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

Nope, that's just an off-the-shelf vanilla G5RV.

In that article, ...

Forget that article which only shows why the *standard* G5RV is
a fairly well matched antenna on 80m and 40m.

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?

I'm sure you know this already. Given an SWR circle on a Smith Chart
that crosses the horizontal resistive line at less than 50 ohms and
given the 1/50 conductance circle, those two circles will cross at
two points. Where they cross in the capacitive reactance region is
the point on the transmission line where a parallel capacitance will
bring the impedance at that point to 50+j0 ohms. This is a common
matching technique for 75m mobile antennas. The same thing can be
done with a coil installed where the circles cross in the inductive
reactance region. This technique is described in the ARRL Antenna
Book.

What I have done on my G5RV is find the point where the SWR circle
intersects the 1/50 conductance circle in the capacitive reactance
region on 3.8 MHz and install a 1000 pf parallel cap there. My series
section line is 22.5 ft. of Wireman #554 at that point. The 50 ohm
SWR is reduced from about 5:1 to 1.3:1 on 3.8 MHz.

Given an SWR circle crossing the 1/50 conductance circle, there's a
point where a cap will result in 50 ohms. A little farther, a cap
will result in 300 ohms. A little farther, a cap will result in
450 ohms, etc. These are the points just past the current maximum
point where one can hang a capacitive stub to achieve a purely
resistive impedance.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 11 Jan 2006 00:13:14 GMT, Cecil Moore wrote:

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

Nope, that's just an off-the-shelf vanilla G5RV.

In that article, ...

Forget that article which only shows why the *standard* G5RV is
a fairly well matched antenna on 80m and 40m.

But... in that article which recommends 28' of 300 ohm ladder line you
say "To improve the 75m SWR, try installing a 1000pF capacitor (mica
or doorknob) in parallel across the ladder line at the ladder line to
coax junction. Remove the capacitor for all other bands."

IMHO, just considering in isolation what is shown on that page there
is something inconsistent about the Smith chart, the impedances,
lengths, and assertions about the SWR improvement.

....

What I have done on my G5RV is find the point where the SWR circle
intersects the 1/50 conductance circle in the capacitive reactance
region on 3.8 MHz and install a 1000 pf parallel cap there. My series
section line is 22.5 ft. of Wireman #554 at that point. The 50 ohm
SWR is reduced from about 5:1 to 1.3:1 on 3.8 MHz.

This implies you are trying to "tune out" the shunt capacitive
reactance at a point on the line where the shunt resistive component
is 50... but you need the opposite sign of reactance reactance (so
that the susceptances subtract), you need an inductive reactance in
that case.

If "your G5RV" has a feedpoint impedance of 36-j324 (that seems
reasonable), your 22.5 ft. of Wireman #554 will transform that to
21.53-j53.33, and the VSWR in 50 ohm line connected at that point
would be 5.3.

A shunt capacitance CANNOT improve the 50 ohm VSWR at that point

The effect of the shunt 1000pF capacitance is to change the impedance
at the junction to around 3.6-j25, which would cause a VSWR of around
17 in the 50 ohm line.

However:

If the ladder line was around 31' in length, then the Z at that point
would be around 21+j25 (equivalent to 50 ohms R in parallel with +43
ohms X), and a shunt 1000pF (~ -42 ohms X) capacitor would give nearly
perfect VSWR on the 50 ohm line.

In summary, in a general sense, if you want to use a shunt capacitor
as you propose, you need to find length of line such that the
admittance at that point is 1/50-jB (negative susceptance is
inductive), and the correct shunt capacitor has a reactance of 1/B.

Flawed explanation aside, the only way that 22.5' works is if your
feedpoint Z is quite different to 36-j324.

Owen
--

Owen Duffy wrote:
But... in that article which recommends 28' of 300 ohm ladder line you
say "To improve the 75m SWR, try installing a 1000pF capacitor (mica
or doorknob) in parallel across the ladder line at the ladder line to
coax junction. Remove the capacitor for all other bands."

Yes, but installing the cap will raise the resonant frequency.

What I have done on my G5RV is find the point where the SWR circle
intersects the 1/50 conductance circle in the capacitive reactance
^^^^^^^^^^
region on 3.8 MHz and install a 1000 pf parallel cap there. My series
section line is 22.5 ft. of Wireman #554 at that point. The 50 ohm
SWR is reduced from about 5:1 to 1.3:1 on 3.8 MHz.

This implies you are trying to "tune out" the shunt capacitive
reactance at a point on the line where the shunt resistive component
is 50... but you need the opposite sign of reactance reactance (so
that the susceptances subtract), you need an inductive reactance in
that case.

Sorry, I misspoke. Where I said "capacitive reactance region"
above, it should have been "inductive reactance region".

If "your G5RV" has a feedpoint impedance of 36-j324 (that seems
reasonable), your 22.5 ft. of Wireman #554 will transform that to
21.53-j53.33, and the VSWR in 50 ohm line connected at that point
would be 5.3.

"If" is the important word. My G5RV is obviously different from your
values. It's made out of insulated wire and I'm not sure it is exactly
102 feet long.

Flawed explanation aside, the only way that 22.5' works is if your
feedpoint Z is quite different to 36-j324.

And it is obvious that's the case. The "450" ohm ladder-line is 22.5 ft.
long and a 1000 pf capacitor resonates it on 3.8 MHz. Whatever the
feedpoint impedance needs to be to cause those conditions, that's
what it is.
--
73, Cecil http://www.qsl.net/w5dxp