The Wonder Whip?
A £10 QRP Portable Multiband Antenna for HF, VHF and UHF
A variation on the “Miracle Whip” and “Wander Wand”.
The Wonder Whip
Context
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.
Existing Products
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.
23 TAPS
Additionally, you may prefer to form small twisted loops to the windings (see below) to make a more mechanically robust soldered joint.
To allow finer tuning, particularly, for 7MHz through to 1.8MHz the “23 TAPS” diagram may be amended as follows:
- inserting an additional single tap at the point marked B;
- connecting the whip to the pole of a S.P.D.T. switch (single throw, double pole switch);
- connecting points A and B to the relevant “Throw contacts” of the S.P.D.T.;
- switching between A and B, reduces the inductance, and effectively gives access to the turn between each physical tap.
Materials
- Telescopic Whip Maplin LB10 HQ 10-section 1.31 metres
- Ferrite ring Fair-rite FT140-43 or RSGB 36mm
- Rotary switch Maplin N89BX 12 way (2 off)
- Black Knobs Maplin KB4
- Antenna wire 16, 18 or 20 gauge
- S0239 Sockets (2 off)
- Binding post 4mm
- Enclosure Floppy disk box
- (Additionally I have included a 3/8th socket for mobile whips)
Construction
From Figure 1 it should be possible to assemble the various elements of the design, however the toroid requires further explanation. The toroid core is wound with 47/48 turns of 16-gauge antenna wire (see note above about smaller diameter Cu wire). The core is tapped at every second turn. Because 24 pole, 1 way rotary switches are not readily or economically available, two 12 pole rotary switches are used. The 12th pole of the first rotary switch is connected to the centre rotor connector of the second switch, to give 23 positions of adjustment. The 23 wire connectors to the toroid are directly soldered to the windings. This was achieved by: a, cleaning and “tinning” the relevant core winding; b, forming a small “tinned” loop to each connecting wire; c, positioning each wire on the tinned core winding, applying the heated soldering iron, which will melt the two tinned surfaces to fuse a secure joint. This process requires patience and care. The telescopic whip is 10mm diameter at the base, with a 4mm hole to accommodate the antenna wire. I modified an PL259 connector by drilling out the cable entry connector to 10mm interference fit, into which I screwed the telescopic whip. The whip is secured to the PL259 connector by finally soldering the antenna wire into position. The green 4mm post identifies the socket for the earth/counterpoise. A counterpoise may assist in lowering the SWR on some frequencies, particularly, below 14Mhz. Above 14MHz the use of a counterpoise does not seem to make any significant difference to on air reports – I disconnected my CP, halfway through a QSO on 18MHz and the other station reported no change in signal strength. I use approximately 10 Metres of 1.5mm diameter wire for all situations. Results suggest that a quarter wave length for the required operating frequency will work satisfactorily.
Prototype – before the “paint job”
Operation
The antenna is tuned by selecting the required band on the radio, and then rotating the two rotary switches on the ATU until the highest background noise or signal is achieved. The antenna peaks on receive and this will provide a good starting point for transmitting. Set your rig to a low power setting (5 watts) and transmit while observing the SWR meter. Rotate the rotary switch knobs until the SWR is optimised at the lowest level. Fine adjustments to the SWR may be made by altering the length of the telescopic whip (not whilst transmitting).
VHF and UHF operation is performed by selecting position one on the control knobs and adjusting the antenna to a quarter wave length.
Horizontal polarisation, particularly for SSB 144MHZ operation is achieved by turning the antenna on its side.
Performance
My subjective feeling is that receive performance is first rate. S meter readings appear very respectable.
Transmit SWR results, using a 5 Watts carrier (23 Taps, without a counterpoise):
- 432MHz 1.5.1
- 144MHz 1.5:1
- 50MHz 1.1
- 28MHz 1:1
- 24MHz 1:1
- 21MHz 1:75
- 18MHz 1:1
- 14MHz 1.5:1
- 10MHz 1.5:1
- 7Mhz 3:1
- 3.5MHz 8:1
SWR results, 5 watts carrier with (23 Taps with a counterpoise):
- 28MHz 1:1
- 24MHz 1:1
- 21MHz 1:1
- 18MHz 1:1
- 14MHz 1:1
- 10MHz 1:1
- 7Mhz 1.5:1
- 3.5MHz 6:1
SWR results, 5 watts carrier (46 “Virtual Taps” with counterpoise):
- 7Mhz 1.3:1
- 3.5MHz 3:1
- 1.8MHz 4:1
On Air Reports
Does it work well? Emphatically YES.
My first portable contact on 40 metres, using 10 Watts, was on Sunday 29th January 2006. Jan, at GB2IWM, gave me a 53 signal report on 7.057MHz.
SM4YPG, Lars, north of Stockholm reported, on the 31st January, 4/7 to 5/7. I was using just 5 watts.
On Wednesday 1st February I contacted AC5N, /Portable from Star Castle G/008/C. Terry, in Oklahoma, USA, on 21MHz, using 15 Watts, reported 59 on his QSL card! A distance of 4400 miles using just 15 Watts into a 51 inch telescopic whip!
Early results are encouraging and I am hopeful that this little box of tricks, and my 51-inch telescopic whip, will serve me well on my next “pedestrian portable expedition” to the remote corners of the islands. My initial enthusiasm is, however, tempered by realism, I have little expectation of making ground breaking DX. But if I can bridge the gap between Scilly and the Mainland and enjoy the occasional QSO when conditions permit, I will be happy. If not, well there is always the view……
Article by M1IOS John originally available at myweb.tiscali.co.uk/m1ios/html/wonder whip.htm
© M1IOS, John Goody January 2006