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Z-Match for 6 meters
By John Haserick W1GPO
The Z-Match is an impedance matching network designed for use with any transmitter that is within the design frequencies of the device. The Z-Match allows the transmitter to operate at a maximum efficiency by matching the source impedance (transmitter output impedance) to the load impedance (antenna, duplexer, isolator, etc.).
I have attempted to make many filters for 6 meters, but none have really worked without excessive loss. I know that the GE MastrII on a local 6 meter repeater required a commercially-built Z-Match between the output of the 100W PA and the Wacom Duplexer, so I thought maybe our 6 meter Micor repeater would also benefit from one. Mine (and probably the commercial version) is just a pi network with input and output variable caps to ground with a coil between the caps. I built it inside a 2"x2-1/4"x4" die cast aluminum box with two N-female chassis mount connectors, #12 enameled wire, and two variable caps, one 50pF and the other 100pF maximum.
The Z-Match coil itself is 5/8" OD, 4 turns spread about 1" long, with about 5/8" pigtails on each end. The #12 wire was close wound over a 1/2" tube then the turns were stretched outside the tube. With the N connectors disconnected, and the Z-Match tuned to 53 MHz, the coil grid dipped at 78 MHz. I would suggest two 75pF caps instead of the 50 and 100pF caps I used, because the 50pF cap ended up close to fully meshed, and while the 100pF cap was not too sharp in adjustment, a 75pF would be less, however I used 100pf to give the Z-Match more range of adjustment, if needed. I recommend a minimum of 500V rated caps for up to 150W and under 2:1 SWR, with 700V rating preferred for up to 250W. Also since the pictured caps are hard to find, 75pF variable caps with two mounting posts and a metal hole plug for shielding over the adjustment shaft slot hole in the aluminum box would be an alternative. Also Teflon insulated N connectors are preferred, otherwise the insulation may melt with soldering.
The tricky part was by trial and mostly error getting the correct inductance on the coil, and that required a sweep generator and return loss bridge. Originally I thought I'd just set the two caps at 35pF (to allow for tuning out different reactances and impedance ratios) and place in a coil that grid-dipped at 53 MHz. That produced a 6 dB loss and 2:1 SWR! I was about to give up, but I discovered the return loss bridge sweep looked good at about 27 MHz, so turns were removed and the coil spread until eventually I got 0 dB loss and 1.0:1 SWR at 53 MHz. For some strange reason the cap settings only changed a small amount. The position of the coil placed the bottom of the 4 turns (about 5/8" OD coil) about 2 mm from the inside of the aluminum case. I do not know if the SWR would have been higher if the coil was positioned more towards the center of the box rather than almost touching the case, but I thought it best to possibly mimic part of a 50 ohm cable between the N connectors.
Connecting it between the 110W output of the PA and the coax to the duplexer actually increased the output of the duplexer from 89 to 92 watts, and dropped the reflected power to the PA from 2 watts to 0.5 watts, not a whole lot, but I expect it would have made more of a difference if there was a bigger mismatch. I believe the output of the PA also increased by a few watts without making any PA tuning adjustments.
I measured the difference in PA current with and without the Z-Match between the continuous duty Micor PA and duplexer. It turned out that the PA drew 14 more watts of input power with the Z-Match inserted to produce about 4 more watts from the PA, and for just 2-3 more watts out of the duplexer, so the Z-Match is affecting the PA tuning and efficiency. Retuning the output cap in the Micor PA made no difference, possibly because it was already at the minimum end of its adjustment range. I suspect the Z-Match is consuming 1-2 watts out of the 110 watts out of the PA, however reflected power from the duplexer to the PA was reduced from 1.8 watts to 0.5 watts by the Z-Match. The bottom line is that for the continuous-duty Micor PA, the repeater was better off without the Z-Match for our low SWR situation. This might not be true for a MastrII PA, or in a higher SWR situation.
Bob WA1MIK suggested that I should try placing the Z-Match exactly 1/2 wavelength electrically from the Tx power sensor, about the equivalent of 5 inches in Teflon coax longer than the Tx BNC output connector. The Z-Match was relocated from the top of the repeater cabinet to the Tx port of the duplexer, and a new longer length of 1/2" Superflex was fabricated using the existing intra-cabinet coaxial cable and rack-mounted Bird wattmeter, plus a Teflon pigtail representing the coax inside the Tx, using a special set-up with the antenna analyzer. This set-up was a long random length of RG213 coax from the analyzer to an N tee, with a 50 ohm dummy load on one port and the 1/2 wavelength of coaxial cable with an open connector on the end opposite the tee connector end. The analyzer was swept to determine the frequency of 1.0:1 SWR and 0 Reactance, and the Superflex trimmed until the Tx output frequency was reached.
The results were much better with the new Z-Match placement and Superflex cable in that the reflected power could now be tuned out to zero reflected and the peak output from the duplexer followed the same Z-Match tuning for zero reflected, which was not the case with the original set-up. Also the Micor forward and reflected readings on the built-in power sensor tracked exactly with the rack-mounted wattmeter in the repeater cabinet. In addition, the Micor PA no longer lost efficiency when the Z-Match was inserted; in fact, about 2 more watts came out of the duplexer with the same PA input with the Z-Match in place (maybe that is where the original 1.8 watts of reflected power went.)
In conclusion, the Z-Match either needs to be placed right at the Tx output connector, or at multiples of 1/2 wavelength electrically away from it.
The author can be contacted at: jhaserick84 [ at ] comcast [ dot ] net.
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This article created on Monday 09-Jan-2017
Article text and photographs © Copyright 2017 by John Haserick W1GPO.
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