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Index |
Multiband G5RV antenne (Published in CQ-QSO, # 9/10, 2006) Introduction The G5RV antenna was originally published by
Louis Varney (G5RV) in Radio Communications in July 1984 and before that in Over time, the concept has been applied by
many radio-hams, in many variations and 'improvements' and has been proven to
be a useful antenna for many applications. Most improvements, both by
amateurs and professionals proved to be minor variations on the theme without
affecting basic behavior. Because of its widespread use, the G5RV
antenna system sometimes has taken on mythical proportions to some
radio-hams, putting the design in the area of 'wonder-antenna's. It therefore
seems a good idea to look again into Louis Varney's original design and
analyze the antenna with contemporary means and methods. According to the original G5RV article, the
antenna has been designed as a symmetrical, center fed dipool,
with 'long-wire' behavior on the From this description one may already deduce
that the G5RV concept will only exhibit (very) low SWR on this In his article Louis Varney presents his
design as to be useful on all HF amateur wave lengths, i.e. 80, 40, 30, 20,
17, 15, 12 and According to Varney, antenna height is
supposed not to be critical although the original antenna has been designed
for Louis further tells us that a balun is not
required between the open feeder line and a coaxial transmission line to the
transceiver. He contradicts this last statement however when he writes: "Under
certain conditions, either due to the inherent
"unbalanced-to-balanced" effect caused by the direct connection of
a coaxial feeder to the base of the (balanced) matching section, or to
pick-up of energy radiated by the antenna, a current may flow on the outside
of the coaxial outer conductor. This effect may be considerably reduced, or
eliminated, by winding the coaxial cable feeder into a coil of 8 to 10 turns
about 6in in diameter immediately below the point of connection of the
coaxial cable to the base of the matching section." This clearly is a description of a 1 : 1
current balun, also known as a 'braiding-choke'. In general it is always a good idea to place
a balance-to-unbalance transformer (balun) at every change-over from a
symmetrical to an a-symmetrical situation to keep currents from following
unwanted paths, so we preferably apply
a balun at the end of the symmetrical feeder line of the G5RV system when
connecting into and a-symmetric coaxial cable or tuner / transceiver. On the characteristic impedance of the
coaxial transmission line to the transceiver Louis prefers this to be between
50 and 80 Ohm, as this line will see a 'rather high SWR' anyway, except for
the 'basic' In figure 1 we see a
diagram of the original G5RV antenna system.
On
the total antenna width of 102' or Fortunately we are currently living in
interesting times, with many antenna calculation programs at our disposal and
computers to perform calculations for us. For this analysis I have been using
the antenna-modeling program EZNEC and transmission-line modeling program
TLW. All further calculations have been performed using elementary math's,
with EXCEL to perform the tiring repetitions. To obtain a fair impression, I will compare
the G5RV behavior with dipole antenna's, each cut for resonance on the
particular HF amateur band. Antenna's are modeled
at
In table 1 some interesting features may be
noticed. Firstly, it appears that a 'standard'
resonant dipole antenna is not bad at all: with a maximum gain of around 7
dBi at each HF amateur frequency band, and a low enough SWR to allow direct
connection to a transistorized transceiver, it is a simple, cheap and
reliable antenna to start-off with. A number of those standard dipoles may be
connected in parallel on the same balun, as only the resonating dipole will
exhibit a low connection impedance and will carry most of the current (and
therefore perform most of the radiation). Such a dipole assembly is called a
cob-web or cats-whiskers antenna system. Secondly,
when comparing maximum antenna gain of the dipole and G5RV antenna, one will
notice a comparable gain below 14 MHz. and more
gain above this frequency for the G5RV, because more wavelengths fit and
contribute. A third observations may be made when
comparing elevation angles; these are remarkably similar for both antenna's.
This is no coincidences but stems from the fact that both are situated at the
same height above the same ground; as the antenna radiation pattern is the
vector summation of both direct and (ground-)reflected radiation, which is
the same for both antenna's, the elevation angels are also the same. The
azimuth patterns will be somewhat different though, again for the multi-wave
fitting at the G5RV. This will result in a multi-lobe pattern, with deep
nulls in between. Looking at SWR re 50 Ohm in the 4th and 7th
column, one will notice big differences, except for 14,175 MHz. the original design frequency for the G5RV. Although not exactly very low, the SWR
figures with respect to 300 Ohm (last column) are already looking somewhat
better. This tells us that a non-resonating dipole antenna in general may be
better connected through high impedance (open) transmission line, resulting
in lower SWR (and so lower cable loss) through mismatch, on top of the
already low losses of these cables when characteristically terminated. Let's take a better look at the second
characteristic element of the G5RV system. This has been modeled as TV-line
with a characteristic impedance of 300 Ohm,
When we start regarding the fourth column, we
find that in spite of the low loss cable the transformer section is showing
some loss. This loss is in closer examination related to SWR at cable input,
as in the second column. The SWR related loss mechanism is explained in more
details in the article "Where
does the power go". Highest
loss (1,12 dB) is at the When comparing 5th and 2nd column we see that
SWR re 300 Ohm is hardly changing from the beginning of the line to the end.
This is a demonstration that SWR is constant along a lossless transmission line,
regardless of the impedance transformation. We do see however small SWR
differences between the two columns as these are related to line losses; the
higher the losses, the higher the SWR differences. The operation of the transformer section is
showing very nicely when comparing SWR re 50 Ohm at the beginning vs. at the
end of the transmission line (3 rd vs. 6 th column). The transformer section performed a good job
that dramatically shows at 3,65 MHz.: SWR has
changed from 1 : 108,6 tot 1 : 4,2. Also on other amateur frequencies this
improvement is evident although to a somewhat lesser extent. When comparing the last two columns, we see
that indeed we may connect 50 or 70 Ohm transmission lines, as differences
are small. As stated above, it is good practice to
install a balun at the intersection of the transforming feeder line and
connecting coax or to an a-symmetric tuner or transceiver. In a different
article, baluns have been described in more detail, see "Baluns".
In design examples this component usually is presented for a 50 Ohm
environment. In case of the G5RV situation however, this is impedance is
variable and depending on the selected amateur frequency. Let's see how this
works out in table 3.
In table 3 we find the SWR figures from table
2, this time decomposed into its real and imaginary constituents and total
impedance. Looking at this last column one immediately finds that the balun
design should differ considerably from 'standard' 50 Ohm values. As stated in
the mentioned article, the parallel impedance of the balun should be at least
four times the system impedance. Considering table 3 we find that when this
balun impedance is around 1200 Ohm @ 10,15 MHz., we
cover most situations although at 28 MHz. the
situation may be somewhat critical. Starting at this 1200 Ohm @ 10,15 MHz., the balun should have an inductance of around 19 μH. When using a popular Total antenna system efficiency is related to
various factors as described earlier. Let's now calculate loss factors and
antenna gain to get an overall impression of the G5RV system performance as
related to our 'standard dipoles'-set in table 4. This time I calculated loss
factors in heat (
Let's start off looking into 'safety factors'
by regarding column six, power lost in the balun. As we selected the
two-ring, In the fourth and fifth column we find power
dissipated in the How this will end-up at the receiving site,
depends very much on 'conditions' and the direction of the receiving station
with regards to the antenna position. As a first and rough comparison, I have
related G5RV efficiency to our standard dipole, in terms of perceived signal
strength (S-points), both in the direction of maximum gain (different angels
for both antenna's). In this comparison (last column) all system losses and
antenna gains have been taken into account except for those in the final
coaxial cable to the TRX. The losses of the latter depend on the type of
cable (unknown) and cable length (unknown) and may not entirely be neglected
anymore when more than a few meters long.
From this comparison it is clear the G5RV
antenna is a compact and efficient 'general purpose' antenna on most HF
amateur frequencies. In the process of calculations for this
article I also performed some modeling on G5RV-variations, for instance the
ZR1DQ proposal. All these variations yield about the same results as the
basic G5RV and/or made trade-off's to favor one
radio-amateur band over the other. Bob J. van
Donselaar, on9cvd@veron.nl |
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