[Elecraft] What is "good quality coax"

[Don Wilhelm]

Martin and all,

Bad quality coax can be lossy (due to age or contamination or manufacturing
problems, or ...) or it can have poor shielding, or it may have very loose
impedance specs, it may not have the standard capacitance per foot, or
resistance per foot, or a lot of other factors.  Good and bad coax is really
hard to judge from appearance.

As an example, I just replaced the coax on my C-band satellite dish here as
part of relocating the receiver to the new house (right next to the mobile
home we had been living in).  The old coax was about 8 years old and was
good quality when it was new (still looked good for the most part too), but
with the new RG-6 triple shielded coax - what a difference in the reception
of some weak signals off the dish!!!  Yes, we do have weak signals from the
Westernmost satellites in the summer due to tree leaves in the reception
pathway, so while this is not strictly measured observation, the picture
quality has improved significantly.  All coax is subject to such degradation
from aging, weathering, etc.  so be careful of coax that you do not know the
source of - such as manufacturer, age, storage/use conditions, etc. because
it can cause you problems.

The bottom line as I see it is to either measure the characteristics of the
coax if you have the equipment and know-how to use it or buy new coax from a
source that you consider trustworthy.

BTW - SWR is NOT a good indicator for judging transmission line - the more
lossy a transmission line is the more it acts like a resistor, and a
resistor will show a low SWR if its value is anywhere near the
characteristic impedance of the line.

73,
Don W3FPR


----- Original Message -----
>
> I see many references to "good quality coax" including in the KAT1
> instruction manual. I'm not sure how to tell "good quality" from the
> "rubbish". I have a pile of RG58, 5mm OD stuff that is cheap (got mine
from
> Electrovalue in UK). Is this good enough for QRP on 40-15m? . The 213 coax
> is much bigger, heavier and expensive.  If I use RG58 and provided I don't
> have long lengths with high SWR I don't think I shouldn't have big losses
> (I am putting up a 40m dipole). Some coax in the UK is advertised as
> "military spec" presumably this is better but in what way?
>