A Grounded Semi-vertical Aerial for the L.F. Bands by G3BDQ

By JOHN D. HEYS, G3BDQ.  This was originally published in the RSGB Bulletin, February 1964, and reproduced with permission.

During the autumn of 1963 the writer decided to try his hand at working some of the 80m S.S.B. DX which was reputed to be available between 07.00 and 08.00 G.M.T. almost daily. Using a horizontal 130 ft. end-fed general purpose aerial proved disappointing and it was most frustrating to give and receive S3 reports or worse when several other British operators with similar transmitting equipment were having S8 and S9 contacts with ZL and VE stations. It did not take long to discover that these successful stations were using quarter-wave vertical aerials. Realizing the limitations of a small urban garden and having a shack on the top floor of a tall Victorian house, G3BDQ almost abandoned the quest for 80m DX.  It was then decided to try a sloping wire quarter-wave suspended from the top of the house, but the problem of feeding the aerial remained. Conventional bottom feeding (Fig. 1(a)) would have required about 80 ft. of coax and the running of this feeder up to the shack almost parallel with the radiator did not seem to be a good idea.

The notion of inverting the aerial (Fig. 1(b)) next occurred but had to be discarded, for such a configuration would certainly not behave as a ground plane and would no doubt just produce high angle radiation. Eventually the idea of top “Zepp” feeding the quarter-wave and earthing its base (Fig. 1(c)) was hit upon. A careful search through the aerial books did not shed any light upon this unorthodox method of feeding a quarter-wave aerial, but it was decided to go ahead and try it.

Fig. 2(a) shows how the aerial is arranged, the top being suspended from a flexible length of bamboo to keep the wire away from the brickwork.

grounded-semi-vertical-g3bdq1The bottom end of the wire is connected to an old galvanized water tank which is buried in the garden and also to four quarter-wave radials which are laid out in the form of a swastika along the edges of paths and fences. These are buried a few inches wherever there is any danger of exposed wires tripping up the unwary.

The aerial length L in feet may be calculated from the Formula 234/Freq. where the frequency is in Megacycles.

Most of the 80m SSB DX working takes place at the top end of the band and for this reason a design frequency of 3.8 Mc/s was chosen.

The radials are each 63ft. long and for a different design frequency their length can be found from the formula 240/Freq. (Mc/s). The earthing arrangements are perhaps the most important factor in this and any quarter-wave aerial system, and the greater the number of earthing rods and radials used the better will the aerial perform.

grounded-semi-vertical-g3bdq2Fig. 2. (a) Simple sketch showing the writer’s arrangement of his aerial, (b) Alternative system using a tree or other high object to support the aerial in which case the aerial can be some considerable distance from the operating position.

Only 15 ft. of open wire feed line was needed, for the top of the aerial came to just above the shack window. With a short length of line there is a high impedance at the ATU. end, but should the alternative layout in Fig. 2(b) be used the longer feeder line could result in a lower impedance and some form of series tuning at the a.ATU.

The radiator and the radials were made from 16 s.w.g. bare copper, which is obtainable from most builders’ merchants. The feeder used 18 s.w.g. enameled wires spaced 2 in. Spacers were placed every 18 in. The polythene ” Comba Roll” hair rollers obtainable from Woolworth’s make excellent feeder spacers, for they are light in weight and are of open geodetic construction.

As soon as the finished aerial was connected up its advantages became apparent. It became possible to work DX stations that could not be heard on the half-wave horizontal wire. The best reports during a month’s operation have been S9 from VE3 and VO1, and S7 from ZL although the mornings paths to these stations go right through the house with its conglomeration of electrical wiring and plumbing. For G working and contacts up to 400 miles the grounded semi-vertical is inferior to the old half-wave aerial with its high vertical radiation angles. Between 400 and 1000 miles the two aerials give similar results, for contacts with stations in excess of 2000 miles the semi-vertical has an advantage of at least two S points. Of course the stations in good locations with properly matched quarter-wave towers and extensive radial systems still lead the race, but there are not more than half a dozen such stations active in the British Isles, and the simple aerial described enables G3BDQ to work his fair share of the DX.

Top Band
Just before the preparation of this article it was thought that perhaps the aerial could be made to work on Top Band. A transmitter was not available so a simple crystal oscillator was wired up (Fig. 3) using the aerial as part of the anode tuned circuit. The ends of the feed line were joined together and a coil LI was connected to the oscillator anode via C1, a blocking capacitor.

The total length of the aerial and its feeder is about an eighth of a wavelength on 160m and the inductance of LI in series with it brings the system to quarter-wave resonance. Fine-tuning is performed by C2, a much smaller capacitor than is usual in Top Band tank circuits. In theory the greatest radiation from an aerial takes place at the point of maximum aerial current, so the inclusion of the inductance LI should not cause much loss in efficiency; the current loop or anti-node being at the earthed end of the aerial.

grounded-semi-vertical-g3bdq3Fig.3.  Circuit diagram of a simple crystal oscillator used for testing the aerial on 160m.  Suggested values: C1,  5000pF;   C2,  47pF;   C3,  twisted wire “gimmick” capacitor to increase feedback,   L1,  40 turns close wound on 1.25” diameter former;   V,  any small output tetrode, 6AQ5,  6V6 or similar.

With an input of 5 watts to the crystal oscillator, contacts were made with many stations including an OK. Operation was restricted to a few hours, but the reports received were as good as or better than those obtained with a quarter-wave horizontal aerial tuned against ground and 10 watts input.

It is hoped that this article will be a ray of hope for flat dwellers, lighthouse keepers, and all those who inhabit upper floors of tall buildings, who previously despaired the likelihood of getting up an efficient aerial system for the LF. bands!

John Heys (G3BDQ) – February 2010.

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