Multi-multi filters for contesting

The specificity of multi-multi operation is the concentration of multiple devices in a small space, usually in one building. This induces relatively high voltage from other bands into the antenna, which affects intermodulation in the receiver. When using high power and an unfortunate coincidence of circumstances, it can lead to the destruction of the receiver's input circuits., obvykle jednej budovy. Do antény sa tak indukuje z iných pásiem pomerne vysoké napätie, ktoré má vplyv na intermodulácie v prijímači. Pri použití veľkých výkonov a nešťastnej zhode náhod môže dôjsť až k zničeniu vstupných obvodov prijímača.

The second factor causing problems are harmonic products from the transmitter on a lower band. The TCVR-e majú potlačenie harmonických produktov pomerne slušné (minimálne 50 až 60dB), horšie je to s koncovými stupňami.

Band filters for contesting

Band filters for contesting are the solution. By connecting such a filter, we narrow the receiver's bandwidth, eliminating signals from other bands and simultaneously suppressing harmonic frequencies emitted by the transmitter.
Manufacturing LC filters, especially for higher performances, is problematic due to the unavailability of suitable capacitors. For a multi-multi workplace MW5W som zhotovil filtre z koaxiálnych pahýľov podľa K2TR. Oproti LC filtrom majú inú frekvenčnú charakteristiku, čo v praxi nie je na závadu. Their advantage is feasibility and efficiency. Výborné výsledky potvrdzujú merania OK1VWK.

Suitable coaxial cable

The quality of the coaxial cable significantly affects the final filter parameters. RG-58 sa radšej vyhnime – potlačenie nežiadúcich frekvencií je nízke a tiež zbytočne časť výkonu stratíme v kábli. Pre výkon cca. 1kW PEP vyhovie RG-213, s ktorým sa aj dobre pracuje. Potlačenie nežiadúcej frekvencie je dostatočné a navyše výhodou je, že filtre na vyššie pásma potláčajú nielen vyššie harmonické, ale pridaním ďalšieho pahýľa, aj nižšie pásma. Výsledné potlačenie je tak veľmi veľké.

For power levels of hundreds of watts, PL T-connectors can be used. All filters should be made from coaxial cable from one manufacturer, preferably from one spool. The shortening factor may slightly differ from the value declared by the manufacturer. It is advantageous to start with the production of a filter for 160 or 80m and measure the filter.

Example of filter production for the 80m band

Na koniec kábla namontujeme konektor. Vypočítame dĺžku štvrťvlnného pahýľa podľa vzorca 75 x V / F (napr. 75 x 0.66 / 3.68MHz = 13.45m). Ako rezervu si necháme 20cm, aby sme mali čo skracovať. Od konektora teda vymeriame 13.65m, kábel odrežeme a koniec skratujeme (poriadne zacínujeme). Filter pripojíme cez PSV If thedeviates from the required, adjust the length of the dipole and repeat the measurement. If it will differ from the calculated length (13.45m, see above), we will also calculate the shortening factor after setting the dipole - making it easier to adjust the filters for other bands. Then seal the end of the cable with insulation tape or stretch heat-shrink tubing.

Connection of the filter to the final stage

The filter can be connected to the final stage directly with a T-connector, or through a short, approximately 1.5m long coaxial cable. The stub can then lie on the ground. Be sure to label all cables well - after making all the filters, you will have a lot of cables. A very nice mechanical solution is to wind the cable into a coil and place it in a large paint can.

Operational results

Filtre sme použili v contestovej prevádzke MW5W počas CQ WW SSB (six separate stations with PA on each band) this year. I only registered two or three frequencies in the 15m band where a signal from another band appeared, but only with a strength of S2. However, it is possible that these were products of the equipment. The other bands were clean.

Dimensions of filters for all bands, CW segments, coaxial cable shortening factor 0.66

	A: suppresses the 20m band, length 3.486m, open end B: suppresses the 40m and 15m bands, length 6.969m, open end
	A: suppresses the 20m and 10m bands, length 6.969m, shorted end
	A: suppresses the 40m and 15m bands, length 6.969m, open end B: suppresses the 10m band, length 3.486m, shorted end
	A: suppresses the 20m and 15m bands, length 6.969m, shorted end B: suppresses the 15m band, length 4.648m, shorted end C: compensates for the reactance of stub B, length 2.337m, open end
	A: suppresses the 40m, 20m, 15m, and 10m bands, length 13.938m, shorted end
	A: suppresses the 80m, 40m, 20m, 15m, and 10m bands, length 27.876m, shorted end

Dimensions of filters for the 160m and 80m bands, SSB segments, coaxial cable shortening factor 0.66

	A: suppresses the 40m, 20m, 15m, and 10m bands, length 13.451m, shorted end
	A: suppresses the 80m, 40m, 20m, 15m, and 10m bands, length 26.470m, shorted end