For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big
HF yagi on its top). The home-made tuner, placed at the tower base, has a
motorized variable vacuum capacitor in series and a (properly selected) fixed
capacitor in parallel. I can so remotely tune the variable capacitor across the
3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the
band. The antenna works fine and I receive good reports.

The only thing which worries me a bit is the very high voltage that develops at
the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to
about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be
a source of significant losses.

To solve my doubt, I started by precisely measuring the capacitance of the tuner
capacitors, and I could then easily calculate the antenna impedance at 3.500
MHz, which resulted to be (18.2 + j 656) ohm.

ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current flowing through
the antenna is about 10.5A, quite higher than the 6.3A figure one would get
should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect
some more loss in the conductors due to the fairly high current.

ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance causes the
antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with
a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am
wondering whether such a high voltage could constitute, by itself, a source of
extra loss. Please note that the wire coming down from the tower is connected
directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do
not see a place where power can get dissipated due to the high RFvoltage, other
than perhaps in the humid air (?).

I could probably avoid this situation by changing the tap height on the tower,
but I would run the risk of not being any longer able to tune across the whole
3500 - 3800 kHz band adjusting one of the two capacitors, and not both.

Any idea on whether the high RF voltage present on the antenna could cause some
significant loss?

Thanks and 73

Tony I0JX
Rome, Italy

On Fri, 9 Sep 2011 21:10:08 +0200, "Antonio Vernucci"
wrote:

For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big
HF yagi on its top). The home-made tuner, placed at the tower base, has a
motorized variable vacuum capacitor in series and a (properly selected) fixed
capacitor in parallel. I can so remotely tune the variable capacitor across the
3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the
band.

Hi Antonio,

Describe the "shunt." (I presume it is a long wire from the tuner,
standing off 10cm or so, up maybe 5 - 10 meters where it attaches to
the tower? Can I also presume that the tower goes to ground at the
base?)

The antenna works fine and I receive good reports.

And so it would seem this would be the end of the post. But no.....

The only thing which worries me a bit is the very high voltage that develops at
the antenna end

End? There are at least two, and perhaps you mean the end of the
shunt at the tuner? If by "end" you mean up at the very top, then
high voltage would be expected (but worry about it would not be
expected).

By this point, there are a lot of gaps in the description (at least
for me).

(about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to
about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be
a source of significant losses.

You worry about loss, when you receive good reports?

High voltage reveals loss in heat. Corona would be an example, but
you don't describe corona. You also don't describe heat.

To solve my doubt, I started by precisely measuring the capacitance of the tuner
capacitors, and I could then easily calculate the antenna impedance at 3.500
MHz, which resulted to be (18.2 + j 656) ohm.

I think you have a sign problem. You are describing a short antenna
with inductive reactance (given the height and wavelength, very
improbable).

However, this might be the feedpoint impedance at the lower end of the
shunt - maybe, maybe not.

ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current flowing through
the antenna is about 10.5A, quite higher than the 6.3A figure one would get
should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect
some more loss in the conductors due to the fairly high current.

Only in the shunt wire. Only if you are using wirewrap wire. Measure
the RESISTANCE. Multiply by CURRENT squared. You get WATTs. How
many? That is your loss. Now, if you can measure the resistance of
your tower, stand very far back because it is about to fall over from
being rusted out.

ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance causes the
antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with
a (well insulated) screwdriver, you would see a nice Tesla-like arc.

With this statement, it is obvious that you are not touching the
screwdriver to the "end" of the antenna (some 15-20 meters above your
head).

Current and voltage are not uniformly equal along the length of the
antenna. Your beam at the top acting as a top hat might yield some
uniformity, but that doesn't come from simple math analysis.

Instead, what you describe is a feedpoint impedance calculation.

Any idea on whether the high RF voltage present on the antenna could cause some
significant loss?

You are working very hard to find a problem that isn't there.

73's
Richard Clark, KB7QHC

Antonio Vernucci wrote:

For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big
HF yagi on its top). The home-made tuner, placed at the tower base, has a
motorized variable vacuum capacitor in series and a (properly selected) fixed
capacitor in parallel. I can so remotely tune the variable capacitor across the
3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the
band. The antenna works fine and I receive good reports.

The only thing which worries me a bit is the very high voltage that develops at
the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to
about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be
a source of significant losses.

To solve my doubt, I started by precisely measuring the capacitance of the tuner
capacitors, and I could then easily calculate the antenna impedance at 3.500
MHz, which resulted to be (18.2 + j 656) ohm.

ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current flowing through
the antenna is about 10.5A, quite higher than the 6.3A figure one would get
should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect
some more loss in the conductors due to the fairly high current.

ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance causes the
antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with
a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am
wondering whether such a high voltage could constitute, by itself, a source of
extra loss. Please note that the wire coming down from the tower is connected
directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do
not see a place where power can get dissipated due to the high RFvoltage, other
than perhaps in the humid air (?).

I could probably avoid this situation by changing the tap height on the tower,
but I would run the risk of not being any longer able to tune across the whole
3500 - 3800 kHz band adjusting one of the two capacitors, and not both.

Any idea on whether the high RF voltage present on the antenna could cause some
significant loss?

Thanks and 73

Tony I0JX
Rome, Italy

You did not describe your RF ground.

I did not describe the RF ground because I consider it not relevant to my
question.

Anyway, the ground system is formed by 64 quarter-wave radials.

73

Tony I0JX
Rome, Italy

"dave" ha scritto nel messaggio
. ..
You did not describe your RF ground.



Antonio Vernucci wrote:

For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly
big
HF yagi on its top). The home-made tuner, placed at the tower base, has a
motorized variable vacuum capacitor in series and a (properly selected) fixed
capacitor in parallel. I can so remotely tune the variable capacitor across
the
3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the
band. The antenna works fine and I receive good reports.

The only thing which worries me a bit is the very high voltage that develops
at
the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to
about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could
be
a source of significant losses.

To solve my doubt, I started by precisely measuring the capacitance of the
tuner
capacitors, and I could then easily calculate the antenna impedance at 3.500
MHz, which resulted to be (18.2 + j 656) ohm.

ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current flowing
through
the antenna is about 10.5A, quite higher than the 6.3A figure one would get
should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must
expect
some more loss in the conductors due to the fairly high current.

ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance causes
the
antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire
with
a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am
wondering whether such a high voltage could constitute, by itself, a source
of
extra loss. Please note that the wire coming down from the tower is connected
directly to the vacuum capacitor terminal, with no stand-off insulator. So, I
do
not see a place where power can get dissipated due to the high RFvoltage,
other
than perhaps in the humid air (?).

I could probably avoid this situation by changing the tap height on the
tower,
but I would run the risk of not being any longer able to tune across the
whole
3500 - 3800 kHz band adjusting one of the two capacitors, and not both.

Any idea on whether the high RF voltage present on the antenna could cause
some
significant loss?

Thanks and 73

Tony I0JX
Rome, Italy

In article ,
"Antonio Vernucci" wrote:

I did not describe the RF ground because I consider it not relevant to my
question.

Anyway, the ground system is formed by 64 quarter-wave radials.

73

Tony I0JX
Rome, Italy

Until the RF Ground is described, You can't make any definitive
decisions, about any kind of Marconi Antenna, no matter how it is feed.
This is common problem with many, less than educated, Hams, that try and
design Antennas Systems for HF. Most of the Losses in these designs are
in the RF Ground System, and not in the Radiators. Just Say'en....
YMMV...

Until the RF Ground is described, You can't make any definitive
decisions, about any kind of Marconi Antenna, no matter how it is feed.
This is common problem with many, less than educated, Hams, that try and
design Antennas Systems for HF. Most of the Losses in these designs are
in the RF Ground System, and not in the Radiators. Just Say'en....
YMMV...

I am not interested in knowing the overall losses and making decisions on my
antenna . I am just interested to know whether someone has experience on the
effect of very high voltage on the antenna terminal. And for that very purpose
the ground system is not relevant.

No useful answer received so far.

The problem of this newsgroup is that people not knowing how to answer a
specific question, reply to other questions, just to write something.

73

Tony I0JX
Rome, Italy

On Mon, 12 Sep 2011 19:59:50 +0200, "Antonio Vernucci"
wrote:

No useful answer received so far.

Hi Antonio,

Then, it would appear, after your having discarded technical points
curiously you want to be validated for thinking your antenna is lossy.

It will be hard to do that without your evidence (back of the envelope
computation does not qualify) that is contrary to everyone's
experience.

73's
Richard Clark, KB7QHC

On 9/9/2011 12:10 PM, Antonio Vernucci wrote:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a
fairly big HF yagi on its top). The home-made tuner, placed at the tower
base, has a motorized variable vacuum capacitor in series and a
(properly selected) fixed capacitor in parallel. I can so remotely tune
the variable capacitor across the 3500 - 3800 kHz range obtaining a
perfect match to 50-ohm everywhere in the band. The antenna works fine
and I receive good reports.

The only thing which worries me a bit is the very high voltage that
develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW
applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure
whether such high voltage could be a source of significant losses.

Generally, your losses are going to be IR losses. High voltage implies
low current, and low loss.

However, when you start talking kilovolts, you might have significant
loss from corona discharge.


To solve my doubt, I started by precisely measuring the capacitance of
the tuner capacitors, and I could then easily calculate the antenna
impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm.


That's the feed point Z of the basic radiator? or of the whole
assembly, including the shunt feed wire/tube?


ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current flowing
through the antenna is about 10.5A, quite higher than the 6.3A figure
one would get should the antenna resistance be 50 ohm instead of 18.2
ohm. So, I must expect some more loss in the conductors due to the
fairly high current.

Yes, that seems reasonable, but what conductor is that 10 amps flowing
in? What is its AC resistance at 3 MHz?


ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance
causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz.
Touching the wire with a (well insulated) screwdriver, you would see a
nice Tesla-like arc. I am wondering whether such a high voltage could
constitute, by itself, a source of extra loss. Please note that the wire
coming down from the tower is connected directly to the vacuum capacitor
terminal, with no stand-off insulator. So, I do not see a place where
power can get dissipated due to the high RFvoltage, other than perhaps
in the humid air (?).

Corona is definitely a source of loss. Humidity doesn't change corona
much, but does change leakage across insulators (i.e. the crackling one
hears under HV power lines on damp mornings).

However, is it a *significant* source of loss? How much stuff is at the
HV? What's the actual surface field? (Running a screwdriver near it
perturbs the field)



I could probably avoid this situation by changing the tap height on the
tower, but I would run the risk of not being any longer able to tune
across the whole 3500 - 3800 kHz band adjusting one of the two
capacitors, and not both.

Any idea on whether the high RF voltage present on the antenna could
cause some significant loss?

HV loss is hard to estimate. You could make your shunt wire a shunt
tube or rod and basically eliminate corona (A rule of thumb is that
10-15 kV/cm radius will have virtually no corona.. so for your 6kV, a
1cm diameter tube is in the right ballpark) Skin depth at 3.5 MHz in
Aluminum is .043mm, and the usual rule of thumb is to make the tubing
wall thickness 3-5 skin depths. 0.12-0.20 mm seems about right. Copper
could be thinner wall (skin depth is less)

I'd worry more about loss across dirty insulators.
And even more about IR losses.




Thanks and 73

Tony I0JX
Rome, Italy

El 09-09-11 21:10, Antonio Vernucci escribió:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a
fairly big HF yagi on its top). The home-made tuner, placed at the
tower base, has a motorized variable vacuum capacitor in series and a
(properly selected) fixed capacitor in parallel. I can so remotely
tune the variable capacitor across the 3500 - 3800 kHz range obtaining
a perfect match to 50-ohm everywhere in the band. The antenna works
fine and I receive good reports.

The only thing which worries me a bit is the very high voltage that
develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW
applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure
whether such high voltage could be a source of significant losses.

To solve my doubt, I started by precisely measuring the capacitance of
the tuner capacitors, and I could then easily calculate the antenna
impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm.

ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current
flowing through the antenna is about 10.5A, quite higher than the 6.3A
figure one would get should the antenna resistance be 50 ohm instead
of 18.2 ohm. So, I must expect some more loss in the conductors due to
the fairly high current.

ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance
causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz.
Touching the wire with a (well insulated) screwdriver, you would see a
nice Tesla-like arc. I am wondering whether such a high voltage could
constitute, by itself, a source of extra loss. Please note that the
wire coming down from the tower is connected directly to the vacuum
capacitor terminal, with no stand-off insulator. So, I do not see a
place where power can get dissipated due to the high RFvoltage, other
than perhaps in the humid air (?).

I could probably avoid this situation by changing the tap height on
the tower, but I would run the risk of not being any longer able to
tune across the whole 3500 - 3800 kHz band adjusting one of the two
capacitors, and not both.

Any idea on whether the high RF voltage present on the antenna could
cause some significant loss?

Thanks and 73

Tony I0JX
Rome, Italy


Hello Tony,

It is not the voltage that will introduce loss (as the field lines
will go into air and frequency is low), but it is (corona) discharge.
Whether this occurs, depends on presence of sharp edges and
especially sharp (double curved) surfaces with small radius, think of
bolts, threaded rods, etc that point away from the tower into the air.

A thin end of an antenne element can also cause breakdown as it points
far into the air (away from other solid structures) and the voltage
maximum will be at the elements of your HF antenna.

In case of your 8 kVrms (that is 12 kVp), an antenna element with
10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at
the end. If the end of the element is just straight (just cut
aluminium), you will exceed 3kVp/mm and corona will occur.

If (corona) discharge occurs, you will notice a sudden change in VSWR
with increasing power.


--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM

HV loss is hard to estimate. You could make your shunt wire a shunt tube or
rod and basically eliminate corona (A rule of thumb is that 10-15 kV/cm radius
will have virtually no corona.. so for your 6kV, a 1cm diameter tube is in the
right ballpark) Skin depth at 3.5 MHz in Aluminum is .043mm, and the usual
rule of thumb is to make the tubing wall thickness 3-5 skin depths. 0.12-0.20
mm seems about right. Copper could be thinner wall (skin depth is less)

I presume that the corona effect should be visible at dark. So far I have seen
none, but we had not a single day of rain since I mounted this antenna
(incredible summer season...). So, I must verify when rain will come, in a few
days from now they say. Or do you think that corona may not be visible?

I am using a 4-mm diameter wire so, if I will really have corona problems, I
could insert the (vertical) wire into a 1-cm aluminum tube connected at the very
bottom of the wire (i.e. at the antenna feed point). Probably a 2-meter long
tube could be sufficient (RF voltage gradually diminishes getting away from the
antenna feed point).

73

Tony I0JX