Why exactly would cable lengths to a duplexer matter?

Question

So a very experienced friend of mine is making a portable suitcase kit with Yaesu FTL-2001 (VHF), Yaesu FTL-7002 (UHF) and DX-720D duplexer* plus power supply and other miscellaneous gear. FTL-2001 and FTL-7002 are connected to inputs of DX-720D and the output is connected to the antenna socket on the suitcase.

Originally, the coaxial jumpers were installed inside of the suitcase so that they fit as neatly as possible. Later on, he decided to change one of the jumpers so that it is of same length as the other jumper because "cables going to a duplexer need to be of same length". He couldn't provide me a better explanation than this.

This set off my WARNING! SUPERSTITION alarm, since it seems to look a lot like this question. Furthermore, to me it seems a bit strange that the same length would be very critical, since the transcievers are operating on different bands. I've read some resources about cavity duplexers where jumpers of length corresponding to certain wavelength fraction are recommended, but I haven't seen anywhere this recommendation of having same length even if the frequency bands are different.

So my question is: Is there actually any real reason to have coaxial jumpers of same physical length for connection to a duplexer?

*Note: this unit is referred to as duplexer on various websites that sell it, but it only has fixed low-pass and high-pass filters from what I see, so perhaps term diplexer would be a better fit.

Answer 1

Ham radio is pretty much the intersection of EE and murphys law. In more detail:

There is no electrical reason but there is an excellent practical reason with feedlines such that if you need a 49 foot cable and a 50 foot cable, murphys law is you'll inevitably find a way to install the 50 foot cable in place of the 49 foot cable leaving a surprising amount of slack and then try to install the 49 foot cable in place of the 50 footer and end up 12 inches short. Very annoying when something like this happens, especially if you have to remove 15 zip ties and rerun both cables thru conduit. All the flexible coax sections on my antennae were precisely my height such that accidents like that can't happen when up in the air, not physically possible to mix up the cable lengths because they're all the same. At a commercial site I visited it was SOP to permanently label cables with their length, which helps both with mixups and with later TDR testing if necessary.

I can totally see a practical use like that getting Urban Legended into a duplexer in a suitcase needs equal length feedlines. If you do accidentally connect the wrong TX to the wrong duplexer port you probably have enough cable slack to fix it without specifically needing to cut to precisely equal length.