By way of agreeing with what Reg posted about capacitance indirectly
adding to the loss, consider that for any TEM line (twin-lead and coax
being two examples), the impedance, Zo, is sqrt(L/C), and the
propagation delay, tau, is sqrt(LC) [neglecting the very small
contribution of R and G for practical lines at HF and above]. From
these two, you can see that C=tau/Zo. If the velocity factor is unity,
then tau for a foot of line is one foot divided by the speed of light,
about 1.017 nanoseconds. If Zo is 50 ohms, then C for that line would
be 20.33pF/foot. If you have line which you know to be 50 ohms and
31.0pF/foot, then you know the v.f. is 20.33/31.0 = 0.656, and by my
other recent posting in this thread, you know that its attenuation will
be about 1/0.656 = 1.52 times as many dB/unit length as the same line
with air dielectric (which would be 50 ohms times 1.52 = 76 ohms).

(The interrelation of tau, Z, C, L, line physical length and velocity
factor suggests that you can determine Z, for example, by measuring C
and v.f. accurately. Some line configurations let you accurately
measure conductor diameters as well. You end up with lots of ways to
determine a set of line parameters.)

But note that a 50 ohm air dielectric coax using the same outer
conductor diameter would have a larger inner conductor, but MORE loss
than the 76 ohm air dielectric line because of the higher capacitance.
Quantitatively, it will have about 1.1 times the dB/unit length loss
compared with the 76 ohm line...so the difference in loss between air
inslated 50 ohm line and solid polyethylene dielectric 50 ohm line
(same OD) will be a ratio of 1.1:1.52, or 1:1.38. Going from 50 ohm
line insulated with solid pe to 50 ohm line of the same OD with air
insulation will cut the dB loss by about 27%. Going from solid to
foamed pe will get you about half that much. There's a bigger effect
going from a solid pe 75 ohm line to an air dielectric 75 ohm line,
cutting the dB loss by over 42% (assuming I didn't screw up the calcs
too badly).

Cheers,
Tom

"Hal Rosser" bravely wrote to "All" (04 Apr 05 20:48:04)
--- on the heady topic of "VF, low-loss line, high-impedence line - =
relationship"

HR Reply-To: "Hal Rosser"
HR Xref: aeinews rec.radio.amateur.antenna:27947

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

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

You are right there is a connection between wire diameter and spacing.
It has to do with the self inductance and resistive losses of two
conductors in proximity. By contrast a balanced line has a wider
spacing and also allows part of the energy to travel unhindered, so to
speak. It helps if the balanced line is designed to be close to the
theoretical impedance of free space. The price to pay is that it is
more susceptible to the environment. The loss in coax is a trade off
to achieve stability.

A*s*i*m*o*v

.... "Beware of all enterprises that require new clothes." -- THOREAU

Asimov wrote:

You are right there is a connection between wire diameter and spacing.
It has to do with the self inductance and resistive losses of two
conductors in proximity. By contrast a balanced line has a wider
spacing and also allows part of the energy to travel unhindered, so to
speak.

Conductors don't "hinder" the traveling of energy. Energy travels just
as well along close spaced conductors as it does along wide spaced ones.
In fact, loss due to radiation is greater with wider spacing than narrow
(although it's still negligible with the lines typically used).

It helps if the balanced line is designed to be close to the
theoretical impedance of free space.

Please explain in what way it "helps". No equation, formula or
theoretical treatment I'm aware of shows any advantage, change, or
anomaly in tranmission line behavior at a value equal to or near the
characteristic impedance of free space. (As has been pointed out many
times before in this newsgroup, the impedance of free space is the ratio
of E/H fields in a plane wave; the impedance of a transmission line is
the ratio of voltage to current of a traveling wave. Although they have
the same unit of measure, they're different things -- like foot-pounds
of work and foot-pounds of torque.)

The price to pay is that it is
more susceptible to the environment.

Do you mean that lines of approximately 377 ohms impedance are more
susceptible to the environment than 200 or 600 ohm lines? In what ways? Why?

The loss in coax is a trade off
to achieve stability.

Coax is more stable than open wire line? Does open wire line drift in
some way?


A*s*i*m*o*v

... "Beware of all enterprises that require new clothes." -- THOREAU

Roy Lewallen, W7EL

"Roy Lewallen" wrote

Energy travels just
as well along close spaced conductors as it does along wide spaced
ones.
In fact, loss due to radiation is greater with wider spacing than
narrow

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

In fact, the field radiated from correctly balanced twin or open-wire
lines is directly proportional to wire spacing.

Radiation resistance is the same as a monopole of the same length as
the wire spacing in terms of wavelength. Rr even at VHF is quite
small.

Radiation is off the ends - i.e., in the same direction as the line.

Polarisation is in the same direction as the wires are spaced.

And, believe it or not, all is independent of the length of the line.
----
Reg, G4FGQ

Reg Edwards wrote:
And, believe it or not, all is independent of the length of the line.

How much does an infinitessimally short
line radiate? :-)
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil,

Almost as much as a "full length" line, if you can feed the power to it.

For details check your favorite antenna book.

73,
Gene
W4SZ



Cecil Moore wrote:
Reg Edwards wrote:

And, believe it or not, all is independent of the length of the line.


How much does an infinitessimally short
line radiate? :-)
--
73, Cecil http://www.qsl.net/w5dxp

On Thu, 07 Apr 2005 09:09:51 -0500, Cecil Moore
wrote:

Reg Edwards wrote:
And, believe it or not, all is independent of the length of the line.

How much does an infinitessimally short
line radiate? :-)

Sterba and Feldman in "Transmission Lines for Short-Wave Radio
Systems", Proceedings of the IRE, Volume 20, No 7., July, 1932 give a
formula for the radiated power in a balanced line.

The line length *is* a factor, however, they give a simplified
approximation for the case of a length more than 20 times the line
spacing and the line spacing less than 1/10 lambda.

P/I^2 = 160 * ( pi * D / lambda)^2

whe

P is in watts

I is the RMS current in a matched line

D / lambda is the wire spacing in wavelengths

"Cecil Moore" asks -
Reg Edwards wrote:
And, believe it or not, all is independent of the length of the
line.

How much does an infinitessimally short
line radiate? :-)

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

Cec, you took the bait.

So just exercise a teeny bit of your imagination.

Suppose you have a generator directly connected to a load resistance
without any line in between.

Let the generator and load terminals both be spaced apart by the same
distance as the conductors of the non-existent line.

The load carries a current along a length equal to the spacing between
its terminals.

The load, by virtue of its length, possesses radiation resistance.

And so radiation occurs with zero line length.

Even a CB-er can understand the obvious.

Can you calculate radiating efficiency?
----
Reg, G4FGQ

"Roy Lewallen" bravely wrote to "All" (06 Apr 05 22:10:02)
--- on the heady topic of " VF, low-loss line, high-impedence line - =
relationship"

RL From: Roy Lewallen
RL Xref: aeinews rec.radio.amateur.antenna:28064

RL Do you mean that lines of approximately 377 ohms impedance are more
RL susceptible to the environment than 200 or 600 ohm lines? In what
RL ways? Why?

Since a portion of the EM field in open wire line is free to travel
outside the conductor into the environment then we may safely assume
there is an exchange between the environment and the conductor. If the
impedance of each is approximately the same then there is less loss in
the interface between the two. It has to do with the reflective
coefficient where the energy is returned. You will note 300 ohm open
line has less loss than 100 ohm open line.


RL The loss in coax is a trade off
to achieve stability.

RL Coax is more stable than open wire line? Does open wire line drift in
RL some way?

It is susceptible to ambient humidity and proximity to conductive
objects (birds, snow, rfi). That is a source of drift in practical
terms.

A*s*i*m*o*v

.... No individual raindrop ever considers itself responsible for the flood

Asimov wrote:

Since a portion of the EM field in open wire line is free to travel
outside the conductor into the environment then we may safely assume
there is an exchange between the environment and the conductor.

If the conductors are perfectly conducting, no part of the field at all
exists within the conductor. With good conductors like copper and at HF
and above, there's very little penetration of the conductor by the
fields, either electric or magnetic. As far as an "exchange" goes, it
sounds like you're trying to describe radiation. If not, what's the
phenomenon you're referring to?

If the
impedance of each is approximately the same then there is less loss in
the interface between the two.

No, that's not true. First of all, a mismatch doesn't cause loss.
Secondly, as I explained in my last posting, the characteristic
impedance of a transmission line isn't the same thing as the
characteristic impedance of free space. If you were to construct a
transmission line with 377 ohms characteristic impedance (numerically
the same as the characteristic impedance of free space), the ratio of
E/H fields between the conductors probably won't be anywhere near 377
ohms, as it is in a plane wave propagating without wires.

It has to do with the reflective
coefficient where the energy is returned.

Well, no. There isn't a bundle of energy trying to escape the line and
bouncing off the air, or bouncing off the air as it travels along the
line, or bouncing off the conductors into the air. So reflection
coefficient isn't applicable here.

You will note 300 ohm open
line has less loss than 100 ohm open line.

Yes, and 600 ohm line has less loss than 377 ohm line. You'll have to
find a way to fit this into your theory if you want to pursue it.

RL The loss in coax is a trade off
to achieve stability.

RL Coax is more stable than open wire line? Does open wire line drift in
RL some way?

It is susceptible to ambient humidity and proximity to conductive
objects (birds, snow, rfi). That is a source of drift in practical
terms.

Thanks for the clarification. Because the differential fields are
completely confined within a coaxial cable, they are indeed more immune
to external influences.

I'm afraid that the conclusions you've reached about loss and
characteristic impedance are based on a poor understanding of
fundamental transmission line operation. The result is some conclusions
that are, and are well known to be, untrue.

If you really feel that you have a viable theory, you should be able to
provide some equations and formulas to quantify the extra loss you're
talking about. The existing theory, formulas and equations, in daily use
for over a hundred years, have been shown countless times to accurately
predict transmission line loss, and they don't include the phenomena
you're describing. So although I think it's highly doubtful that your
formulations will prove more accurate, if you post them they can pretty
easily be tested by actual cable measurement.

Roy Lewallen, W7EL