The legacy of Louis Varney (I), by Louis Varney, G5RV
On 28 June 2000, at 89, Louis Varney, alias G5RV, rejoined the large family of silent keys hams. To pay tribute to this famous amateur radio, I would like to present you one of the main work he shared with the ham community : the invention of the G5RV dipole antenna.
The G5RV, with its special feeder arrangement (ladder line), is a multi-band centre-fed antenna capable of very efficient operation on all HF bands, specifically designed with dimensions which allow it to be installed in gardens and other open spaces which accommodate a reasonably-straight run of 31.1m (102 ft) for the "flat-top" standard model.
Because the most useful radiation from a horizontal or inverted-V resonant antenna takes place from the center two-thirds of its total length, up to one-sixth of this total length at each end of the antenna may be dropped vertically, semi-vertically, or bent at some convenient angle to the main body of the antenna without significant loss of effective radiation efficiency. For installation in a very limited space, the dimensions of both the "flat-top" and the matching section can be divided be a factor of two to make the half-size G5RV, which is a very efficient antenna from 40 to 10m.
In contradistinction to multi-band antennas in general, the full size G5RV antenna was not designed as a half-wave dipole on the lowest frequency of operation, but as a 1/2l centre-fed longwire antenna on the 20m band, where the 10.36m (34 ft) matching section (the open-wire or ladder line) functions as a 1:1 impedance transformer, enabling the 75W Twin-Lead or 50/80W coaxial cable feeder to "see" a close impedance match on that band with a consequently low VSWR on the feeder.
However, on all the other HF bands the function of this section is to act as a " make-up" section to accommodate that part of the standing-wave (current and voltage components) which, on certain of the operating frequencies, cannot be completely accommodated on the "flat-top" (or inverted-V) radiation portion. The design centre frequency for the full-size version is 14.150 MHz, and the dimensions of 31.1m (102 ft) is derived from the formula for longwire antennas which is :
Length (ft) = 492 (n - 0.015) / f (MHz) = 492 x 2.95 / 14.150 = ~102 ft (31.1m)
where n is the number of half-wavelengths of the wire (flat-top).
In practice, since the whole system will be brought to resonance by the use of an antenna tuner, the antenna is cut to 31.1m (102 ft).
As it does not make use of traps or ferrite beads, the "dipole" portion becomes progressively longer in electrical length with increasing frequency.
This effect confers certain advantages over a trap or ferrite-bead loaded dipole because, with increasing electrical length, the major lobes of the vertical component of the polar diagram tend to be lowered as the operating frequency is increased. Thus, from 20m up, most of the energy radiated in the vertical plane is at angles suitable for DX working. Furthermore, the polar diagram changes with increasing frequency from a typical half-wave dipole pattern on 80m and a 2-1/2l wave in-phase pattern on 40 and 30m to that of a "longwire" antenna on 20, 17, 15, 12 and 10m bands (Fig.1 below).
Although the impedance match for 75W Twin-Lead or 80W coaxial cable at the base of the matching-section is very good on 20m band, and even the use of 50W coax cable results in a VSWR of only 1.8:1 on this band, the use of a suitable antenna tuner is necessary on all the other HF bands. Why ? Because on those bands the antenna plus the matching-section will present a reactive load to the feeder. Thus the use of the correct type of antenna tuner (unbalanced input to balanced output if twin-wire feeder is used, or unbalanced to unbalanced if coaxial feeder is used) is essential in order to ensure the maximum transfer of power to the antenna from a typical transceiver having a 50W coaxial (unbalanced) output.
Also to satisfy the stringent load conditions demanded by such modern equipment employing an ALC system which "senses" the VSWR condition presented to the solidstate transmitter output stage so as to protect it from damage which could be caused by a reactive load having a VSWR of more than about 2:1 (Fig. 2 above).
The above reasoning does not apply to the use of the full-size G5RV antenna on the 160 meters band, or to the use of the half-size version on 80 and 160 meters. In these cases the station end of the feeder conductors should be "strapped" and the system tuned to resonance by a suitable series-connected inductance and capacitance circuit connected to a good earth or counterpoise wire. Without this change, all your emitting power will be converted in heat before reaching the antenna...
Alternately, an "unbalanced-to-unbalanced" type of antenna tuner such as a "T" or "L" matching circuit can be used. Under these conditions the "flat-top" (or inverted-V) portion of the antenna plus the matching section and feeder function as a "Marconi" or "T" antenna, with most of the effective radiation taking place from the vertical, or near vertical, portion of the system; the "flat-top" acting as a top-capacitance loading element. However, with the system fed as described above, very effective radiation on these two bands is obtainable even when the "flat-top" is as low as 7.6m (25 ft) above ground.
The next chart (Fig.3) displays the gain and takeoff angle variations according the length of the G5RV (from 1/2l to 10l). Like in the previous diagram, longer is the antenna, better is the gain and lower is the takeoff angle. These curves explain also why the longer is the G5RV dipole better will be results in DXing in the upper bands. Therefore beverage antennas, which length can sometimes exceed 200 meters are so appreciated.
Theory of operation
The general theory of operation has been explained above; the detailed theory of operation on each band from 80 to 10m follows, aided by figures showing the current standing wave conditions on the "flat-top" and the matching (or make-up) section. The relevant theorical horizontal plane polar diagrams for each band may be found in any specialized antenna handbooks. However, it must be borne in mind that :
(a) the polar diagrams generally shown in two dimensional form are, in fact, three dimensional (i.e. solid) figures around the plane of the antenna;
(b) all theoretical polar diagrams are modified by reflection and absorption effects of near-by conducting objects such as wire fences, metal house guttering, overhead electric power and telephone wires, house electric wiring system, house plumbing systems, metal masts, guy wires, and large trees.
Also the local earth conductivity will materially affect the actual polar radiation pattern produced by an antenna.
Theoretical polar diagrams are based on the assumptions that an antenna is supported in "free space" above a perfectly conducting ground. Such conditions are obviously impossible of attainment in the case of typical amateur installations. What this means in practice is that the reader should not be surprised if any particular antenna in a typical amateur location produces contacts in directions where a null is indicated in the theoretical polar diagram and perhaps not such effective radiation in the directions of the major lobes as theory would indicate...
160m. On this band and without modifications the dipole does not work at all (at least in emission). Its length is about 0.183l, three time shorter than half a wavelength. Even placed high over the ground (10-15m) and using a ladder line 10.35m long for the matching section to get the lowest loss, completed with a short coax of low loss too, the antenna impedance (Z = R±jX) is sky high, above 1800W, and thus infinite. It presents a highly capacitive load to the tuner because it is not resonant ! If your tuner is able to match this load, you will get an extremely high SWR on the feed line (VSWR > 160:1, thus infinite using a ladder line), and very high currents will flow in the tuner's inductive component(s). In other words your transmitter's power will be dissipated as heat due to the resistive losses in the feed line and in the tuner inductor(s). The overall efficiency (power in/power out) of your antenna system will be near 0 due to losses that can exceed 35 dB or 6 S-points !
NB. In the field, do never try to drive a too short antenna. Even if you read on your built-in S-meter a weak signal of about S5 in emission (instead of S9+60 dB at resonance !), the RF will not go further than the antenna tuner and your 100 watts will dissipate as heat. This silly action can damage your PA transistors as well as your antenna tuner : the heat dissipation can crack toroid cores, melt the plastic forms in roller inductors, etc. So take care. Therefore I warmly suggest you to buy an external SWR-meter before purchasing any other accessory for your shack. With the dummy load these are the two main devices to buy with your RTX and your antenna.
80m. On this band each half of the "flat-top" plus about 5.18m (17 ft) of each leg on the matching-section forms a fore-shortened or slightly folded up half-wave dipole. The remainder of the matching-section acts as an unwanted but unavoidable reactance between the electrical centre of the dipole and the feeder to the antenna tuner. The polar diagram is effectively that of a half-wave antenna (Fig.4 below).
40m. The "flat-top" plus 4.87m (16 ft) of the matching section now functions as a partially-folded-up "two half-wave in phase" antenna producing a polar diagram with a somewhat sharper lobe pattern than a half-wave dipole due to its colinear characteristics. Again, the matching to a 75W Twin-Lead or 50/80 ohm coaxial feeder at the base of the matching section is degraded somewhat by the unwanted reactance of the lower half of the matching section but, despite this, by using a suitable antenna tuner the system loads well and radiates very effectively on this band (Fig.5 below)
30m. On this band the antenna functions as a two half-wave in-phase colinear array, producing a polar diagram virtually the same as on 40m. A reactive load is presented to the feeder at the base of the matching section but, as for 40m, the performance is very effective (Fig.6 above).
20m. At this frequency the conditions are ideal. The "flat-top" forms a three-half-wave long centre-fed antenna which produces a multi-lobe polar diagram with most of its radiated energy in the vertical plane at an angle of about 14°, which is very effective for DXing. Since the radiation resistance at the centre of a three-half-wave longwire antenna supported at a height of 1/2l above ground of average conductivity is about 90W, and the 10.36m (34 ft) matching section now functions as a 1:1 impedance transformer, a feeder of anything between 75 and 80W characteristic impedance will "see" a non-reactive (i.e. resistive) load of about this value at the base of the matching section, so that the VSWR on the feeder will be very nearly 1:1. Even the use of 50W coaxial feeder will result in a VSWR of only about 1.8:1. It is here assumed that 10.36m (34 ft) is a reasonable average antenna height in amateur installations (Fig.7 below).
17m. The antenna functions as two full-wave antennas fed in phase; combining the broadside gain of a two-element colinear array with somewhat lower zenith angle radiation than a half-wave dipole due to its longwire characteristic (Fig.8 above).
15m. On this band the antenna works as a "longwire" of five half-waves, producing a multilobe polar diagram with very effective low zenith angle radiation. Although a high resistive load is presented to the feeder at the base of the make-up section, the system loads very well when used in conjunction with a suitable antenna tuner and radiates very effectively for DX contacts (Fig.9 below).
12m. The antenna again functions effectively as a five-half-wave "longwire" but, because of the shift in the positions of the current anti-nodes on the flat-top and the matching section, as may be seen from Figure 10 below the matching or "make-up" section now presents a much lower resistive load condition to the feeder connected to its lower end than it does on the 15m band. Again, the polar diagram is multilobed with low zenith angle radiation.
10m. On this band, the antenna functions as two "longwire" antennas, each of three half-waves, fed in-phase. The polar diagram is similar to that of a three half-wave "longwire" but with even more gain over a half-wave dipole due to the colinear effect obtained by feeding two three-half-wave antennas, in line and in close proximity, in-phase (Fig.11 below).