This article describes the design of an antenna for "local" contacts on the lower HF bands (mainly 3.5-14MHz).
The design objectives are:
The antenna is intended to serve mainly local VK contacts. The requirement can be simply met by an antenna with approximately omni-directional characteristics. Even though the location is a semi rural one, a horizontal antenna is chosen for best noise performance.
VK1OD is located in Bowral (about 100km SW of Sydney), roughly in the centre of the south east corner of Australia where well over half of Australia's population reside, as illustrated in the map to the right.
The table shows the nearby state capital cities (2002) and the path parameters for 7MHz communications at 0500 UTC with SSN=100.
Note that radiation angles from 31 to 82 degrees suit these cities.
A dipole mounted low to the ground with its legs sloped downwards from the centre was chosen as a reasonably omni-directional horizontal antenna. A mast to support the centre of the dipole at a height of 11m is available, and the legs can be conveniently sloped downwards at about 30° to the horizontal.
A dipole of length greater than about 35% of a wavelength at the lowest frequency of operation and fed with low loss open wire feedline provides a basis for a reasonably efficient multi band antenna system, albeit requiring an ATU to present the required 50Ω to the transmitter. So, a dipole length of 31m was chosen to allow operation down to 3.5MHz with reasonable efficiency.
On 30 and below, the radiation pattern has two main lobes, though on the lowest bands there is not much variation with azimuth, ie the pattern tends towards omni directional by 80m. The dipole is 3λ/2 on 20m, and so has four major lobes. The pattern develops more lobes at higher frequencies. It is anyone's guess whether one of the narrow lobes on 17m or above is pointing in a useful direction, so use will be mainly on the lower bands and accordingly, this article concentrates on those lower bands.
The dipole will be fed with 10m of home made open wire line using 2mm 7 strand copper wire and commercial spacers.
This antenna system is as described by (Varney 1958), the famous G5RV. Fig 1 shows Fig 2 from (Varney 1958) where he details the purely open wire feed (or "tuned feeders") configuration. (The scan is poor, the right hand side of L2 connects to the right hand side of the variable capacitor at the bottom of the schematic.) The ATU used the VK1OD implementation though is a T match with an effective 1:1 current balun and 50Ω coax to the transceiver.
Fig 2 shows the calculated antenna system efficiency using an NEC model of the structure, TWLLC model of the transmission line, and W9CF's ATU model of an ATU (adjusted for 500pF caps and Ql=80).
Efficiency is one of the common trade-offs in multiband antenna systems, though in this case, efficiency is better than 80% on the low bands, and on 40m at 90% is only very marginally behind the optimised half wave dipole antenna it replaced (at 92%).
The avoidance of coax improves efficiency of the common hybrid feed configuration of the G5RV where antenna system efficiency on 80m and 40m is often around 60% or 1.5-2.0dB worse.
Fig 3 shows where the transmitter power goes on 40m, the main band of operation, Total system loss is 0.45dB.
Fig 4 shows expected peak voltage at the ATU terminals at a nominal 1kW. Figures of more than 1kV are potentially excessive for mid range ATUs (eg MFJ949E), but within the capability of the so called 3kW class ATUs most of which withstand 3kV or so. It can be seen from Fig 3 that the pre-WARC bands are less challenging of the ATU in this case. The higher voltages would be likely to produce excessive dissipation in ATUs that use a ferrite cored voltage balun at their output, a very popular but quite poor choice.
Fig 5 shows the feed point detail. The top end of the feed line can be seen with the Tet-Emtron spacers. The spacers and conductor had problems and will not be considered for future projects. The wire projections above the gibbet are to discourage birds, they are not intended as an air termination for lightning!
The halyard and guy lower down is synthetic rope to avoid interaction with the feedline. Measured common mode feedline current at the entrance panel on 7MHz with 100W is 30mA, quite acceptable.
Fig 6 shows the end insulator termination detail. The copper is terminated using crimp sleeves particular to the purpose. These were widely used on open wire telephone lines, but probably not obtainable now. The tail is 2mm FSWR and a reusable fence strainer used to tension the dipole span to 40N for a design wind speed of 40m/s.
Fig 7 shows a closer view of the strainer.
Fig 8 shows the earthing of the supporting mast to a 2.4m long 16mm copper clad ground rod driven into wet clay. The 35mm2 conductor bond the mast to the earth rod using a Cadweld below ground in the valve box. The ground rod consistently measures around 12Ω resistance indicating a soil resistivity of around 20Ωm (which is quite low). When connected to the mast in its foundation, the combined resistance consistently measures 9Ω to 10Ω.
Fig 9 shows the feed line entrance panel mounted high on the external wall. The grey box contains a Guanella 1:1 current balun designed for tuner application (high voltage withstand, high choking impedance).
Fig 10 shows the feed line entrance panel mounted high on the external wall. The panel is grounded via a 6mm^2 conductor on the inside of the wall. The ATU sits on a purpose built shelf unit, and the connection from the coax jack to the to the ATU terminals is short and direct.
The dipole legs are rigged to allow adequate sag to survive high wind, see Sufficient sag for wire antenna spans for wind survival. A wind design was done for each span with Antenna wire catenary calculator for the materials used and a wind design speed of 40m/s. The required wire tension unloaded by wind is 39N, so the wires were tensioned to just less than that requirement.
Fig 11 shows a shot along the dipole leg, and the sag can be seen clearly.
To rig the span flatter would reduce the capacity to withstand high wind. If the ARRL guideline of 10% of the GBS (1370N) was used, the wind rating under the same conditions would be less than half at around 19m/s or 68km/h... manifestly inadequate.
On-line ham radio forums abound with opinions which naturally tend to echo the folk lore of ham radio.
At (eHam 2013), a poster asked for advice on commissioning his G5RV (Jr) and
was offered the traditional
use something else solution, and from
seasoned author K2DC:
You're getting good advice here. I've tried a G5RV a couple of times at two different QTH's. Every time I took it down and put the 40/80M inverted vee pair back in its place, I was amazed at what I could hear and work. A 40M dipole or inverted vee should work very well on 15M, even without a tuner in part of the band. And it should tune easily on 10M. Some guys swear by G5RV's. I only swear at 'em.
Prejudice abounds (here from Extra grade hams... whatever that means), and for a supposedly scientific based hobby, science is sometimes quite absent.
Many QSO partners insist that the antenna system is not a G5RV because it doesn't use the hybrid open wire / coax feed arrangement. It is doubtful that they have actually read G5RV's writing on the antenna, particularly (Varney 1958). Fig 1 above is from G5RV's article, he tested and described two feed system variants, the "tuned feeder" arrangement as used here and the hybrid open wire / coax feed.
In unstructured QSOs, signal reports on 40m both ways compare with those for the previous half wave dipole.
The antenna system was compared over a 14,700km path on 40m with a quarter wave vertical over elevated radials at a nearby station and the results were comparable with those obtained on previous comparisons with a half wave dipole, see A comparison of a G5RV at VK1OD and QW vertical at VK2DVK on 40m over a 14,700km path using QRSS for a detailed report.
Let me preface this by stating my opinion that WSPR is not a good tool for assessing system performance for a host of reasons.
Nevertheless, a brief WSPR test was run on 40m around grayline hours to observe the reports. Three different transmit frequencies were used to minimise the effect of interference.
Table 1 shows the WSPR spots.
Of the 25 spots, 6 were other stations hearing VK1OD and 19 were VK1OD hearing other stations. Despite a transmit ratio of 33%, rx spots were three times tx spots but this is quite usual as VK1OD has low ambient noise and it seems many if not most WSPR participants are in noisy locations and with poor antenna systems (eg the attic dipole seems very popular in the UK).
Note that all stations were running 5W, which is high power in the mindset of the new breed of train-spotters who have appropriated WSPR for their own narrow purposes.
© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.