Subject: Re: [AMPS] Power Handling Capability of Coax Cable
>
> > Running maximum legal input power and assuming ~ 750 Watts key down
> output,
> > the "real numbers" you need to look at for voltage breakdwon in the
> > dielectric is....194 Volts (assuming a 50 ohm system). Not much danger I
> > would say as I conduct 4,000 volts over a RG-59U coax cable from my power
> > supply.
>
> You are probabably OK. One thing where you need to be careful though with
> RF voltages is that you need to look at the peak voltage instead of just the
> RMS voltages. The 194 volts you give is an RMS voltage. However, at 750
> Watts, your peak voltage is not all that high. RG-58/59 type cables will
> easily handle up to 800 Watts CW.
Okay...it seems nobody buys ARRL handbooks anymore!
There are different kinds of RG-58 and RG-59. TMS RG-58C is only good
for 300 v rms, while Belden RG-58C (8262) is good for 1400 v rms. Plain
old RG-58 (solid center conductor) is also good for 1400 v rms.
Now let us consider RF. 30 mhz is the worst case for HF amp use.
All ratings below are assuming 1:1 swr (VERY IMPORTANT) and 40 deg. C ambient.
RG-58 350 watts
RG-213 1500 watts
RG-8X 300 watts
RG-217 3900 watts
All coax has loss, which is dissapated in heat. Add significant swr losses and
the dielectric heating begins. This causes migration of the center conductor
toward
the outer conductor, especially where the coax makes a turn. Then you have an
increasing spiral of more heating at the point of the impedance bump in the
line.
>
> > My application is a 12" run of coax from the output of my Pi-L tank to the
> > vacuum antenna relay which could handle at least 1500 watts out, key down.
Even the low cost under-engineered designs have always used solid dielectric
RG-213, or at least Teflon RG-58.
>
> 12" of what sort of coax? Even with RG-58, I'd bet you would be OK. I used
> RG-8 in my amp, but it is admittedly very difficult to route. A very good
> choice might be RG-8X.
The RG-8X is a foam dielectric cable, and is less robust @ RF and only .3 db
less loss per 100' than solid dielectric RG-58. Making bends with foam
dielectric
cable inside an RF deck is an invitation to conductor migration due to the
ambient
heat in the deck. We found out years ago that coax running in the engine
compartment
of autos needs to be Teflon type. The failure rate was cut to zero when the foam
coax
was replaced.
(((73)))
Phil, K5PC
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