[TMC] 50 ohms vs. 70 ohms

[Roy Morgan]

On Sep 8, 2009, at 3:21 PM, H.L. wrote:

> ....  How serious is the mismatch between 50-Ohm and
> 70-Ohm systems and cables?

The mismatch can be useful at times.  In particular, a piece of 70 ohm  
coax between a 50 ohm one and a 90 or 100 ohm load can act as a  
matching section/"transformer".  Collins used this principle in an  
antenna.

As I understand it, however, at frequencies below 30 mc, with  
relatively low loss coax, mismatches, or running the coax at non 1:1  
SWR levels usually does no harm.  (200 feet of RG-58 at 144 mc with  
SWR at 3:1 gets you rather big losses, however.)


> Many TMC transmitters and linear
> amplifiers are 70 ohms,

70  ohm coax shows the lowest losses, most all factors being normal  
(SWR, dielectric losses, I squared R losses and so on).  TMC  
transmitters are normally NOT amateur power levels, also.

> ...
> useful one-foot up to ten-foot coax cables found at ham flea markets
> are usually 50 ohms. Can we ignore the differences?

Yes, especially at HF with short runs of cable.

HOWEVER: the 70 ohm BNC connectors may damage the 50 ohm ones, or vice  
versa. (The center pin diameter is not the same and can spread out the  
female part, resulting in bad connections when the right connector is  
used with it.)  I don't know of 70 o hm type N connectors, but if  
there area any, the same warning would apply.

Roy

Roy Morgan
k1lky at earthlink.net
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Groton NY, 13073
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