The Wonder Whip?
A £10 QRP Portable Multiband Antenna for HF, VHF and UHF
A variation on the “Miracle Whip” and “Wander Wand”.
I enjoy low power, portable operations, from various headlands on St Mary’s and also from some of the more remote off-islands around my home, on the Islands of Scilly. My Yaesu FT857D acquires its RF signals from an ad-hoc antenna system, based around; a telescopic fishing pole, various bits and pieces of wire, a lightweight MFJ-901B ATU for HF; and a SOTA beam for 144MHz operations. Many of the locations I visit are remote, rocky and without topsoil. In many circumstances, it is almost impossible to erect any form of antenna. Often the soil is only millimetres thick and it is not possible to guy the telescopic pole. Vegetation is low lying and making attachments to the granite rocks, to gain height, is not really feasible. I wish to combine my interests in amateur radio with my desire to operate from the picturesque locations these islands have to offer.
There are several commercial solutions to my problem: Miracle Whip, Wander Wand, Buddistick, ATX Walk-about and various Maldol and Diamond systems. Reviews of these products suggest that they all work well, within the limits of a very short antenna and radiating elements. It is notable that many of the reviews, highlight the lack of any facility to attach a counterpoise. These ready made, commercial solutions, come at a high economic price. Against the background of what I need, I have set myself the task of designing and constructing a portable multiband QRP antenna system. The main constraints are economic, my limited antenna design expertise, the availability of materials (no radio shops in the Atlantic!) and my very limited test equipment.
The Autotransformer – Miracle Whip
Trawling through the Internet I discovered an article by Robert Victor, VA2ERY, who described his investigations with an autotransformer, as a solution to achieving multiband, QRP, portable, HF operations. Briefly, he describes methods of transforming impedance, using a transformer rather than a loading coil to match a short whip over a wide range of frequencies. The autotransformer may be considered as a double wound transformer; the bottom, primary part being connected to the rig, whilst the secondary is connected to the whip. The impedance transformation is the square of the ratio between the secondary and primary windings. The tapping point varies the ratio between the secondary and primary windings and establishes an impedance transformation. The article can be read in full in QST, July 2001 © ARRL.
The VA2ERY design is a continuously variable autotransformer, based on a coil transformer of 60 turns with the impedance ratio selected by a wiper sourced from an “Ohmite” rheostat. Clearly there are considerable manufacturing difficulties in producing and winding the transformer core and the need for accurate construction and placement of a wiper mechanism.
Using VA2ERY’s autotransformer concept as a starting point, I experimented with a compromise design which would allow me incremented tuning (rather than continuously variable), with the advantage of less rigorous mechanical complexity. After some experimentation (trial and error) my final design employs a 36mm outside diameter, ferrite ring and is wound with 48 turns of either 16, 18 or 20 gauge copper wire. The core is tapped at every second turn. Fine adjustment is achieved by altering the length of the whip.