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The ultimate mini-telescopic vertical antenna for the travelling ham
(A SCOTIA team design)

We at SCOTIA recently designed a primarily portable, but efficient, multi-band vertical. It was so compact (3.6m, telescoping in 12 sections to 40cm) and weighing only just over 200g, that your XYL (or OM!) might not have discovered you were carrying it until you reached your holiday destination!. It could be erected in minutes and be resonated on 15m, 17m and 20m (as well as on 10m and 12m with a shorter radiator) by means of its continuously adjustable loading coil. We called it the SCOTIA "Traveller" (as distinct from the SCOTIA "Islander" - see below). NO ATU WAS NEEDED, the S.W.R. being at, or near 1 : 1, on all bands if properly adjusted for resonance. Various optional accessories were available (guying kit, ground-mounting spike, 10m/12m radiator, velocity factor-corrected feedlines in either one or two 1/2-wavelengths).

The current SCOTIA "Islander", a 5m tall multi-band vertical, is derived from the "Traveller" but is based on a succession of prototypes developed and tested by André GM3VLB as part of the process of activating in excess of 220 islands, both in GM-land and overseas, over the past 10 "seasons". Many of these designs have also been used by other well-known GM island activators such as Alex GM0DHZ, Peter GM3OFT, Keith MM0BPP and Lorraine MM0BCR.

SOME THEORETICAL CONSIDERATIONS

Many commercial mobile, portable or other short vertical antennas use some form of base-loading. At the centre of the 20m band, the radiation-resistance of a typical 8-foot "whip", is less than 5 ohms. If the manufacturer claims an S.W.R. of 1 : 1, thus implying an input-resistance (at resonance) of 50 ohms, this can only mean there is a combined loss-resistance (ground + others) of some 45 ohms. The resulting efficiency [(radiation resistance ÷ input resistance) x 100%] is thus less than 10%. In other words, 90% of the input power is wasted as heat. Remember, a 50-ohm dummy load also gives an S.W.R. of 1 : 1 but is not too efficient as a radiator of electro-magnetic energy!

When designing the "Traveller", we increased the length of the 8-foot whip by a factor of almost 1.5, bringing it to nearly 12' (3.6m). Theory tells us that the radiation-resistance increases by the square of this, namely by about 2.2. By also using centre-loading rather than base-loading, theory also predicts a further increase of up to 2.25 times, giving a possible radiation-resistance of some 23 ohms on 20m.

If the "Traveller" then showed an S.W.R. of around 1 : 1 (as it did in most situations), this now equates to an efficiency of some 46% - almost 5 times higher than the 8-foot whip! We will not make any rash claims, but clearly theSCOTIA "Traveller" design achieved a very respectable efficiency and performance.The latter is perhaps best exemplified by the following details from CT/GM3VLB's log. They describe the very first 2-way contacts with the antenna prototype, ground-mounted over arid soil in the middle of a Portuguese orange grove, miles from the sea. The QSOs were witnessed by Don CT1IUW/G3TIR and Ray G3ASG.

Notes:

(i) On the 17m and 15m bands, the efficiencies are even higher, as the "Traveller" is proportionately longer. On the 10m and 12m bands, using the accessory radiator, efficiencies will be higher still.

(ii) A limited number (less than 20) of SCOTIA "Travellers" was available from either André GM3VLB or Alex GM0DHZ. As the prices reflected, all items were home-brew although from new parts. Proceeds went towards expedition funds. They were be sold "as seen" and without guarantee, other than the personal reassurances of two "old-timers" (combined ages approaching 140!), backed by well-documented, successful expeditions to over 220 islands and elsewhere, when this and similar short verticals (as well as the GM3VLB multi-band "inverted V" - see RSGB RadCom Mar/Apr 2005) have given outstanding performance in well over 100,000 QSOs.

Please contact either AndréGM3VLB or Alex GM0DHZ (both OK on <QRZ.com>) for further information. Although the "Traveller" is no longer available, a limited number of its highly successful "offspring" the "Islander" is available from Andre GM3VLB.

28 February 2007