Figure 2 of the MATCHBOX manual shows a 6 inch to 12 inch length specification for interconnecting 50 ohm coaxial cables, one at the output of the transmitter and one at the input of the MATCHBOX. ?There is no coaxial cable length specification for interconnecting cables between accessories, like the No. 250-20 low pass filter or No. 250-37 directional coupler. ?Apparently, Johnson considered the length of these two particular cables critical. ?On page 1, section II (Installation), the manual states that overall length of interconnecting coaxial ?(transmission) line [TL] should be as short as practical because ¡°input reactance varies more rapidly with [respect to] frequency as the length of the line increases.¡± ?So, it is clear to understand as the length of interconnecting coaxial TL increases with respect to the wavelength in use, any MATCHBOX input reactance present will vary increasingly quicker as the frequency is adjusted. ?The manual then states, ¡°If a low pass filter and a TR switch are used,¡± these two length-specified interconnecting TL¡¯s show in Figure 2 should be used.
Why are the lengths of these two TL¡¯s particularly critical under the stated condition?
If a TR switch, either a No. 250-39 or some other like found in the ARRL Handbook, is not used, i.e. use of a transmitter or transceiver containing TR switching, would the 6¡± to 12¡± TL length requirement following the transmitter be relaxed? ?Under this condition and using a low pass filter such as the No. 250-20, can I assume the 6¡± to 12¡± TL length at the MATCHBOX input, at least, must be maintained, but the rule of shortest practical interconnecting TL could be applied for any other TL¡¯s necessary?
Thank you.
