RadioBanter - View Single Post

December 5th 05, 06:13 PM posted to rec.radio.shortwave

In article .com,
wrote:

Telamon wrote:

So concerning the "transfer Impedance" link to the Belden site, the
chart of the various cables shows some of the expected (by me that
is) shield performance. The shielding improves at higher
frequencies due to the skin depth of the shield was expected but by
around 100 MHz the shielding starts getting worse again. Can you
explain this apparently consistent result over a number of
different cables? It must be due to a physical characteristic of
coax cable construction. Maybe the frequency response of the cable?

Terry wrote

I have assumed that the inter braid openings, the lower effeciency of
mylar foil, and the basic losses of the cable foil account for this.
A fellow enginer insists it is an effect of self inductance.

Other coax companies charts show very simlar cuirves.

The holes in the coax braid are a tiny fraction of the 1/4 wave of the
highest frequency on the chart so I'm doubtful of that.

The coax cable self inductance of the outer braid/shield? Well we
should be able to see an effect of coax diameter size where the larger
diameter should have lower self inductance.

The larger diameter and other wise similar cable has the dip at a
higher frequency. Maybe your buddy is right about the coax outer shield
self inductance.

Many of the coax on the charts are not current but I compared two,
which are 9104 and 9116. Both have the same electrical characteristics
except for the inner and outer DC resistance. 9116 is the larger
diameter cable with lower DC resistance for both conductors. This cable
has its dip on the chart at a higher frequency than the 9104. The 9116
has lower insertion loss (S21) at higher frequencies than the 9104 at
the 6 dB point of about 200 MHz.

If this was a function of Q then it would make sense that the dip would
be at a higher frequency on the larger / lower DC resistance cable but
this is not a function of Q as far as I can see.

I don't know maybe it has something to do with the traveling wave being
T.E.M. in the cable but that should be opposite of the chart result.
Smaller diameter should be better at higher frequency.

I expect that the 9116 is better at higher frequencies because it is
less dispersive at higher frequencies due to the lower ohms per inch of
the conductors. You always have two losses in a transmission line,
which are the conductor and dielectric loss. One or the other can
dominate depending on materials, dimensions and frequency. Here I
expect that the 9116 cable dielectric loss is higher than the 9104 but
the 9116 conductor loss is lower, which in this case dominates.

The greater dispersion of the 9104 coax at higher frequency may cause
the RF energy in the cable to radiate some and not just turn into heat
in the cable or your buddy is right about the outer shield inductance
per foot?

--
Telamon
Ventura, California