HOT NEWS
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.
| # |
Date |
Time |
Callsign |
Sent |
Rcvd |
Information |
| 1 |
7 Dec 2005 |
1655Z |
D44AC |
59+ |
59 |
CAPE VERDE islands |
| 2 |
7 Dec 2005 |
1658Z |
FG1GW |
59 |
59 |
GUADELOUPE |
| 3 |
7 Dec 2005 |
1701Z |
VP5/W2VD |
59 |
57 |
TURKS & CAICOS |
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 |