Sunday, December 28, 2008
A Tuning Knob Control for the CARC Buildathon Magnetic Loop Antenna
Here is a useful tuning aid for the CARC Buildathon Loop Antenna that allows the loop to be tuned by rotating a knob rather than compressing or sliding the braid. The control is insulated from the RF parts of the antenna and therefore allows tuning to be carried out while applying a few Watts of RF. The addition of a tuning knob is a great improvement over the sliding cloths peg and while there is still some hand capacity effect, this may be easily compensated for during adjustment Figure 1 shows the tuning control in situ on the author's 40m/30m/20m triband version of the CARC Buildathon Loop Antenna. The centre-zero meter on the base is a simple phase meter that allows precise 'on-the-nose' tuning. The phase meter is another very useful addition to the loop that will be described in a future newsletter article.
The trolley was designed using a PCB layout program. Two identical templates were then printed, one for each side plate and glued to two slightly oversized pieces of acrylic sheet. I used a 'Pritt Stick' but any similar adhesive may be used that is easy to peel off later. My template (not to scale) is shown in figure 2. Detailed dimensions are not shown since it is assumed that others wishing to make a similar device will explore their own junk boxes to use of whatever hardware might be available.
The two rollers will probably present the greatest challenge to those not having access to a lathe. However, it may be possible to spin a piece of nylon rod or similar in a drilling machine against a round file to cut the grooves, (although I couldn't possibly recommend such a method on health and safety grounds). Each nylon roller is grooved to a depth of 1mm and is of the same radius as the polythene 'inner' of the coaxial cable. Having a round file of the same diameter as the coax inner helps here. A 5mm holes through the centre of each roller makes a sleeve bearing over 5mm diameter tubular spacers.
The Drive Spindle
The drive spindle is made from ¼-inch diameter paxolin rod to allow a standard ¼ inch control knob to be fitted. A narrow groove was cut first about 3/16 inches from one end. Then the trolley was assembled on a scrap piece of cable to determining where to cut the second groove. The holes in each plate were then elongated to allow the spindle to be tensioned against the polythene inner using two springs fashioned from safety pins as shown in photographs 3 and 4.
The CARC Buildathon Coaxial Loop Antenna was initially conceived as a CARC fun project. However, its performance has exceeded all expectations, despite flouting the golden rules of loop design that shy away from using braid as the radiating element. Those that have tried this small loop will testify to its effectiveness. Running just 2½ W, the author's loop has been heard at over 3600 miles by W3CSW using the WSPR program (Weak signal Propagation Reporter, courtesy of Joe Taylor, K1JT). At the time, the loop was indoors, stood on a sideboard against a wall (see Figure 1).
The addition of a tuning knob in conjunction with a simple tuning phase meter has greatly enhanced the usefulness of this remarkable antenna. Watch out for a description of the associated tuning phase meter in a future article.