On one or two bands, I couldn’t get it completely extinguished, but it did give a definite null. On most bands, I could get the LED indicator to go completely out. The little Z-match loaded up the downspout on 40 through 10 meters with no problems. There may be some interaction between the two capacitors, so you might have to go back and forth between them a time or two.įor an initial test, I hooked it up to the famous-in my mind, at least- WB3GCK Downspout Antenna. Then, switch in the SWR bridge, apply some RF, and tweak the capacitors for minimum brightness on the LED. To use the Z-Match, adjust the capacitors for a null in the background noise in your transceiver. It certainly could have been built into a smaller package, but I had this enclosure on hand and decided to put it to use. The whole thing was packaged in an enclosure which measures 3 x 5 x 2 inches. The bridge will handle a typical 5-watt QRP rig without flinching and could probably handle a bit more than that.
For the 50-ohm resistors in the bridge, I substituted 2 100-ohm, 1-watt resistors. It uses a resistive bridge circuit with a single LED to indicate a null when the bridge is balanced. The SWR bridge I used is a Dan Tayloe LED SWR indicator from a kit that was offered years ago by the Arizona scQRPions. I should have purchased a truckload of them when they were available! Similar capacitors with smaller values are still available if you look around. Unfortunately, Mouser no longer carries them, and I don’t know of another commercial source. The capacitors are poly film variable capacitors (2 sections 365pF each), which were originally purchased from Mouser Electronics. The extra input capacitance can sometimes be helpful on the lower frequencies.
Using this switching arrangement, I can select between one section of the capacitor, both sections in parallel, or both sections in parallel with a fixed 470pF mica capacitor. How convenient!Īnother W6JJZ modification I used was the inclusion of a DPDT (center off) toggle switch to provide some flexibility with the input capacitor. Another reason for choosing the T-200-6 core was that I happened to have one in my junk box. Here again, I went with W6JJZ’s suggested turns count. The number of turns has to be adjusted for the Type-6 core, due to differences in permeability. W6JJZ recommends the Type-6 core over the Type-2 because it provides a higher Q over most of the HF range. First, instead of the T-200-2 toroid specified in the SPRAT article, I used a T-200-6 core. I incorporated a few modifications in my version, based on an article by W6JJZ (“The Z-Match: An Update”, QRP Quarterly, July 1995, pp 10-11). This design, by the way, is similar to the one used in the Emtech ZM-2. I based it on a classic design which was first appeared in SPRAT #84 (see the G3YCC web site for a schematic of the original design). I make no claims of originality for anything in my version of the Z-match. The range of impedances that can be matched is not as great as in other designs, such as the “T” configuration.Tuning is usually very narrow and can be a bit touchy sometimes.Here are some disadvantages of the Z-match design:
This sort of thing usually appeals to QRPers.
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