[TowerTalk] Where to get LMR 400

[jimlux]

On 11/3/20 9:09 AM, jimlux wrote:
> On 11/3/20 8:24 AM, dj7ww at t-online.de wrote:
>> RG213 is specified with 1300W on 6m
>>
>> 73
>> Peter
> 
> 
> MIL-C-17/74 (RG 213 spec) sort of says that..
> 
> It says 3700V rms maximum
> 
> Figure 2 shows the "straightline" from 1200W (@50 MHz) down to 180 W 
> (@1000 MHz), probably based on thermal limits from the current in the 
> center conductor - 0.0888" OD seven strands that are 0.0296" each 
> (between AWG 20 and AWG 21)
> 

We can sort of back into what the current limit is

Skin depth at 50 MHz in copper is 0.36 mils (0.00036") so significantly 
smaller than the wire diameter. At 1 GHz it's more like 0.081 mils.

At 1200 W and 50 ohms the current is 4.9 amps.  The DC resistance is 
0.176 ohms/100 ft (MIL- C17/74) - That's the full cross sectional area 
(7* strand area)..

So the power dissipation per foot is 8.6 mW/foot at DC..
But at 50 MHz, it kind of looks like about 59 mW/ft (because the 
resistance is 1.2 ohms/100ft)
and at 1GHz it's 263 mW/ft.

Limiting the power to 180W would change the dissipation to about 100 mW/ft.


---
One important thing - this is in a matched system.
If the SWR is, say, 2:1 or 5:1, the current peaks are higher. In the 
worst case, the current is twice (100% reflection) - which quadruples 
the power dissipation (at those points)

  There are instances of high power microwave transmitters running close 
to the edge, and when the line was unterminated, there were nice burned 
spots every 1/2 wavelength along the line.  (a lot of high power 
transmitters have a circulator at the output so the reflected power goes 
to a load)

So a system where you're running 100W at 1GHz through RG-213 meets the 
data sheet if the antenna's hooked up, but if it doesn't it could melt.