
not only without a tuner, but with built-in tuners. When using
a built-in tuner, the antenna configuration doesn't have to
be changed as often. My IC756PRO will tune my present configuration
from 3.72-4.0 MHz. or 280 kHz.

I plotted the impedance presented to the coax for a range of
frequencies from 3.5 to 3.8MHz, they are at
http://www.vk1od.net/temp/G5RV-W5DXP.GIF . The Smith chart is
normalised to 50 ohms. The solution seems fairly narrow band, the VSWR
at 3.55 was 6, at 3.6 it was 1.3, and at 3.65 it was 5.

Changing the length of the series section will shift the resonant
frequency. I can vary mine from 22.5 ft. to 38.5 ft for a near-
perfect SWR on all HF ham frequencies.

Of course, implementations will have slight differences in actual
feedpoint impedances, and the outcome is very sensitive to slight
differences in feedpoint Z. This "no-tuner" matching scheme will
probably need significant customisation for each implementation.

IMO, that is what ham radio is all about - warm up the old MFJ-259B
and get with the program. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Owen Duffy wrote:
http://www.vk1od.net/temp/G5RV-W5DXP.GIF

Owen, what software did you use to generate that graphic?
--
TNX & 73, Cecil, http://www.qsl.net/w5dxp