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., usually of one building. A relatively high voltage is thus induced into the antenna from other bands, which affects the intermodulation in the receiver. When using large powers and an unfortunate coincidence, the input circuits of the receiver may be destroyed.
The second factor causing problems are harmonic products from the transmitter on a lower band. The TCVR-s have a relatively decent suppression of harmonic products (at least 50 to 60dB), it is worse with the final stages.
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 I made filters from coaxial stubs according to K2TR. In comparison to LC filters, they have a different frequency characteristic, which is not a disadvantage in practice. Their advantage is feasibility and efficiency. Excellent results are confirmed by measurements of OK1VWK.
Suitable coaxial cable
The quality of the coaxial cable significantly affects the final filter parameters. RG-58 let's avoid - the suppression of unwanted frequencies is low and also unnecessarily we lose a part of the power in the cable. For power around 1kW PEP, RG-213 is suitable, which also works well. The suppression of unwanted frequency is sufficient and moreover the advantage is that filters for higher bands suppress not only higher harmonics, but by adding another branch, also lower bands. The resulting suppression is very significant.
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
At the end of the cable, we mount the connector. We calculate the length of the quarter-wavelength branch according to the formula 75 x V / F (e.g. 75 x 0.66 / 3.68MHz = 13.45m). We leave 20cm as a reserve to have something to shorten. From the connector, we measure 13.65m, cut the cable and shorten the end (properly insulate). Connect the filter via 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
We used filters in the MW5W contest operation during 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 |