W1fb Spc Transmatch

With the abundance of reasonably priced commercial transmatches and an equally large number of articles on inexpensive home-brew units, ihere seems little that anyone could add to antenna Euncr ideas. Little, that is, unless you use an all-band wire antenna. Then the limitations of commercial and general ham designs begin to show up, On 10 meters, one of the capacitors runs out of minimum capacitance a shade before the SWR drops to 1:1. The coil matches only as it hits the stop. Converting the 10:1 SWR to 50 ohms for the transmitter seems to exceed the unit's abilities, even though it performs well on every other band.

If you only wish to flatten a 2:1 or 3:1 SWR, you may never encounter these problems on any band, However, center-fed Zepp and variations on the G5RV antennas can present the transmatch with complex combinations of resistance and reactance. With enough reactance at the transmatch, normal all-band components and construction may provide a poor match at 10, 12 and 15 meters.

The problem is with the all-band design concept. My commercial transmatch covers 160 to 10 meters, with a 36 |JH inductor and a pair of 240 pF air variables in a standard T circuit. This is all enclosed in a case allowing a half-inch space between the components and the chassis or case metal. Although the tuner is compact and versatile, the high-value components have high minimum values as well. The capacitors are rated at 40 pF minimum; which looks more like 45 or 50 pF with the case closed. Minimum inductance is not listed for the rotary inductor, but between I and 2 \lH would be a good guess, especially with its 2'/>-ineh external '¡ead to ground,

Transmatch Performance

The standard T circuit {with series capacitors and a shunt coil) is a high-pass filter. It does little to suppress harmonic energy. The SPC circuit (as W1FB calls it), shown for comparison with the T in Figure 1, is claimed to provide an estimated 20 dB of harmonic suppression. Of course, we will only achieve the rated selectivity if we can maintain a high-loaded circuit Q. Normally, stray capaci-


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