Antenna-Tuner for 50 MHz

Direction Finding Antenna

 

Modified CB-Antenna-Tuner for 50 MHz

A lot of hams have no special antenna for the 6-m-Band, but a transceiver which has this band build in without an internal antenna tuner. During ES-openings it is possible to work stations with the first hop all over Europe with simple antennas, in some cases DX can be reached. With a cheap antenna tuner you may work on the magic band!

In principle, wire antennas such as monoband dipoles for different bands, Windoms, G5RV and W3DZZ for 50MHz can be used. It comes to the aid of the fact that most baluns (for example those of Fritzel in Germany) contain toroid material that is specified and usable up to 50MHz. The same rules are valid for beam- and quad-antennas.

The praxis shows that due to transformation effects on the feeding coax-cable a SWR of >3, in most cases about 2 can be seen.The used coax cable should be of better quality, not just RG-58, to minimize the losses due to a higher SWR. Predictions can hardly be made about the respective impedance at 50 MHz since these depend very much on the individual lengths, the height above ground and the ground conditions. Nevertheless, there is a high probability that a relatively low-impedance feeding point will be available at the station. With a simple antenna tuner then an adjustment is possible.

As an example the Azimuth pattern of a 18,1-MHz-dipole shows the practicability of such an antenna (Fig. 1). For a W3DZZ I have plotted the expected Azimuth pattern for 50,15 MHz (Fig. 2). The axis of the antenna is 0-180. Four peaks with a gain of 5 dBd and several others are to be seen. The elevation lobes for a 10 m high W3DZZ as example are not to bad as Fig. 3 shows.

Fig. 1

                                                                  Fig. 2

For years, I have been using a modified "CB Matcher" antenna tuner (Fig. 4). These cheap and simple parts are very useful for amateur purposes! They originally contain an air coil with 10 turns. I have installed a new inductor, it consists of 5 turns CuAg with 1.5 mm thickness with an inner diameter of 10 mm. Two mica tuning capacitors with a final capacity of about 120 pF are build in and can withstand more than 100 Watts RF on the 10-m-band. Disadvantage of these capacitors is their very nonlinear characteristic and the fact that several turns are tunable. Because of this a scale is useless. But once you have found the right tuning points, the settings can be retained and a re-tuning is usually omitted. Fig. 5 shows a view into the converted matchbox, which is a classic PI- or Collins-filter (Figure 6).

It is of course possible to build your own fitting device. The tuning capacitors used should have 100-150pF capacity. Broadcasting rotary types 2x 300-500  pF are sufficient with the dielectric strength, if two disk packs are connected in series. This halves the capacity and doubles the voltage they can withstand.