[Elecraft] Co-ax for the K2

[Jack Brindle]

This whole discussion amazes me. 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. In these cases, the feedline will see high  
voltages (perhaps very high), which causes heating in the dielectric  
and will eventually destroy the coax. It doesn't matter if it is 75  
ohm or 50 ohm coax, it will happen. What kind of voltages could we  
see? Depending on the mismatch, potentially into the thousands with a  
100 watt transmitter. That does real damage in low-cost coax no matter  
how much braid coverage.

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

And next, why is open-wire even better? It has even less non-air  
dielectric than hardline. Like hardline, its loss is so low that you  
can run long lengths with very little loss.

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.

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...

On Nov 12, 2007, at 8:33 PM, Don Wilhelm wrote:

> Doug,
>
> I beg to differ.  There are physical properties that give rise to  
> the characteristic impedance which are related to the dielectric  
> properties and the relative conductor diameters for the center  
> conductor and the braid.  These same properties will create a  
> particular RF voltage handling characteristic for the coax in  
> question (or any coax for that matter).
>
> The real difference in coax quality is the shielding percentage of  
> the outer braid.  That has nothing to do with the RF voltage  
> handling, but it certainly has a lot to do with the leakage from the  
> coax.  In the extreme, a coaxial cable could have similar leakage  
> characteristics as open wire line, but again that is not related to  
> the RF voltage handling characteristics.
>
> So look for a coax that has 90% or greater braid coverage.  There is  
> the 'quad shield' RG6 that is quite good in shielding  
> characteristics, but I do not know if it is available in white  
> jacket material.
>
> As it has been mentioned, the white jacket is not as UV protective  
> as the black, so for use indoors, it really does not matter, but  
> outside, use the cable jacket that is rated for UV protection.
>
> 73,
> Don W3FPR
>
> Doug Person wrote:
>> There are many types of cable designated RG-6 and RG-59.  Many are  
>> not designed to handle rf voltages typical of ham transmissions.   
>> My suggestion is to be *careful* what you select.  Good quality  
>> RG-59, the type broadly used by hams many years ago, is hard to  
>> find right now.  We're not talking about what the cable companies  
>> are using - we're talking about what you can buy at Wal-Mart or  
>> Home Depot.
>>
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-Jack Brindle, W6FB
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