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.

"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

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

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

"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

"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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"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

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

"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