[Elecraft] Co-ax for the K2

[N2EY at aol.com]

In a message dated 11/13/07 3:19:30 PM Eastern Standard Time, 
jackbrindle at earthlink.net writes:


> Almost universally, there seems to be  
> no recognition that even though the feedline is matched at the radio,  
> it is indeed NOT matched beyond the tuner. The feedline can see  
> humongous mismatch (high SWR) and the operator doesn't know because  
> the tuner hides it. 

That's only true if the load impedance is far from 50-75 ohms, or is highly 
reactive.

In these cases, the feedline will see high  
> 
> voltages (perhaps very high), which causes heating in the dielectric  
> and will eventually destroy the coax. 

*If* the mismatch is considerable and the power is high enough. But at the 
hundred-watt level and SWRs under about 4 to 1, that's not going to be an issue 
with most coax.

> So why is hardline so much preferred? It has next to no non-air  
> dielectric to be heated/destroyed.
> 

The reason hardline is preferred is that its loss is less. All the TV 
hardline I've seen has foam dielectric, btw. 

> And last, what is the best solution? Most likely to remote the antenna  
> tuner so that it always provides a good match on the local feedline  
> for the transmitter to see. The ultimate, of course, is to place the  
> tuner at the antenna. Why doesn't everybody do this? Because it is  
> rather difficult to do. Ham radio, along with the rest of engineering,  
> is all about finding the best compromise that provides the best results.

And knowing what is really going on in the system. An SWR of 2:1 at HF on 100 
feet of most coax types isn't a big deal. 10:1 is a different thing 
completely!

> 
> So, if you can assure a good match between the antenna and the coax,  
> or perhaps even one that transforms the antenna feed impedance to the  
> transmitter's at all operating frequencies, then you have found the  
> right solution for you and should go forward with whatever coax fits  
> that solution...

Agreed - but a good match doesn't have to be a perfect match. 

73 de Jim, N2EY




**************************************
 See what's new at http://www.aol.com