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My
G5RV page
related
short dipole link
related
external G5RV analysis
88 ft or
44 ft Dipole
Recently I’ve been fielding service problems from people using 88-foot long
dipoles. I also see people recommending this antenna when there is limited
antenna space, even when there is sufficient room for a normal dipole. The
problems I hear of generally center around tuner damage, balun damage, or
failure of a tuner to match the antenna. An analysis of various installations
has shown that an 88-foot
dipole is too short
to be a reliable
antenna on 80 meters (or a 44 foot dipole on 40 meters) without
some form of
low-loss reactance
compensation at the
antenna feedpoint or
very near the
antenna.
While pattern
looks excellent on
higher bands, lowest
band performance of
a short dipole (in
this case 88 feet
long on 80 meters or
44 feet long on 40)
certainly leaves a
great deal to be
desired. feed line
VSWR is over
100:1 with
an 88 ft long dipole
30 feet high using
typical ladder line
feeders (400 ohm
nominal impedance)
on the bottom of 80
meters.
Looking at an 88
foot long dipole of
#16 wire in EZnec 4:
EZnec4 predicts:
(I used 400 ohms
in the model because
most 450 ohm ladder
line is really
around 400 ohms.)
This means
the balun and
antenna tuner see an
impedance somewhere
between 3 ohms to
50k ohms depending
on feed line length.
This is in stark
contrast to 22:1
maximum possible VSWR and 18 to
8800 ohms
impedance range
(depending on
feed line length)
when a dipole or doublet is
full
length!
Let’s look at a
Smith Chart of that
antenna on 80 meters
as feed line length
is varied:
Minimum Z to
tuner shown above.
Maximum Z
to tuner shown
above.
It isn’t the
efficiency of the antenna
that is in question.
The problem is
feed line efficiency,
balun requirements,
and high VAR power
levels in the tuner
system. When balun,
feed line and tuner
losses are included,
losses can be
terrible. In
many cases 80-meter
efficiency of an 88
ft dipole, or
40-meter efficiency
of a 44 ft dipole,
can be 10% or
less. Of course
you can get lucky
and make as high as
75% efficiency with
a sweet length of
feed line. The main
point is system like
this are very tough
on components.
The text below
actually applies to
any short dipole. If
you don’t like the
dB or so loss of a
G5RV system
(including tuner and
feed line losses),
you really
won’t like the
performance of an
antenna
significantly
shorter than a 102
ft long G5RV.
Loss in a Short
Dipole
Power loss in
short dipoles
primarily comes from
compensating
reactances and
matching to 50 ohms.
This is because VAR
(volt-amperes
reactive) power is
very high.
See
VAR power
In a system with
reactance, current
and voltage are not
in phase or in step.
Because maximum
current does not
occur at the same
time as maximum
voltage, the simple
product of current
times voltage
(I*E=P) can be much
higher than the
actual useful power.
The higher reactive
current causes
increased current
squared times
resistance (I^2*R=P)
heating and loss.
The higher reactive
voltage, in a
similar fashion,
causes increased
dielectric losses.
In the worse case,
power loss caused by
increased voltage
and current in
reactive systems can
actually cause
component failure.
Short dipoles and
verticals (like a 44-ft vertical) will
almost always model
with very good
efficiency. The is
because the losses associated with a short antenna have little to do with the
antenna itself. Nearly all losses in very short antennas are in the feed line and
matching system. Modeling programs have a
perfect lossless feed and matching system unless they include the real
mismatched feed line
losses and matching
system losses.
Let’s look at an
example by looking
at the suggestion of
shorting a dipole to
approximately .33
wavelengths, or 44 feet, on 40
meters.
Dipole Impedance
The analysis in
the following text
is not a worse case
analysis, but rather
a typical best
case analysis.
In the real world
VSWR could exceed
100:1 when the
antenna is closer to
earth if losses are
low or modest.
The Eznec 40
meter model of a 44
ft dipole is shown
here:
From this we see
the feedpoint
impedance is 30 ohms
resistive, which we
should all agree is
not too bad. The
problem is the
feedpoint reactance of -j655 ohms.
This reactance
results in a VSWR
of over 48:1
on a typical
“window ladder
line”, or 46:1
on a real open wire
450 ohm line.
I’ve verified the
results of the older TLA
(supplied with older ARRL
Antenna Handbooks)
for dry new
transmission lines,
and the older versions of TLA
closely agree with
actual measurements
using network
analyzers. Unfortunately some later versions of TLA and some other ARRL
publications or articles are not so good, and greatly underestimate ladder line
losses. Real 450-ohm window line loss is about double from TLA predictions, probably because
copper losses and effects of reduced impedance from the dielectric
were
underestimated. Real
line impedance of 450-ohm ladder line is
generally much lower than
450 ohms. We can
consider TLA to be a
very optimistic result,
keeping in mind
real-world 450 ohm
ladder lines can be quite a
bit worse.
Using TLA we find
the following
feed line losses for
a 75-foot feed line:
Note the
following from the
above:
- Line loss is
now just under
2dB for TLA’s
estimate of a
perfectly dry
brand-new
feed line (it’s
actually just
over 2 dB if
we use real
measured losses
on aged dry line
rather than TLA’s values).
This does not
include tuner
losses, balun
losses, or
additional
losses from
less-than-perfect
installation.
- Maximum RMS
voltage on the
transmission
line is a
staggering 4424
volts with 1500
watts. This is
over 6250 volts
peak voltage.
(Even with 100
watts peak
voltage would be
1600 volts!)
- Line input
impedance is 21
-j295, this is
what the tuner
must match.
Suppose we add a
well-designed
“3KW”
tuner to this
system. Typical
better tuners have
the following
component values and
Q’s:
- Capacitors:
4.5kV, 250 pF,
Q=5000 at
mid-setting
- Inductors:
maximum
inductance 38uH
with Q=200
We will assume a
perfect balun
without any loss.
(It makes very
little difference by
the way if the balun
is on the tuner
input. Don’t think
moving a current or
choke balun to the
input of a
tuner is a solution.
The balun is under
the same common mode
stress at ether end
of the
tuner.)
With 250pF
maximum capacitance,
matching the odd
impedance of this
short dipole
requires a tuner
with the above
component
limitations to
operate with a Q of
30.4. The resulting
loss is 233.6 watts or 15% of applied power,
and maximum voltage
across a capacitor
is 4250 volts! You
are almost at the
voltage rating of
4.5kV, so any small
imperfections will
cause a capacitor to
arc. The inductor in
the tuner would also
dissipate 192 watts,
which would damage
most inductors
unless duty-cycle
was very low.
Some tuners (like
MFJ Tuners) would be
worse than the above
values, Dentron
tuners would be
close to the above
values, and many
tuners would not
even be able to
match the load
impedances.
Automatic tuners and
pi-network tuners
would have a
difficult time
matching an
impedance like this.
A More Accurate Transmission Line Calculator
While the older version of TLA shown above is acceptable, newer versions are
not. VK1OD has a much more accurate transmission line calculator
at
this link. It is a little tricky to enter data in the VK1OD
calculator, so enter impedance exactly as I did below in the format used with
30.51-655 (no spaces):
| Parameters |
|
| Transmission Line |
Wireman 551 |
| Code |
W551 |
| Data source |
Wireman / N7WS |
| Frequency |
7.000 MHz |
| Length |
75.000 ft |
| Zload |
30.51-j655.00 Ω |
| Yload |
0.000071+j0.001523 S |
| Results |
|
| Zo |
400.00-j1.14 Ω |
| Velocity Factor, VF -2 |
0.903, 1.227 |
| Length |
212.89 °, 0.591 λ, 22.860 m |
| Line Loss (matched) |
0.105 dB |
| Line Loss |
2.015 dB |
| Efficiency |
62.87 % |
| Zin |
16.21-j192.64 Ω |
| Yin |
0.000434+j0.005154 S |
| VSWR(50)in |
49.17 |
|
With new dry heavy duty transmission line, we would lose about 2dB of our
power in the feed line alone, and another 1.6 dB in a big well-designed manual
tuner! This is 3.6 dB loss, more than half our power, and we have not included
balun losses which could amount to another 1-3 dB loss with this load impedance.
It is quite possible to have in excess of 6 dB loss on the lowest band using
good heavy ladder line and a good tuner with an 88ft or 44ft dipole.
Conclusion
The lower
limit in size of a
multiband dipole
before feed system
and matching
losses move to
the edge of severe
problems is about
200 feet on 160
meters, 100 feet on
80 meters, 50 feet
on 40 meters, and so
on.
A
good rule of thumb
is length in feet
must equal 1.25
times the band in
meters. The result
is the minimum
dipole length you
can use without
using a good
matching system in,
at, or near the
antenna!
160
meters= minimum
efficient length
160*1.25=200 feet
80
meters= minimum
efficient length
80*1.25=100 feet
The G5RV length
of feed line and
antenna is the lower
limit in size. A
normal G5RV system,
including tuner,
typically has about
1dB of loss on 80
meters and less than
2dB system
loss (including
loss from coax and
matching) on
80,40, and 20
meters. People seem
to hate G5RV’s, yet
they now seem to be
willing to further
shorten the G5RV and
recommend others do
the same!
As an antenna is
shortened from that
length, losses in
the feed system
(even what Hams
consider a
good one) climb
rapidly. 88 feet is
just too short for
an 80 meters
antenna, because as
you see above it is
at the limit of what
most tuners will
match. It also
places most tuners
at their power limit
at a few hundred
watts of applied
power.
The optimum
length for a
multiband dipole is
near 1/2 wl on the
LOWEST band, and the
optimum open-wire
feed line length is
any odd multiple of
1/8th wavelength on
the lowest band.
This means an
optimum 80-meter
dipole would be
about 125ft long,
and the feed line
would be 25-30ft,
75-90ft, or
125-150ft long. The
longer the feeder,
the more likely you
are to having to
trim it for optimum
tuner performance.
Most tuners like
to see impedances
HIGHER than 50 ohms,
and inductive loads
at low impedances.
Pi’s and L’s are NOT
a solution to
matching problems.
They actually are
significantly more
restricted in
matching range than
a conventional T-network
using the same
general style and
quality components!
feed line voltage
is also a good way
to estimate
wet-weather
performance of
“window”
ladder line. If
voltages are more
than 1000 volts RMS
at 100 watts,
operation in wet
weather will
certainly cause
tuning or loss
problems. Use TLA
and other tools as a
way to plan
antennas. Remember,
there are more
important things
than pattern! A good
pattern is useless
if we cannot efficiently feed
power to the
antenna.
©2004 W8JI
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