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

Hi Wim,

despite my wire is only 4mm in diameter, SWR does not vary at all applying a
steady full-power carrier (not even some small oscillations of the reflected
power meter needle). I then presume that no big corona effect takes place (also
because I see no corona light at dark). I am not sure whether a light corona
effect (i.e neither visible nor influencing SWR) could take place, but if so I
presume loss would anyway be very low.

I will check again under rain conditions (test not done so far), but I am not
sure whether rain increases susceptibility to corona, with respect to hot summer
days (with umidity close to 80% sometimes). Any idea at that regard?

73

Tony I0JX

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

Antonio Vernucci wrote:

Hi Wim,

despite my wire is only 4mm in diameter, SWR does not vary at all applying a
steady full-power carrier (not even some small oscillations of the reflected
power meter needle). I then presume that no big corona effect takes place (also
because I see no corona light at dark). I am not sure whether a light corona
effect (i.e neither visible nor influencing SWR) could take place, but if so I
presume loss would anyway be very low.

I will check again under rain conditions (test not done so far), but I am not
sure whether rain increases susceptibility to corona, with respect to hot summer
days (with umidity close to 80% sometimes). Any idea at that regard?

What happens if you move the slant-wire up or down on the tower?

El 13-09-11 17:30, Antonio Vernucci escribió:
Hi Wim,

despite my wire is only 4mm in diameter, SWR does not vary at all
applying a steady full-power carrier (not even some small oscillations
of the reflected power meter needle). I then presume that no big
corona effect takes place (also because I see no corona light at
dark). I am not sure whether a light corona effect (i.e neither
visible nor influencing SWR) could take place, but if so I presume
loss would anyway be very low.

I will check again under rain conditions (test not done so far), but I
am not sure whether rain increases susceptibility to corona, with
respect to hot summer days (with umidity close to 80% sometimes). Any
idea at that regard?

73

Tony I0JX

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


Hello Tony,

As long as your wire with D=4mm is not close to other structures (say
0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is
carrying 12kVp. You still have some margin before getting close to 3
kV/mm.

At ground level, influence of HF frequency on dielectric breakdown
voltage of air is negligible. If the wire is coated, the diameter
increases, hence reducing the E-field at the plastic-air interface.

When VSWR doesn't, change (with increasing power), the loss due to
corona discharge will be minimal. Of course when you lose 10W from
2kW, you will not notice this probably. However if you get a discharge
between metal and plastic insulation, the plastic will fail in the end
(due to spark erosion).

conducting Droplets (rain) do reduce the breakdown voltage as it
distorts the field. This introduces 'hot spots'. Unfortunately I don't
have data on this.

I could imagine that moisture/water layer on the wire would introduces
some loss (as you get a current through the moisture layer). But this
is not due to corona effect.

It can be calculated, as you know the voltage and you can calculate
the capacitance to ground (so you know the capacitive current that
"leaves" the wire.


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

Hello Tony,

As long as your wire with D=4mm is not close to other structures (say
0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is carrying
12kVp. You still have some margin before getting close to 3 kV/mm.

At ground level, influence of HF frequency on dielectric breakdown voltage of
air is negligible. If the wire is coated, the diameter increases, hence
reducing the E-field at the plastic-air interface.

When VSWR doesn't, change (with increasing power), the loss due to corona
discharge will be minimal. Of course when you lose 10W from 2kW, you will not
notice this probably. However if you get a discharge between metal and plastic
insulation, the plastic will fail in the end (due to spark erosion).

conducting Droplets (rain) do reduce the breakdown voltage as it distorts the
field. This introduces 'hot spots'. Unfortunately I don't have data on this.

I could imagine that moisture/water layer on the wire would introduces some
loss (as you get a current through the moisture layer). But this is not due to
corona effect.

It can be calculated, as you know the voltage and you can calculate the
capacitance to ground (so you know the capacitive current that "leaves" the
wire.


--
Wim
PA3DJS

Thanks for info. When things go down to the physics level, it is not so easy to
understand phaenomena, unless one has specific knowledge on them.

73

Tony I0JX

El 13-09-11 20:32, Wimpie escribió:
El 13-09-11 17:30, Antonio Vernucci escribió:
Hi Wim,

despite my wire is only 4mm in diameter, SWR does not vary at all
applying a steady full-power carrier (not even some small oscillations
of the reflected power meter needle). I then presume that no big
corona effect takes place (also because I see no corona light at
dark). I am not sure whether a light corona effect (i.e neither
visible nor influencing SWR) could take place, but if so I presume
loss would anyway be very low.

I will check again under rain conditions (test not done so far), but I
am not sure whether rain increases susceptibility to corona, with
respect to hot summer days (with umidity close to 80% sometimes). Any
idea at that regard?

73

Tony I0JX

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


Hello Tony,

As long as your wire with D=4mm is not close to other structures (say
0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is
carrying 12kVp. You still have some margin before getting close to 3
kV/mm.

At ground level, influence of HF frequency on dielectric breakdown
voltage of air is negligible. If the wire is coated, the diameter
increases, hence reducing the E-field at the plastic-air interface.

When VSWR doesn't, change (with increasing power), the loss due to
corona discharge will be minimal. Of course when you lose 10W from
2kW, you will not notice this probably. However if you get a discharge
between metal and plastic insulation, the plastic will fail in the end
(due to spark erosion).

conducting Droplets (rain) do reduce the breakdown voltage as it
distorts the field. This introduces 'hot spots'. Unfortunately I don't
have data on this.

I could imagine that moisture/water layer on the wire would introduces
some loss (as you get a current through the moisture layer). But this
is not due to corona effect.

It can be calculated, as you know the voltage and you can calculate
the capacitance to ground (so you know the capacitive current that
"leaves" the wire.



Short note on change in VSWR due to corona.

Many reflectometers use diode rectifiers with relative heavy load. If
there is no bias (or the bias is wrong), you will not notice small
variations in VSWR, when VSWR = (almost) 1.

If you have a directional coupler you may use your oscilloscope or
spectrum analyser as detector device. The oscilloscope (50 Ohms input
impedance) you can use to verify if the residual VSWR is because of
higher harmonics or real reflection. Of course the spectrum analyzer,
oscilloscope with FFT or a receiver (with heavy attenuation) will show
you the desired signal directly.

If you match to very low return signal, even a few Watts loss due to
corona you can detect at full power. you may need to do this during
very quiet weather as small mechanical changes due to elasticity of
the construction materials will lead to small changes in reflection
coefficient.

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