Many antennas and
antenna designers
neglect the true
cause of loss. The
major problem using
short antennas is
the reactance, not
the length!
What is
Reactance?
Reactance really
means the voltage
and current at that
point of the system
are no longer
in-phase or 180
degrees
out-of-phase. The
result is, for the
same real work
power, the reactive
power or
Volt-Amperes-Reactive
power is higher. We
can say the
“apparent
power” is
higher while the
“power that
does real work”
remains the same.
Let’s assume we have
a transmitter with
100 watts output.
With a zero
reactance load the
RMS voltage times
RMS current will be
100. Remember the
formula P= I E ? If
the load is 50 ohms,
we would have 70.7
volts times 1.414
amperes.
When we add
reactance to the
system, the voltage
and current are no
longer in step at
that point in the
system. To have 100
watts of real useful
power, we might have
1000 volts and 2
amperes! That’s 2kW
VAR power with a
100W transmitter.
This is really why
feed lines get lossy
and tuners burn up
when an antenna
system is very
reactive.
What Does
Reactive Power Do?
My first lesson
in reactive power
occurred around 1962
or 63. I was a
Novice with a single
807 in my homebrew
transmitter. It ran
60-75 watts input,
or about 30-50 watts
output. I tried to
load my 40 meter
dipole on 80 meters,
and my feed line
actually melted!
Let’s look at an 80
meter dipole
example. With 100
watts and a modest
height 80 meter
dipole at resonance
we have 1.32 amperes
at 75.9 volts at the
feedpoint. The 50
ohm SWR is 1.15:1
Frequency = 3.72
MHz
Source 1 Voltage =
75.89 V. at -0.1
deg.
Current = 1.318 A.
at 0.0 deg.
Impedance = 57.59 –
J 0.09832 ohms
Power = 100 watts
SWR (50 ohm system)
= 1.152 (450 ohm
system) = 7.813
Let’s Shorten
the Dipole….
For the 102-foot
G5RV we have:
Frequency = 3.72
MHz
Source 1 Voltage =
143.5 V. at -61.09
deg.
Current = 1.441 A.
at 0.0 deg.
Impedance = 48.16 –
J 87.19 ohms
Power = 100 watts
SWR (50 ohm system)
= 4.957 (450 ohm
system) = 9.699
Notice if we
multiply 1.441
amperes times 143.5
volts, we have 206.8
watts of reactive
power.
With the dipole
at G5RV length of
102 feet, we have
1.44 amperes and
143.5 volts. The VAR
power is 206.6 watts
with 100 watts of
real power applied.
The VSWR referenced
to 50 ohms is 5:1.
Losses are not
significantly
different, because
VAR power is only
207 watts.
Let’s Shorten
the Dipole even
more……..
When we shorten
the antenna to 88
feet, we have a
problem.
Frequency = 3.72
MHz
Source 1 Voltage =
1078 V. at -87.53
deg.
Current = 2.153 A.
at 0.0 deg.
Impedance = 21.58 –
J 500.3 ohms
Power = 100 watts
SWR (50 ohm system)
> 100 (450 ohm
system) = 46.651
What a change in the
system! With a 100
watt transmitter
current is now 2.15
amperes while
voltage is 1078
volts. 50 ohm SWR is
over 100:1. Antenna
feedpoint VAR power
is 2317 watts with
only 100 real watts
applied. So far as
the overall system,
including the
matching, feed line,
conductors, and
insulators are
concerned, it is
like we have
increased power
level 23 times.
What About an
Extreme?
Let’s look at a
20 meter dipole on
80 meters.
Frequency = 3.72
MHz
Source 1 Voltage =
14570 V. at -89.94
deg.
Current = 6.233 A.
at 0.0 deg.
Impedance = 2.574 –
J 2338 ohms
Power = 100 watts
SWR (50 ohm system)
> 100 (450 ohm
system) > 100
VAR power is now
9.1 kilowatts with a
100 watt
transmitter.
Conclusion
VAR power is the
real killer in
system efficiency
and component
damage. VAR
power is the
real reason a very
short dipole taxes
system components
and has terrible
efficiency.
We can get away
with a 88-foot
dipole on 80 meters
if we have proper
reactance
compensation. Proper
reactance
compensation brings
voltage and current
back into phase,
reducing the
apparent power
handled by the
system. This reduces
losses.
The components
required must be
sized to handle the
increased voltage
and current. A 100
watt transmitter
operating into a
highly reactive load
can be like running
9kW, so far as the
components in the
reactive part of the
system are
concerned. This
means the feed line,
balun, and tuner
must be
conservatively rated
when using a short
dipole, or you have
to run very low
power. It also means
losses will be high.
We have to use very
low loss components
when reactive power
levels are high,
otherwise we convert
energy into heat.
If we look at the
antenna by itself,
we hardly notice any
change in antenna
performance when
length is decreased
from 130 feet to 50
feet or less on 80
meters. The problem
is the reactive
power of a very
short antenna
stresses other
components,
including the
feed line, balun, and
tuner.
We probably
wouldn’t notice 5dB
of power loss, after
all conditions can
vary many decibel
from day to
day. This is
why QRP operators
often enjoy using
inefficient
antennas. Operators
who place their
antennas in
competitive
situations would get
the feeling
something is not
quite right.
Of course with QRO,
poor efficiency
shows up like a slap
across the face. A
melted tuner, balun,
or feed line will
pique someone’s
interest.
Power loss might
easily slip under
our
“RADAR” at
100 watts or less,
but at high power
the additional
component stresses
will capture our
attention!
