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+---- Thread: A New Project for the GBARC Builder Group (/showthread.php?tid=271)
A New Project for the GBARC Builder Group - VA3KOT John - 2020-11-22
Good news - we are moving out of the solar cycle minimum. Sunspots are back and the Solar Flux Index is rising. We have a long way to go before we hit the solar cycle peak again (around 2025) but propagation conditions are improving.
During the solar cycle peak we are able to make DX contacts with simpler antennas and less power. With that in mind I decided to revisit an old project for pedestrian mobile HF operations. The project involves what is commonly called a buddistick antenna. It is basically one half of the popular Buddipole antenna. A Buddipole antenna is a horizontal portable HF dipole. A buddistick is a vertically oriented monopole that is electrically one quarter wave long. A full length monopole would be too long to be easily portable, but it can be shortened with a loading coil. That makes it an ideal candidate for pedestrian mobile operations.
I have previously used a home-made centre loaded buddistick and made a QRP contact with Italy from the southern shore of Georgian Bay. The design involved separate coils for each band and that made it a little inconvenient to use. I propose to redesign the buddistick with a bottom loading coil that is tunable for multiband operation. And yes, bottom loading is supposedly less efficient than centre loading but it has other advantages like stability that make it more suitable for pedestrian mobile.
Building the coil poses a challenge. There are online calculators for determining the required inductance for each band. Other builder groups, such as Barrie ARC's WAX group 3D-printed a grooved coil former and used zinc-coated steel wire for the coil. A shorting lead allows them to select the required inductance tap for each band. VK3YE in Melbourne, Australia has experimented with concentric coils that can be overlapped to vary the inductance. The commercially available Wolf River coils use a shorting collar that can be slid along the coil to select inductance.
I had a different idea that will be easier to build. In fact, I have built it already. Some time ago I published a post on these forums concerning a Ground Tuning Unit (GTU). It comprises a series L-C tuner for resonating a ground connection. A series L-C circuit is an acceptor circuit; i.e. it will pass signals at the resonant frequency and reject others. If we turn a GTU upside down and use it to resonate an antenna instead of ground it will serve our purpose here.
But, we have one more problem to solve yet. A monopole cannot be used alone. It needs another monopole for the ground side. We could simply use two series L-C circuits; one for the antenna and another for the ground, but that gets cumbersome. The HFPack group of pedestrian mobile operators favour trailing counterpoise wires. I don't personally like that idea because a dragged counterpoise wire is detuned randomly depending on the terrain on which it is laid. In any event, I prefer to operate from a fixed location and even then it is difficult and time-consuming to select the correct counterpoise length.
An alternative is to use a raised counterpoise. A raised counterpoise is less susceptible to detuning by variations in the terrain. On the other hand, on 40-metres we would need a 33ft wire stretched out above ground, fixed to a tree or some other support. That seems perverse to me; the buddipole occupies very little space but it needs a long counterpoise wire? No, that makes it impractical in a public space like a park or a beach.
Fortunately, there is a solution. Yes, we need about about 33ft of wire for a counterpoise on 40 metres, but it does not have to be stretched out straight. It can be wound into a shape called a Petlowany coil. The wire is wound concentrically with each turn spaced from the next to reduce mutual L, C effects. A Petlowany coil is a flat, 2-dimensional counterpoise that is only a couple of feet in diameter for 40 metres. It can be hung on a mounting pole below the antenna. Sounds strange? yes, but I have tried it and QSOd with it.
Over the winter I will work on the detailed design and in the spring I will demonstrate it to the club with a recommendation that the project be adopted by the GBARC Builder Group. It will be a useful project for those of us who like to set up a station while camping, or during a trip to the beach.
Anybody wishing to join the GBARC Builder Group can add a comment or email me at va3kot@gmail.com. No experience necessary.
RE: A New Project for the GBARC Builder Group - VA3KOT John - 2020-11-23
First design thoughts:
The tuning must be as simple as possible. Although a variable capacitor seems like the best solution, high voltage air-spaced variable capacitors are becoming very rare since manufacturers stopped producing tuned RF radios (replaced with VFOs and rotary encoders). One possible solution being considered is to use a fixed inductance and a fixed capacitance that are close to resonance then adjusting the length of the whip for fine tuning.
For a dual band buddistick for 20m and 40m, a 20 microhenry coil in series with a short coax open stub will theoretically work. For the 20m band, 3 inches of RG-8X coax will provide a capacitance of approximately 6.25pF. For the 40m band, 12 inches of RG-8X will provide a capacitance of approximately 25pF. In each case, the core of the coax will be connected to the coil and the braid will be connected to the whip. The coax stubs would be plug-in to facilitate easy band switching.
The inductance and capacitance calculations are based on using a Buddipole 9ft whip (which is actually about 9ft 4in long). If the L-C circuit is resonant at a slightly lower frequency than required, the whip can be shortened to bring the antenna resonant frequency up a little.