On Tue, 10 Jan 2006 21:14:40 GMT, Owen Duffy wrote:

On Mon, 09 Jan 2006 15:50:20 GMT, Cecil Moore wrote:

Ricke wrote:
If SETUP right , There the best HF I've ever used.

Maybe the only one? :-) The G5RV, with tuner, is a pretty
good 80m, 40m, 20m, and 12m antenna. If the series section
is varied from 20 feet to 36 feet, it becomes a very good
all-HF-band antenna. With the addition of a parallel 1000pf
capacitor with the series section at 22 feet, on 75m my "G5RV"
has SWR of 1.3:1 and works as well as a 75m 1/2WL dipole.

Is this the antenna described at http://www.qsl.net/w5dxp/G5RV.HTM ?

I have made a mistake during my analysis, let me try again:

In that article, on 75m you model a feedpoint impedance of 36-j324,
28' of 300 ohm ladder line, for a Z of 15+j4 (seems to indicate 48.2
deg length of 300 ohm line with 0.007dB loss (optimistic)).

At that point, were 50 ohm coax connected directly, the VSWR at the
load end of the 50 ohm coax would be 3, however you shunt the 17+j4
with 1000pF to give a new Z of 17.3-j3.0 that results in a VSWR at the
load end of the 50 ohm coax of around 2.9, almost identical to the
case without the capacitor.

Presumably when you say that the capacitor improves the VSWR on 75m,
you mean the VSWR on the coax. Did I miss something, how does the
capacitor improve the VSWR on 75m?

Owen

PS I couldn't make the numbers work for 22' as in your quote, where I
got a VSWR at the load end of the coax of 27. I couldn't see where the
VSWR of 1.3 comes from?
--
--

Owen Duffy wrote:

On Tue, 10 Jan 2006 21:14:40 GMT, Owen Duffy wrote:


On Mon, 09 Jan 2006 15:50:20 GMT, Cecil Moore wrote:


Ricke wrote:

If SETUP right , There the best HF I've ever used.

Maybe the only one? :-) The G5RV, with tuner, is a pretty
good 80m, 40m, 20m, and 12m antenna. If the series section
is varied from 20 feet to 36 feet, it becomes a very good
all-HF-band antenna. With the addition of a parallel 1000pf
capacitor with the series section at 22 feet, on 75m my "G5RV"
has SWR of 1.3:1 and works as well as a 75m 1/2WL dipole.

Is this the antenna described at http://www.qsl.net/w5dxp/G5RV.HTM ?


I have made a mistake during my analysis, let me try again:

Now you tell me after I spent 15 minutes replying to it. :-)
The details are there so I won't repeat it here.

In that article, on 75m you model a feedpoint impedance of 36-j324,
28' of 300 ohm ladder line, for a Z of 15+j4 (seems to indicate 48.2
deg length of 300 ohm line with 0.007dB loss (optimistic)).

At that point, were 50 ohm coax connected directly, the VSWR at the
load end of the 50 ohm coax would be 3, however you shunt the 17+j4
with 1000pF to give a new Z of 17.3-j3.0 that results in a VSWR at the
load end of the 50 ohm coax of around 2.9, almost identical to the
case without the capacitor.

Presumably when you say that the capacitor improves the VSWR on 75m,
you mean the VSWR on the coax. Did I miss something, how does the
capacitor improve the VSWR on 75m?

What you missed is that the frequency must be changed to obtain the
benefit. The capacitor is *not* installed at the 17+j4 point. It is
installed at the 1/50 + j1/X admittance point. You can either increase
the length of the feedline past the 17+j4 point to the 1/50 + j1/X
admittance point or increase the frequency thus electrically lengthening
the feedline to the 1/50 + j1/X admittance point. You cannot keep both of
those values constant as you tried to do above.

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.
--
73, Cecil http://www.qsl.net/w5dxp

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

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