A Z-Match type Antenna Matching Unit for the AM broadcast band by VE3EAR
Like many who hunt for DX on the broadcast band, either with a crystal set or a modern solid state digital wonder, I use a random length wire as my antenna, along with a good ground. We all know that the last bit of signal that can be squeezed out of an antenna can only happen if the antenna is "matched" to the radio. I decided to try and adapt the popular Z-Match circuit used by hams on 80 thru 10 metres for use on the AM broadcast band. The beauty of the Z-Match circuit is that it does not require an expensive roller inductor or a lossy, multiple-tapped coil. All tuning adjustments are done with just two controls, both of which are on variable capacitors.
A quick Google search for Z-Match will turn up a vast number of sites with details on building them for the ham bands. Aside from changes to the values of the two capacitors, and the number of turns on the toroid inductor, the only other change I make to the basic circuit is to eliminate the tap for the ground connection. Since the unit would not be used with a balanced feed line, there was no need to tap the inductor so that the link coil "saw" a more or less balanced coupling to the main coil. This also made construction easier, as only C1 needed to be insulated from chassis ground. The frame of C2 and the "cold" end of L1 both go to ground.
I raided my junk box and found a couple of variable capacitors, or varicaps as I like to call them, and a T-200-2 powdered iron toroid core. For C1, a three-gang varicap from an old BC receiver was chosen, and with all of the sections paralleled it varied in capacity from 75 to 775 pF. For C2, a nice four-gang unit was chosen, each section varying from a minimum of 25 pF. to a maximum of 560 pF. I wired the front two sections in parallel for tuning the centre tap to ground portion of L1 and the rear two sections where paralleled for tuning the full coil. A few moments spent with an on-line L-C resonance calculator told me that I needed about 40 uH. in total for the main coil, L1.
Using some #22 insulated hookup wire, I wound the main coil, L1, using the figure 8 style to have the two ends of the coil on opposite sides of the toroid. It turned out that a total of 60 turns filled the inner diameter of the toroid nicely and the crossover made for a convenient place to locate the centre tap. I measured the full coil with my LCR meter and by plain luck it came out to 42.5 uH., not far at all from the calculated 40 uH., and with no sign of Murphy getting into the act! The link winding, L2, which is for connection of the antenna and ground, is inter-wound with the "cold" end of L1 and it consists of 9 turns of the same wire. I connected the leads of L2 to a BNC jack, but you could use binding posts or whatever other type of connections that suit your own application.
The connection to the radio is done with a short length of coax connected to another BNC jack on the unit. The centre pin of the BNC jack is connected to the frame of C1 and its stator is connected to the centre tap of L1. I mounted both jacks on a small piece of aluminum angle stock which also serves as the common ground connection of the unit. I included a pair of 400 pF. fixed capacitors which can be connected with clip leads to be in parallel with C1 and/or C2 if additional capacity is required By varying the connections to the two fixed caps they can be configured for 200, 400 or 800 pF. of additional capacity. With my 200 foot long wire, I found I couldn't quite make it to the bottom of the band, but connecting an extra 200 pF. across C2A solved that problem.
I chose to build my unit "breadboard style" and I used a 1/4 inch thick piece of Lexan as the chassis. This also served as the insulator for C1 which has to be mounted above ground. It would be a good idea to include an insulated shaft extension and knob on C1 as well, to minimize any hand capacity effect. Tuning is pretty sharp, and a pair reduction drives with vernier dials would add a nice touch, but regular large diameter knobs do a good job too. If you decide to put the unit into a metal box, be sure to leave some clearance around the varicaps and the toroid and insulate C1's frame and shaft.
My description and pictures are intended as just some guidelines, and you can tailor your own unit to whatever components your junk box yields. I think you'll find the Z-Match a useful addition to your DXing hardware!73, J. Bruce McCreath, VE3EAR