I hope I'm not treading on anybody's toes here..
This is one page of a publication I have on hand.
Mike at Repeater Builder: not sure if "the duplexer tuning guide"
by Bob Majewski is still in publication, as the copy right is dated 1991.
I can fwd further info if you want to chase it up for your web page.
It's in a 20 page bound A4.
rgds John
VK4JKL
"Critical Cable Cutting Instructions For Duplexers"
The following are instructions for cutting the critical lengths of
interconnecting cables between cavities. Please note, that there are a number
of
factors involved (such as the loop length in the cavity)
that make the following calculations just a good estimate.
As in laboratory conditions, the lab technician may
try a different number of cable lengths to obtain optimum performance.
1. Calculate the wavelength in air of the frequency in inches by the using the
following formula {result in inches}:
11808 / Frequency in MHz =
2. If the frequency is VHF use a quarter wavelength calculation. Divide the
results in step one by four.
3. If the frequency is UHF you may wish to convert to a three-quarter
wavelength
calculation using the following formula:
3 times Calculated Wavelength in inches divided by 4
i.e. (3 X W.L. / 4)
4. Add up all connectors and deduct (in inches) from calculated length of cable
(see table below).
5. Determine the velocity factor of the cable (see table below) and multiply
this factor to the cable length.
Example:
To cut a length of RG-8 cable three-quarter wavelength at 464.500 MHz having
two
"tee" connectors and two PL-259 connectors.
1. calculate full wavelength (11808 / 464.500 = 25.42 inches)
2. calculate 3/4 wavelength (25.42 x 3/4 = 19.06 inches)
3. deduct length for connectors (19.06 inches - 1.5 inches (2 tees) - 1 inch
(2 PL-259s) = 16.56 inches}
4. deduct for cable velocity factor (16.56 x 0.66 (RG-8) = 10.92 inches)
5. 10.92 inches is critical cable length for 3/4 wavelength at 464.500 MHz
Common Factors Deductions
Connectors: (deduct in inches)
PL -259 1/2 inch
Tee 3/4 inch
N 1/2 inch
Elbows 1 1/2 inches
Velocity Factors: (estimates only)
Polyethylene (RG8, 214, 213) =.66, Teflon =.7, Hardline foam =.8
