Operating out in the field with your trusty QRP transceiver nearly always requires an ATU. However, those in the know favour a near resonant antenna as there is less power to be wasted in matching. I have had good results with resonant dipoles and end-fed antennas, the latter always wins in ease of deployment. I see lots of radio amateurs using an Auto ATU which require a power source and are generally quite limited in their impedance working range.
Over the years I've built several miniature antenna match boxes, and I can say that what follows is the best of the bunch; light, versatile and easy to use. It's not just an L-match, as the variable capacitor and inductor can be configured in to three different networks, enabling it to match into just about any impedance, from tens of Ohms to a couple of kilo Ohms on 80m through to 10m.
Circuit
The circuit detailed in Figure 1. shows a 3 pole 3-way switch which enables the variable inductor and capacitor to be configured into one of three networks (Figure 2); making the most of the components and providing maximum versatility. The match box is equipped with ports to terminate coax, single wire, and balanced antenna feeds. The coaxial input being wired in parallel with the random wire connection point and earth.
There is nothing new about the circuit design and has been culled from a copy of the 73 Magazine published in the USA: author being KB4ZGC. In my version I've chosen to use an LED type VSWR indicator lifted from a an excellent QRP ATU designed by DF3OS. I decided not to build the DF3OS design as I thought I could get more versatility out of the components using the KB4ZGC matcher. The only other circuit change is the addition of a 1:1 balun so balanced line antennas could be catered for.
A moving coil meter was used in the original circuit to show reflected power, but these are now expensive and miniature examples are quite rare. The 1:1 balun is a current type of device, wound on a type 43 material ferrite ring (FT50-43), using the design shown on VK6YFS web pages.
Figure 2. Showing three possible types of tuning network offered by the matchbox.
The dual section 280pF capacitor was purchased from Spectrum Communication. This could be replaced with a similar value single section variable with a fixed 180pF capacitor to be switched in parallel. The variable inductor was wound on an iron dust toroid (FT82-43) as detailed in Figures 3. You will notice that the number of turns needed are not equal on the core, this was to provide a logarithmic type of control, quickly giving a larger inductance for lower frequencies.
Figure 3a. Photo not showing the exact number of turns.
Figure 3b. The number of turns wound on the FT82-43 toroid are shown in each bubble, turns are counted by the number of times the wire passes through the toroid.
I hope you enjoy the project. I have built two of the matchers, the first using a series of toroidal inductors wound in a binary sequence, 2, 4, 8, 16 uH etc. Although it worked very well as a matcher, it was over complex and difficult to operate, plus the extra components took up space and increased the cost of building. The single control for varying the inductance makes for very slick operation.
73 Rick G6AKG
References:
https://archive.org/details/73-magazine-1996-10/page/18/mode/2up
https://vk6ysf.com/balun_choke_balun_hf.htm
https://sem.pl/sp5jnw/konstrukcje/atudf3oshtm/atudf3oseng.pdf
No comments:
Post a Comment