On Tue, 05 Apr 2005 18:07:57 -0700, Roy Lewallen
wrote:

K7ITM wrote:

. . .
But beware that you are more
likely to have dielectric loss in open-wire line for a variety of
reasons...
. . .

Yes, this is something I didn't mention and should have. My statement
about the negligibility of dielectric loss below 1 - 10 GHz is strictly
true only for coax with decent (common) dielectric material (e.g., PE or
PTFE). When the impedance is higher, as it is with ladder line, the
effect of the dielectric loss is proportionally higher. On the other
hand, a good part of the ladder line field is in the air (although it's
most intense directly between conductors, where any insulation typically
is), which reduces the effect of loss in the dielectric.

Many years ago I measured the attenuation of some common 300 ohm TV
twinlead, and found that in some cases when wet its attenuation could
exceed that of RG-58 coax. The extra loss is intirely due to degradation
of the quality of the dielectric between conductors. See
http://eznec.com/Amateur/Articles/Po...Feed_Lines.pdf. I know Wes
has done similar measurements on window line and has posted the results
at his web site; perhaps he'll remind us again of the URL.

Su

http://users.triconet.org/wesandlinda/ladder.htm

Wes

The relationship between the three characteristics is more imaginary
than real. It amounts to little more than an old-wives' tale.

The reason attenuation is usually smaller for twin line than coax is
because the twin line conductors are usually of greater diameter than
the coax inner conductor.

And the reason twin line usually has a greater velocity is because the
conductors are spaced further apart and usually there's less
insulating material between them.

But it's quite easy to reverse the situation by obtaining large
diameter, high impedance coax and flimsy close-together twin line.
----
Reg, G4FGQ

===============================

"Hal Rosser" wrote in message
. ..
I've noticed, (but have not studied), some loose relationships in
transmission line characteristics (and I guess waveguides fit in
here).
From an observer's point of view, it seems that a high
characteristic
impedence line (like 400-ohm or 600-ohm ladder line) also is usually
a
lower-loss line, and has a higher velocity factor.
It also seems that some coax may have a low VF and high loss.

Is there a real cause for the relationship of these 3
characteristics of
transmission lines ? Is it something we can generalize ?
It makes some sense to say that the faster a signal gets through the
line,
the less loss it will have - and that gives some credence to the
relationship in VF and loss being inversely associated.

"Reg Edwards" wrote in message
...
The relationship between the three characteristics is more imaginary
than real. It amounts to little more than an old-wives' tale.

The reason attenuation is usually smaller for twin line than coax is
because the twin line conductors are usually of greater diameter than
the coax inner conductor.

*** Thanks - good point
and as Roy pointed out - the voltage would be higher - so the loss would be
lower.
***

And the reason twin line usually has a greater velocity is because the
conductors are spaced further apart and usually there's less
insulating material between them.
****
Does that mean that more insulaton material between the conductors decreases
the velocity factor ?
Ok - its making more sense. Ladder line just happens to have a high VF and
low loss - each for different reasons.
****

But it's quite easy to reverse the situation by obtaining large
diameter, high impedance coax and flimsy close-together twin line.
***
I guess using zip-cord (rubber lamp cord) would be an example.
*********

You guys are good.
Thanks for the info.

Snip...


But it's quite easy to reverse the situation by obtaining large
diameter, high impedance coax and flimsy close-together twin line.
***
I guess using zip-cord (rubber lamp cord) would be an example.
*********

Snip...

The type of lamp cord common in South Africa (don't know about other
countries): Two conductors of 0.75mm^2 cross sectional area insulated with
about 1mm of white pvc and a spacing of around 2.5mm has an impedance of
aproximately 60 Ohms. Close enough to 50 to use for quick&dirty dipoles
without balun or tuner. Though have no idea of the velocity factor and don't
really need to bother as I just pull apart the cord until I have what looks
like enough to get a good swr. Then fine tune by pulling more or cutting. A
swr of about 1.3 is achievable.

73
Roger ZR3RC

The type of lamp cord common in South Africa (don't know about other
countries): Two conductors of 0.75mm^2 cross sectional area insulated with
about 1mm of white pvc and a spacing of around 2.5mm has an impedance of
aproximately 60 Ohms. Close enough to 50 to use for quick&dirty dipoles
without balun or tuner. Though have no idea of the velocity factor and
don't
really need to bother as I just pull apart the cord until I have what
looks
like enough to get a good swr. Then fine tune by pulling more or cutting.
A
swr of about 1.3 is achievable.

73
Roger ZR3RC

I've heard that lamp cord was low-impedence but had forgotten what the
impedence was.
Do you just use some tape once you unzip the length you need - to keep it
from self-zipping from the tension?
I also heard it had a pretty high loss - But like you say - for a quick-and
dirty antenna and feedline, its a good trick for a ham's bag.
Thanks for the info.

Hal Rosser wrote:
Do you just use some tape once you unzip the length you need - to keep it
from self-zipping from the tension?

Just tie a knot at that point.
--
73, Cecil http://www.qsl.net/w5dxp

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"Hal Rosser" wrote in message
. ..


The type of lamp cord common in South Africa (don't know about other
countries): Two conductors of 0.75mm^2 cross sectional area insulated
with
about 1mm of white pvc and a spacing of around 2.5mm has an impedance of
aproximately 60 Ohms. Close enough to 50 to use for quick&dirty dipoles
without balun or tuner. Though have no idea of the velocity factor and
don't
really need to bother as I just pull apart the cord until I have what
looks
like enough to get a good swr. Then fine tune by pulling more or
cutting.
A
swr of about 1.3 is achievable.

73
Roger ZR3RC

I've heard that lamp cord was low-impedence but had forgotten what the
impedence was.
Do you just use some tape once you unzip the length you need - to keep it
from self-zipping from the tension?
I also heard it had a pretty high loss - But like you say - for a
quick-and
dirty antenna and feedline, its a good trick for a ham's bag.
Thanks for the info.


Duct tape, insulation tape, etc. or my personal favourite - a cable tie.

Confuscious Say: Ham who leaves home without screwdriver, duct tape and
cable tie, is same as doctor without stethoscope and syringe.

73 Roger ZR3RC

From all the responses, I got a lot to think about. Thanks.
I got the impression that resistance in the wires is the main cause for loss
in a transmission line.
NOTE: I recall some line having much higher losses at higher frequencies.
(so substitute x for R ??)
Another note - I noticed some coax has different capacitance rating per ft.
depending on the type and brand, etc.
(I thought about using a length of coax for a capacitor in a trap at one
time).
Question: could some of this loss be caused by the capacitance in the line ?

There is no power loss in either pure capacitance or pure inductance.
There is loss only in the resistive (or conductive) components: the RF
resistance in the wire and the RF conductance in the dielectric. It is
fundamental that the inductance and capacitance in a TEM transmission
line are just what cause the energy to propagate from one end to the
other...or I suppose if you view it at a higher level, you could say
that the same fields which yield the effects we call capacitance and
inductance also cause the propagation of energy when they result from a
TEM transmission line configuration. I'm sure other valid ways of
looking at the situation exist too. (I should also mention that there
can be some power lost to radiation, but in most cases that's quite
small.)

Increased loss at high frequencies comes from several sources: smaller
skin depth at higher frequencies means higher resistance in the wires.
That goes up as the square root of frequency, once you get to a skin
depth which is small compared with the thickness of the copper. Higher
frequencies result in higher dielectric loss, though that's generally
not an issue below a few GHz. But imperfections along a line can cause
significant attenuation because of multiple reflections; dozens of
small reflections can add up to a big problem.

Cheers,
Tom


Cheers,
Tom

Question: could some of this loss be caused by the capacitance in
the line ?

=================================

Yes. It's another way of looking at it.

In addition to current in the load, there is a current which flows
between the pair of wires through the capacitance.

Increase the capacitance and this current increases.

There is negligible loss in the capacitance itself.

But the capacitor current has to flow along the wires to get there.

And so the additional capacitor-current loss actually occurs in the
wire resistance.

But this is just the same as saying that loss is greater because the
impedance Zo is lower (due to the increase in capacitance).

The opposite effect occurs by increasing inductance. An increase in
inductance increases Zo and so much reduces attenuation. That's why
88 mH inductive loading coils were used at intervals of 2000 yards at
audio frequencies in very long telephone cables. An invention of the
great but modest Oliver Heaviside which I think somebody else patented
and manufactured by many millions.

88 mH loading coils, spaced at 2000 yards, increases Zo from about 300
ohms to 1100 ohms, thus reducing loss in dB per mile to about one
third.
----
Reg, G4FGQ.