G5RV Antenna
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G5RV Antenna |
The G5RV antenna is
probably one of the
most maligned
antennas in the
world. I’m not quite
sure how it got its
reputation, but if
there was ever a
national association
for fair treatment of
antennas the G5RV
would be the poster
child! Part of this
is because people
install G5RV’s in
very sloppy
installations and
then assume all
G5RV’s will perform
like their badly
installed antenna.
Another part of the
problem is people
repeat inaccurate
claims or
repeat folklore.
The 102 foot long “G5RV” was not actually invented by G5RV.
The 102′ flattop appears in books as
far back as the 30’s as an all band antenna. G5RV published changes to the
basic 102′ 80/20M antenna to optimize it for coaxial feed on 20 Meters. Over time, changes have been made to “tweak”
operation as a multiband antenna.
The earliest reference in my books is
in a 1937 Handbook. This configuration was optimized to match a 625 ohm line:
So why 102′ length? The 100 – 105 feet long doublet length reduces large
impedance excursions present in half wave 80 meter doublets when used on
harmonics. For example, a half wave center fed doublet or dipole appears as 40-100 ohms on the band
where it is 1/2 wave long. If we connect it to a 1/4 wave long 400 ohm 1/4 wave
feedline, the feedline SWR is between 4:1 and 10:1. This means the impedance at
the transmitter end is somewhere between 1600 and 4000 ohms! On 40 meters the
antenna becomes a full wave. It is two half waves fed in series, the impedances
of each end fed half wave adding at the feedpoint. The feed impedance becomes several thousand ohms. Making the antenna 102ft long
moderates that impedance, especially if the balanced feeder is shortened to
about 30-35 feet.
Moderating doublet impedance requires planning
between element length and feedline length and impedance. The G5RV antenna and
feedline length just happens to work out on several, but not on all,
bands.
The balanced line in systems like this acts as a
non-radiating matching transformer. The balanced line does not
significantly “radiate”. The balanced line acts like a distributed matching network, loosely correcting impedance
errors on several bands. (You’ll see more of that down page.) A few bands are not corrected,
actually being made worse. On most bands the G5RV works fairly well when mixed
with a 50 ohm feeder. The G5RV has a modest SWR
in the 80, 40, 20, and 12 meter bands, is usable on 15 meters. It is not a good choice for 30 or 10 meters. On 30 and 10 meters, feedline
loss from high standing wave is a worry.
The ZS6BKW is another iteration that
makes different bands better, but again does not correct all bands.
Some people believe an antenna not being a resonant length hurts radiation.
That isn’t the case at all. The system from source to load generally must be
reasonably well matched matched for maximum energy transfer, but non-resonance
in an antenna itself does not hurt reception or transmission. Lack of matching
reduces energy transfer, but only dissipation from increased current in loss
resistances wastes power or hurts efficiency.
Most important to remember, all antennas are compromises in cost,
performance, complexity, and size. A properly built and installed G5RV is a
better option than an end fed half wave so long as the central feedpoint can be
tolerated.
My G5RV Practical Installation
I installed my
first G5RV antenna
in the mid-90’s. It
was about 102 feet
long. It was fed with 30-feet of
300-ohm
transmitting type
ladder line. I had a
choke balun made of
20-30 feet of RG8X
wound on a thin wall
4″ PVC drain pipe.
This choke was at
the transition
between the twinlead
and coax. My antenna
was installed about
80-100 feet above
ground between two
tall Pine trees.
No, the 300 ohm (or less ideal 450 ohm) does not radiate
(except when fed as a T for 160M). The G5RV, fed the normal way, is a
simple doublet antenna and the 300 ohm section is just a non-radiating matching
section. The idea the matching section radiates a vertical signal is just some
silly Internet myth.
To work 160, I
built a small box
that contained a
small vacuum relay.
With the relay
engaged the antenna
is worked against
ground.
This made it a 100-foot
tall “T” antenna
that I worked
against a radial
system. My final
system looked like
this:
It isn’t
necessary to have
100 feet of height.
Even heights as low
as 30-feet will
work, but C1 might
have to be changed
to an inductor, L network, or
some other simple antenna
matching system. The 160-meter matching system must not create a path to ground
when in the HF position. In my system, I really didn’t care if the feeder
radiated a little or not. The antenna was a safe distance from the house and any
noise sources.
G5RV SWR
Here are plots of G5RV
feed line SWR:
The G5RV has a
reasonable SWR on
80, 40, 20, 15, and
12 meters. It is not
an acceptable
performer on 30, 17,
or 10 meters. SWR is actually
better with 300-ohm
transmitting line
than with the 450
ohm line normally
used.
While the G5RV
requires an antenna
tuner, it is very
efficient on at
least 5 bands. This
is true even when
fed through fairly long
runs of RG-213
feed line.
Feedpoint
Impedance
There is a myth
that the G5RV works
better with a 75 ohm
feed line. This myth
stems from the fact
Varney, G5RV,
originally used 70
ohm twinlead to feed
the antenna.
Here are the
impedances of the
G5RV at resonance
near each band where
it has a reasonable
SWR. This is with no
feed line loss,
measured right at
the twinlead end
where coax would
connect:
Frequency = 3.7
MHz
Impedance = 17.17 –
J 0.2957 ohms
SWR (50 ohm system)
= 2.912 (75 ohm
system) = 4.368
Frequency = 7.8 MHz
Impedance = 23.7 + J
1.429 ohms
SWR (50 ohm system)
= 2.112 (75 ohm
system) = 3.166
Frequency = 14.26
MHz
Impedance = 95.9 – J
0.05889 ohms
SWR (50 ohm system)
= 1.918 (75 ohm
system) = 1.279
Frequency = 20.3
MHz
Impedance = 21.74 –
J 0.835 ohms
SWR (50 ohm system)
= 2.301 (75 ohm
system) = 3.451
Frequency = 24.73
MHz
Impedance = 93.96 –
J 0.3803 ohms
SWR (50 ohm system)
= 1.879 (75 ohm
system) = 1.253
You can see the
antenna slightly
misses resonance on
several bands, but
not so far that SWR
is intolerable.
Either 50 or 75 ohm
feed line would work,
either being about
equal with the
other.
feed lines and feed line Losses
You should use
good quality coax on
the G5RV, and a good
antenna tuner.
There isn’t any
reason to run a
certain length 50
ohm feeder (it can
be 75 ohms also, if
good quality). The
shorter and better
the feed line, the
better the antenna
will work.
Assuming a
100-foot long
LMR-400 feed line we
have the following
mid-band feed line
losses when a G5RV
is compared to the
same transmission
line with a
perfectly matched
antenna:
| Band | 80 | 40 | 30 | 20 | 17 | 15 | 12 | 10 |
| SWR ? : 1 | 2.71 | 4.1 | 16.5 |
1.85 | 9.9 |
5.5 | 2.6 | 10.2 |
| Dipole Loss -dB | 0.25 | 0.35 | 0.4 | 0.49 | 0.56 | 0.6 | 0.65 | 0.7 |
| G5RV Loss -dB | 0.37 | 0.92 |
6.3 |
0.6 |
4.46 |
2.1 | 1.0 |
7.4 |
Note: SWR is at
the radio end
of the 100 ft long
50 ohm feed line!
We can see on 80,
40, 20, and 12
meters the antenna
above has very good
efficiency.
Efficiency be
essentially just as
good as a dipole. On
15 meters
performance is
still good. The
antenna is
poor on
30 and 10 meters.
On The Air
Testing
One of the only ways to
reliably test an
HF antenna is a
blind A-B comparison
against another
known good reference
antenna. Since I
also had
a dipole at 100 feet
a few hundred feet
away from the G5RV
(similar height), I
had an ideal
testing situation.
Blind Test
I checked into
various groups on 75
meters and did a
blind A B antenna
test. I would say
“this is antenna one
one one, this is
antenna two two two”
while switching
antennas. The
results were:
- Nearly ALL of the
reports indicated
NO
difference in
signals! (58 equal
reports) - When a difference
in signal was
reported, about
equally split either
the full size dipole
or G5RV would win.
(7 G5RV better, 9
dipole better)
Announced Tests
I also did tests
that called the
antennas by name. I
would say “This is
the the G5RV,G5RV,
G5RV. This is the
dipole, dipole,
dipole”. The results
were rather
revealing.
- When I would call
each antenna by the
correct name, the
G5RV was reported
“weaker” or with
“poorer audio” or
some other negative
comment more than
80% of the time. The
remaining stations
reported no
difference! - When I would call
each antenna by the
opposite name,
the antenna I was
calling the G5RV was
still reported
in some negative
context about 80% of
the time. The
remaining percentage
again reported no
real difference.
Conclusions
Based on models
and air testing the
G5RV performs quite
well as a 5-band
antenna. Those bands
include 80, 40, 20,
15, and 12 meters.
The antenna will NOT
work as well as a
dipole on any other
HF band.
By adding a relay
and a ground system
the G5RV can be made
to work quite well
as a “T” antenna on
160 meters.
People have an
overwhelming
prejudice against
the G5RV. This
prejudice even
biases signal
reports when they
think a G5RV is
being used. Like any
antenna, the G5RV
will work best when
up in the clear. Run
it through the
bushes or like a
clothesline and it
might be a
disappointing
antenna, but then so
would a dipole or
Rhombic at the same
crummy height.
since 9/9/06 page
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