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Coax notch filters for multi – multi contesting

When you run more than one station in the multi – multi cathegory you might experienced a receiver overload problems. Antennas are too close to each other and running high power could damage receiver nor overload it!

We have developed coax notch filters which performed well in MW5W last year. It’s a K2TR design – easy to built, not very expensive but really effective.

Construction an usual LC filters (pi – network) for high power is a serious problem. Capacitors which can handle these power levels are very rare and expensive. I have worry about coaxial stubs which acts like a notch filters due the different frequency response. They could only reject odd or even harmonic frequencies.

The quality of coaxial cable is very important. It designates the bandwidth, rejection and losses.. The RG-58 coax is a bad choice. Much better is RG-213 which can handle without problems 1kW PEP.

Filters are inserted in the line with a SO-259 Female Tee adaptors. I am recommending to construct all filters from one roll of coax cable and all connectors from one reseller. The velocity factor of coax cable could vary a litlle bit. Building and tunning the stub for 160m or 80m band can give us an answer about the real velocity factor of coaxial cable. Little inaccuracy on these bands aren’t problem but on the higher bands is accurancy importnant.

Example 80m band filter: mount on the coax PL-259 connector. Count the lambda/4 stub with a formula 75 x V / F (napr. 75 x 0.66 / 3.68MHz = 13.45m). Add 20cm more, cut the cable and short the end. Connect the stub to the transceiver output and attach a dummy load and wattmeter behind the filter. In the 80m band must be indicated full power in the dummy load. Switch for 40m band and check the power handled to the dummy load – it should be zero. I fit is so, no tunning is required. If the rejected frequency is out of the band cut the coaxial cable and measure again. When it is OK, measure the exact lenght of coaxial cable and calculate the velocity factor.

Coaxial notch filters, CW segments, velocity factor 0.66

	A: nulls 20m band, lenght 3,486m, end open B: nulls 40m and 15m band, lenght 6,969m, end open
	A: nulls 20m a 10m band, lenght 6,969m, end shorted
	A: nulls 40m and 15m band, lenght 6,969m, end open B: nulls 10m band, lenght 3,486m, end shorted
	A: nulls 20m and 15m band, lenght 6,969m, end shorted B: nulls 15m band, lenght 4,648m, end shorted C: compensates for reactance from filter B, lenght 2,337m, end open
	A: nulls 40m, 20m, 15m and 10m band, lenght 13,938m, end shorted
	A: nulls 80m, 40m, 20m, 15m aj 10m band, lenght 27,876m, end shorted

Coaxial notch filters for 160m and 80m bands, SSB segments, velocity factor 0.66

	A: nulls 40m, 20m, 15m and 10m band, lenght 13,451m, end shorted
	A: nulls 80m, 40m, 20m, 15m and 10m band, lenght 26,470m, end shorted

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