Double shielding reduces the amount of RF that passes through the shield, either on TX or RX.
It is particularly useful in applications such repeaters where maximum isolation is needed to
keep out strong signals, or to keep from radiating signals that could be picked up by a
In most ham applications there will be more radiation from the coax due to common mode
current on the outside of the shield than radiation passing through the shield.
The other parameters in question:
Vf = velocity factor. Use this number if you have to calculate a particular length of cable,
such as 1/4 wavelength. (The electrical length times the velocity factor gives the physical length.)
Center conductor consists of 19 strands of #29 wire.
Dielectric is the type of plastic used in the center insulator.
Cap is the capacitance in picofarads per foot. Usually this doesn't matter.
Shield looks like Aluminum foil with a copper wire (
Jacket is the type of outer plastic cover - some types are more resistant to UV radiation, etc.
Outer diameter is the size in inches.
Voltage rating is the maximum rated voltage between the two conductors. For 300V peak
at a 50 ohm impedance the line should handle about 900 watts maximum with a perfect SWR.
While some people have pushed RG-8X beyond this, the line may overheat on the higher bands
where the loss per foot is higher, and it doesn't allow any safety margin in case the inner
conductor migrates through the foam center insulation when you have a bend in the cable
in the hot sun, or someone tightens a tie wrap down too tight securing it to a tower leg.
Wrapping coax around a metal mast to make a choke is convenient, but not very effective.
RG-8/U and RG-8A/U are former military coax type designators. They are no longer active,
but have been superseded by RG-213. Now anybody can use those numbers for their products
without having to meet the former standard. RG-8X ("Mini-8") was an attempt to make a smaller
cable that was less lossy than RG-58: it was never a military standard, and there never has been
a standard specification for that particular part number.
I think the difference between RG-8/U and RG-8A/U was that the latter had a stranded
center conductor while the former was solid copper. The nominal impedance was 52 ohms,
while the new RG-213 is 50ohms and has a non-contaminating jacket material.
Here's a convenient calculator for coax losses:http://www.vk1od.net/calc/tl/tllc.php
You'll find RG-8X listed as Belden 9258: that is just one manufacturer, and since there is no
standard spec for the stuff you'll have to guess close the particular variant you are considering
is to this Belden type. The most important factor for your use is probably the loss, usually given
as dB / 100' at specific frequencies. In this case, 100' of Belden 9258 RG-8/X at 29 MHz will have
1.6dB loss, so about 70% of your power will reach the antenna. Using the larger RG-213 would
bring that up to 80%. If you really want to be cheap you could use RG-6A TV coax, with about
the same loss as RG-213 or RG-8, though with some possible variation in SWR.