[BC] RG-8

[Gregory Muir]

Bob,

Voltage breakdown is somewhat moot.  As it has already been mentioned, RG8 
is rated for very high DC voltage breakdown.  Harris uses it for transport 
of the high voltage for the final in their transmitters from the power 
supply cabinet to the transmitter cabinet.  We're looking at constant 
operation at levels a little less than 10 KV.

The problem is the connectors that are usable with this cable.  PL259s were 
originally developed during the WWII days when we knew little about anything 
in the UHF frequencies.  This connector was originally and still is rated to 
only 300 MHz not to mention that it presents a very non-uniform impedance to 
the transmission line.  Typical maximum voltage rating is in the 
neighborhood of 500 volts peak.

The N connector was developed to allow a robust connection at frequencies up 
to 11 GHz with a resultant peak voltage rating of 1500 volts.  While a much 
better connector, it normally is not recommended for use in the lower 
frequencies at power levels over 1 KW.  I happen to work with a small 
combined FM station with two transmitters operating at TPO of 1 KW.  The 
original installation uses Andrew LDF5-50A 7/8 Heliax for all of the runs 
between the combiner and antenna except for the last foot at the 
transmitter.  The transmitter (Marti PNP1000) uses N style bulkhead 
connectors for the RF out and are connected to a N connector on the end of 
the Heliax with a 1 foot piece of RG8 cable.  At 1 KW the cable runs warm 
but the connectors are a little more toasty than that - proof enough to me 
that the cable (and especially the connectors) are running in the lower 
portion of the comfort curve.

It's interesting to note that Andrew rates the N connector that interfaces 
to their 7/8" cable (L4PNM-RC, etc.) at an average power rating of 600 watts 
at 900 MHz and a peak power rating of 10 KW (!!!).  While the average level 
looks acceptable, I would approach the peak level with tongue in cheek.

Greg


----- Original Message ----- 
From: "Bob Groome" <bobgroome at yahoo.com>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Sent: Thursday, April 03, 2008 11:43 AM
Subject: Re: [BC] RG-8


> This begs a question:
>
> What is the power rating for RG8? Will it handlle
> 1.5 kW 1 kw 100% Modulated) and a little for
> safety in case there is a VSWR? My old books
> don't have plane RG8. A little math says you'd
> have about 225 volts of RF for 1,000 watts and
> 4.45 amps. I seem to recall that the coax is good
> to 500 volts. Maybe it's the PL259 that has the
> voltage limit. But I sure remember RG8 running
> very warm at 1000 watts.

<snip>