PRIMARY CONCERN: I am in the process of building a 70cm repeater and
need to know how to measure and test the physical and electrical
length of the short coax cables linking the (band-pass/notch) duplexer
cavities.
I read somewhere that these cables should be an odd multiple of 1/4
wavelength at the operational frequency. In my case, a 1/4 wave cable
won't easily reach between cavities; so I am using one that is
approximately 3/4 wavelength.
QUESTION #1: Due to the Tx/Rx split, I would expect these cable
lengths might need to be different between the Tx cavities and the Rx
cavities, assuming that I can make the cables this close to a target
length. However, I am now wondering how much this small of a
difference might matter. Should I be concerned about this small of a
difference in the cable lengths (i.e., approximately 0.2")?
QUESTION #2: My next problem came when I tried calculating the
"physical length" for these cables. I did the calculations for my
operational frequency on the test pair, even trying to allow for the
velocity factor (Vf = 0.695) for the RG-400 cable; but I'm not sure
where to measure the cable length with respect to the type-N
connectors. At 70cm even a 1/4" will impact the effective wavelength
of these jumper cables; so this measurement seems relativel critical.
CALCULATIONS:
wavelength(ft) = 983.6 / [frequency(MHz) * Vf]
for 3/4 wavelength cables,...
Tx @ 446.860MHz (L = 3 * 4.5893" = 13.7679")
Rx @ 441.860MHz (L = 3 * 4.6413" = 13.9239")
If I install the type-N connector on one end of a 14" piece of RG-400,
at what point should I measure and cut it for the other type-N connector?
QUESTION #3: When I prototyped up a couple cables, I tried to confirm
that I was in the right ball-park for their length. I tried measuring
their electrical using my MFJ-269 analyzer.
For reference, I set the analyzer for a 270 degree (i.e., 3/4
wavelength) cable length, and tried to measure the electrical cable
length of the initial (approximately 14" long) cables; before I cut
and installed the second type-N connectors. Adjusting the frequency
even closer, I came out with a resonant frequency (i.e., lowest SWR)
at around 420 MHz, presumably indicating that the cables were long.
The analyzer also indicated a 1.2' length for the initial setting of
270 degrees; which seemed about right. Assuming a target frequency at
around 444 MHz (i.e., mid-way between Tx and Rx frequencies for test
case), a 420 MHz resonance would indicated that the cables should be
cut down to approximately 95% of their length. If this is correct, I
should cut the cables down to somewhere around (14" * 0.95) 13.3".
This seemed quite a bit shorter than the caculated length (around
13.8" (+/- 0.1")); so wanted to validate my test methodology as well.
Am I doing this correctly?
Thank-you in advance for your assistance.
