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The SCOTIA "Bandhopper" - a mobile multi-band vertical antenna


See also the GM3VLB Short Multi-Band Vertical
the GM3VLB Multi-Band Inverted-Vee Dipole
the GM3VLB Mini-Delta

the SCOTIA Poor Man's Antenna Analyser

and the GM3VLB Expedition Equipment List

 

(The RSGB and RadCom were given priority over publication of this article, but after many months of “inaction”, I have decided, for the benefit of fellow amateurs, to publish it myself on this web-site)

 

In the search for an effective and efficient aerial for /M and /P operation on all bands 10m to 80m, SCOTtish Islands Award team members Alex GM0DHZ, Niall GM6GMZ and myself have come up with the novel design described below.

Back in my 5Z4KL days in the mid-60s, I had acquired, second-hand and free of charge, an American mobile antenna known as the Webster “Bandspanner”. Telephone communication was then very limited throughout East Africa and indeed most of the African continent. Man had not yet landed on the moon and the “mobile phone” was certainly unheard of! Communication across the 3 former colonies of Kenya, Uganda and Tanzania relied almost entirely on 2-way radio, mostly HF “mobile” (hence the availability of these /M antennae), be it the police, the army, the emergency services such as the Flying Doctor or indeed private business. The road network was also very limited (almost no tarmac to speak off) resulting in Nairobi’s Wilson Airport having the biggest number of small aircraft movements in the world.

/M with Floors castle in the background

As we know, maintaining reliable radio communication 24 hours a day over distances ranging from the limit of the VHF range to 1000 miles or more, requires the ability to change frequency rapidly and over several bands.

The “Bandspanner” remains today a unique, non-automatic /M aerial which covers 10m to 80m with one simple adjustment, at a readily achievable SWR of 1 : 1. There is no need to change loading coils or whip sections, or to have coil taps (whose positions are never quite right), or to have large, high drag, physically unwieldy, potentially troublesome, screwdriver-type motorised tuning. In effect, tuning is done by shorting out more or less turns by simple raising or lowering a high-tensile tapered whip passing through a clever locking “clutch” arrangement. Marks are etched on the whip at the approximate positions for the 5 bands available at the time. Moving from one band to another can be achieved virtually in seconds. The only minor problem is the “U.S.A.” tuning range on 80m – 3.8 to 4.0MHz! However, with a small capacity hat (a piece of wire clipped on just above the coil), operation across the Region 1 80m band is possible.

The original “Bandspanner” consists of a 5-foot by 1” diameter high-quality fibreglass tube at the top of which is wound a 2’ long, spaced, loading coil, part of which protrudes inside the tube. At the top is a locking “clutch” mechanism through which a 57” tapered steel whip moves up and down, with a clever sliding contact internally shorting out turns.

Its near-centre loading results in a greater (theoretically up to 2.25 times) radiation resistance than base-loading, thus achieving improved efficiency. Again, at some 10’ when fully extended (on 80m) and mounted on a suitable mobile mount, it far out-performs its 4 or 5-foot long rivals, or indeed even the ubiquitous, “standard” 8-foot mobile whip. The slim design offers minimal drag and, if suitably anchored, causes no concern, even at sustained, top European motorway speeds.

The problem today is that, despite having designed arguably the best-ever mobile aerial of its day, the Webster company went out of business in the late 60s. If ever you see a Bandspanner for sale, grab it!

In the meantime, we at SCOTIA, using our considerable experience with short verticals, both in activating islands and in over 40 years of /M and /P operating, have designed an aerial, the SCOTIA “Bandhopper” fit to challenge the best available on the market today

This is not an idle claim!

We may perhaps be a little limited by “kitchen-table” or “garden-shed” engineering but, this said, we believe our design to be, in some respects, even superior to the “Bandspanner” in that, unlike the latter, it uses a considerable part of its maximum available “support-tube + whip” length of around 9’9” on all bands except 10m and 12m where it is shortened to act as a full ¼-wave on 10m.

Indeed, from 15m up to 80m , it is some 9’7” long. The variable inductance loading-coil tuning uses a novel “fixed spring contact” arrangement with the coil/whip assembly moving past this, “as one”.

Two sets of contacts are used, one for 10/12m and one for 15m to 80m. The lower contacts also allow operation (with a shorter aerial overall) on bands up to 40m.

The sliding-coil itself, whilst less than half the length of that of the “Bandspanner”, allows continuous tuning across all bands. On 80m, an additional fixed inductance wound on the outside of the base of the support-tube, is brought into play, the sliding-coil movement then allowing tuning from 3.5MHz to 3.8MHz.

As with the Bandspanner efficiency is again enhanced by the use of near centre-loading, as well as a relatively low-loss sliding-coil.

In mobile use (or if necessary when /P), the author favours a simple, base-mounted, switched-ratio toroidal transformer, (see web-site) matching aerial input impedances between about 5Ω and 50Ω.

Other matching methods are of course possible.

Basic construction

Some constructional details follow for those wishing to “have a go”. Possible sources of some of the materials used are listed at the end of the article. The main items required are a rigid insulating pole 33mm in diameter by about 1.7m tall, a 53” tapered, high tensile steel whip, a 25mm diameter coil former (solid or tube) grooved at 14 T.P.I. and just deep enough to hold the turns in place, a 3/8” x 24 UNF bolt about 50mm long, about 7m of 20SWG enamelled copper wire for the 80m loading coil (initially about 65t/60µH) wound on and near the tube base, about 11m of 18 SWG bare, tinned copper wire for the sliding coil, and miscellaneous small hardware.

The recommended pole is what is known as a boundary marker pole and is used in the sporting world (alternatives could conceivably be used, but the individual constructor would have to determine new dimensions and performance might then be affected). The marker poles come with a plug at the top end and a thick (ground) spike embedded in a solid 40mm plug at the other. The spike and its plug can be removed by gripping the spike in a vice and pulling the pole with a twisting action to crack the adhesive. The other plug can then be knocked out with a long broom handle, or similar. The embedded spike has two small “flanges” which prevent it coming out, but if a 10mm hole is drilled in the base of the plug until is meets the spike, and the hole is cleaned around the spike edge, the spike can be gently hammered out, whilst suitably supporting the plug it to prevent it shattering. (Although not tried, heating the spike might help).

The 3/8” x 24 UNF bolt can now be made to fit the plug and the assembly used to attach the finished antenna to standard mobile mounts. The plug is held in place with a small self-tapping screw to prevent rotation. The head of the bolt is cleaned and soldered (or drilled and tapped to take a self-tapper and solder tag) to connect to the base of the antenna (i.e. the 80m coil). When complete, it is advisable to cover this with silicone sealant (or similar) and to drill a small drain hole level with the top of the “plug”.

The other plug is modified with a suitable bush (e.g. dowelling) insert about 20mm long, which is itself drilled to accept a TV aerial plug modified to act as a locking clutch through which the sliding tapered whip passes. The whole assembly is then suitably “locked” to prevent rotation.

The 10 – 80m sliding coil is tightly wound with 18 SWG bare tinned copper wire, onto a 250mm length of either PVC (or other) tubing or solid rod grooved at 14 T.P.I.. The tubing is “plugged” at the top end with a 20mm length of suitable “dowelling”, “epoxied” into place. A brass panel pin in each end of the coil-former can be used to anchor the ends of the coil. The plug is drilled to accept the telescopic whip. This must be connected electrically to the top of the coil. Because of the difficulty in soldering to the stainless steel whip, the author made a short (3 or 4mm) bush to be a very tight fit over the whip, hammering it on the last 2 or 3cm. A soldered connection can then be made to the top of the coil).

Contact with the sliding coil is maintained by two sets of a novel, but very simple, “spring contact” arrangement. At each position, three 4mm x 5mm rectangular holes are made at 120° round the circumference. Into each of these is placed a small 5-turn “spring” wound with 20 SWG tinned copper on a 2mm drill-bit, 5mm “tails” are left at each end, parallel with the outer surface of the “spring”. The three springs are joined to form a closed loop, using thin flexible wire soldered to the tails.

The actual “springiness” (as well as waterproofing) is provided by a 20mm length of discarded bicycle inner tube placed across the top of the “springs” It does work!
In the unlikely event of the system wearing out, replacement of both the “springs” and the tubing is simple and virtually cost-free !

The base loading coil consists of some 65 turns of close-wound 20 SWG enamelled copper – an inductance of about 60µH is required (the inductance being important, rather than the number of turns). Initially it’s best to wind on 2 or 3 extra turns.

The top of the coil must be connected to both sets of “spring contacts”. The author used copper sheath removed from RG-213 (or similar) coax, flattened and stuck to a strip of double-sided carpet tape running up the tube and between the spring contacts Additional protection (e.g. fibreglass, epoxy glue, varnish, tape etc.) can be used later.

With the aerial completed and suitably vehicle-mounted , the fixed 80m coil can be adjusted. Make sure the shorting link is not in place!). Set the sliding coil/whip so that its top two turns are resting on the upper set of spring contacts (i.e. minimum inductance). Check the resonant frequency (ideally with a suitable aerial analyser) – this should be below 3.8MHz. Now remove turns (to be safe, one at a time!) until the resonant frequency rises to 3.8MHz or just above. The 80m coil adjustment is complete. Moving the sliding coil/whip to its highest (fully extended) position should allow tuning down to 3.5MHz and below (and of course all frequencies in between).

Whilst some measurements are clearly critical (e.g. tube and whip lengths, the sliding coil turns and spacing, the 80m coil inductance etc.), much of the constructional detail is left to the ingenuity and the junk-box of the individual constructor. This surely ultimately allows for greater personal satisfaction than when copying every last minute detail. Some photographs are included below to provide some inspiration.

Best of all, this aerial can be made for £20 or less, depending on your junk box and perhaps powers of persuasion in getting fellow hams to share your project costs. Please feel free to contact André by e-mail at gm3vlb@btinternet.com (but please don’t expect an unlimited free consultancy service!!).

N.B. The “Bandhopper” can be used /P using a unique conversion base-plate (borrowed from the “Islander”) clipped to the base of the tube. This takes the coax from the rig and a twin-radial system. The aerial is then mounted on a short support and held in place, for example, by three guy wires/tent pegs, or other means (e.g. strapped to a hotel balcony). The two opposing radials should be kept about 30/40cm above ground, e.g. with garden canes.

 

SUGGESTED SUPPLIERS OF PARTS FOR THE SCOTIA “Bandhopper”

Prices shown were as of mid-2007 - contact individual suppliers for the latest information
(Clearly, cost can be greatly reduced if made as a group/club project)

1. PLASTIC BASE TUBES: These are “Boundary Marker Poles” obtainable from Valentine Distribution, 85 Amberley Road, PORTSMOUTH, Hampshire, PO2 0TQ. Web-site www.valentinedistribution.co.uk . They come in 4 colours (yellow, red, blue and orange) and cost £3.50 each. The price for 4 (one of each colour, including P&P and VAT) was £18.95

2. TINNED COPPER WIRE: The Scientific Wire co., 18 Raven Road, LONDON, E18 1HW (Tel: 020 8505 0002) Web-site www.wires.co.uk (e-mail: dan@wires.co.uk). The cost for TWO x 500g reels of 1.25 mm tinned copper wire was £15.07 including P&P and VAT). They can also supply the 0.8 to 0.9mm enamelled copper wire needed for the 80m coil (old motors, dynamos, alternators etc. are also a good source of enamelled wire).

3. 53” TAPERED STAINLESS STEEL WHIPS: Sandpiper Aerial Technology, web-site www.sandpiperaerials.co.uk Tel: 01685 870 425 (contact Chris, mention GM3VLB).These were available at £3 each at amateur rallies (on prior request) - Extra P&P and VAT if ordered by post.

4. 3/8” x 24 x 2.5” PLATED BOLTS: Aircraft quality - some available from GM3VLB - £3.00 each (inc. P&P)

5. COIL FORMERS: This is the tricky item. This has to be 9.5” long and almost exactly 1” outside diameter and shallow-grooved at 14 T.P.I. Rapid Electronics Ltd., Severalls Lane, COLCHESTER, Essex, CO4 5JS, web-site www.rapidelectronics.co.uk e-mail: sales@rapidelec.co.uk , Tel:01206 751166 - Rapid have 1” diameter, thick-walled plastic conduit, Order Code 04-5415 at £1.65 + P&P + VAT for 3m .
DAVIS INDUSTRIAL PLASTICS (contact Gavin Davis) do a 1m 1” grey PVC solid rod. See web-site www.davis-plastics.co.uk and their eBay shop < http://stores.eBay.co.uk/Fantastic-plastic>

6. 2mm PCB SOCKETS: These are available from Rapid Electronics (see above). Part No. 17-1042 (tinned) or 17-1040 (gold-plated)

7. 2mm “PLUGS”: Actual 2mm plugs are relatively costly. GM3VLB uses a crimp-on pin (again available from Rapid Electronics, Part No. 33-1274, “red”) which is a good, and much cheaper, alternative.

8. LOCKING CLUTCH SYSTEM: This consists of a high-quality TV aerial plug (Maplin No. FD85G - see www.maplin.co.uk) fitted with  a twin rubber “collet” made from a small rubber cabinet foot sliced in two. GM3VLB has a limited number these for £5.00 (inc. P&P)

9. OTHER HARDWARE: Junk-box, or wherever !!

Good luck !!

See also the GM3VLB Short Multi-Band Vertical
the GM3VLB Multi-Band Inverted-Vee Dipole
the GM3VLB Mini-Delta

the SCOTIA Poor Man's Antenna Analyser

and the GM3VLB Expedition Equipment List

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