Multiband G5RV antenne
(Published in CQ-QSO, # 9/10, 2006)
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
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
In figure 1 we see a diagram of the original G5RV antenna system.
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
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