On Apr 5, 3:03 pm, Owen Duffy wrote:
Owen Duffy wrote :

...

So for small diameter 50 ohm polyethylene dielectric line at 1.8MHz,
the worst case for most ham applications, Xo/Ro is about .18. For
that, I used 2.7dB/100ft for RG174 type line. That's getting to be

I make the loss/100' of RG174 to be 1.1dB and from that I get Xo=-3.6
ohms. (Did you get loss/100m from somewhere? This is probably the
answer to Cecil's diligent spot of an apparent error.)

Tom, in view of your comment about skin effect not being well developed
in RG174, I went to Belden's data sheet for 8216 (RG174 type) and sure
enough, the loss below 30 MHZ does not track the loss=k1*f^0.5+k2*f
model. Your loss figure of 2.7dB/100' may well be correct, and my line
loss calculator is in error below 30MHz for this particular cable due to
the thin copper plating and steel core of the inner conductor. TLDETAILS
and other calculators based on the same loss model will also be in error.
I note that the ARRL TLW shows 1.8dB/100' for 8216 at 1.8MHz.

This sets me thinking of a way to calculate a lower frequency limit to
the loss model when I generate it, so that I can store that limit in the
database and prevent calculation below that frequency.

Owen


Hi Owen,

Yes, I've played with the same model (k1*sqrt(f)+k2*f) for loss. For
things below 500MHz or so--and generally above--the k2 term seemed to
always be so low as to not be worth including. For one thing, the
high-frequency apparent loss may well NOT be due to increased loss in
the dielectric, but rather to variations in impedance along the line
or some similar phenomenon. Published specs for precision lines seem
to be a lot closer than those for "garden variety coax" to what I'd
expect based on theory and what I believe to be the dielectric's power
factor for polyethylene and PTFE. Assuming that all types of line
from a given reputable manufacturer use the same quality polyethylene
and the same quality PTFE, we should see the same contribution from
dielectric, taking into account solid versus foam, for all types: at
a given frequency, the dielectric loss of the poly or PTFE should be
the same. If that's the case, then for some lines at least, the k2
factor must not be due entirely to dielectric loss.

A note on my simple formula: one of the approximations in it is that
the phase angle of Zo is assumed to be close to zero, so that
sqrt(1+j*x) can be reasonably approximated as 1+j*x/2. For x=+/- 0.2,
the error magnitude is less than 0.005, and the error in the imaginary
part is less than 0.0005. But for line that has a seriously reactive
Zo, the error can be large.

We have Roy to thank for pointing out to me, several years ago, that
characteristic of RG-174. I knew I'd have a chance to thank him in
public for it sometime. ;-)

Cheers,
Tom