[TowerTalk] Coax specifications, etc.

[Jim Lux <jimlux@earthlink.net>]

I just looked up the MIL spec

MIL-C-17G (9 March 1990 + 1 supplement and 3 amendments) is the standard (I 
have it in pdf if anyone needs it)

Interestingly, there is no such thing as Mil-Spec RG-213 or RG-58, since 
the "slash sheet" for that coax has been invalidated since the early 
90's(although it's available for reference).  The military no longer buys 
PVC jacketed coax, so there's no need for a procurement spec.  MIL-C-17/74C 
for RG-213, -17/28C for RG-58


When it comes to RF properties, there are requirements on the nominal 
impedance, and more stringently, on the VSWR/return loss and 
attenuation.  Since they define the conductor diameters and the dielectric 
in the spec, the nominal impedance falls out of that by arithmetic. (50 
ohms +/- 2 ohms for both)

The impedance is to be measured by using a time domain reflectometer.

The attenuation and return loss specs are of the most interest from an RF 
standpoint:

RG-58 50 MHz 4.dB/100ft, 28 dB return loss(1:1.1), rising to about 23 dB @ 
1GHz(1.15:1), 28dB/100ft

RG-213 50 MHz, 1.2 dB/100ft,about 29 dB return loss (1.07:1) 1GHz 
9dB/100ft,23.5 dB RL

These are measured with the usual technique of a source, a precision power 
meter, and calibration standards for attenuation, and a slotted line, 
bridge, or network analyzer for the return loss. The standard calls out 
most of the specific measurement technique.   The source and meter would 
all be terminated in 50 ohms (or the coax characteristic impedance)

It doesn't say whether the sample has to meet spec over all temperatures, 
or just at "room temp" (25C, 60%RH, 760torr).  This is sort of interesting, 
because temperature affects the loss fairly significantly.


With respect to voltage and power handling, there are several things:

There's an "engineering working voltage", 1400Vrms for RG-58, 3700Vrms for 
RG-213, which you could use for "design purposes".

The actual performance spec is at 60Hz  and applies to the voltage applied 
between the shield and the center conductor (there's another spec for the 
jacket).

RG-58 5kV rms +10%, -0%
RG-213 10 kVrms +10%, -0%

You have to apply that voltage for no less than a minute, and have no 
breakdown.

There's also a corona extinction voltage, which is somewhat lower, and a 
spark test, which applies to the jacket.

As far as power handling goes, there's a chart for each type of cable.
For RG-213, the lowest number in the chart is 50 MHz and it's 1.2 kW, 
likewise RG-58 is 50MHz and 300W. Power rating is at 25C at sea level.

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