The loss has nothing to do with the speed of travel, except that the
effective dielectric constant has a direct effect on speed and an
indirect effect on loss.

At frequencies from at least HF well into the UHF range or higher, the
loss in transmission lines having decent insulation (e.g., PE or PTFE)
is almost all due to conductor loss rather than dielectric loss. Higher
impedance line has lower loss simply because for a given amount of power
being conveyed, the current is lower. Therefore, the conductor I^2 * R
loss (which is nearly the total loss) is lower.

If you introduce a dielectric material (other than air) between
conductors, the characteristic impedance drops and the velocity factor
increases, due to the same effect. Only in that way are they related in
a ladder line.

In a coax cable, some of the plastic insulation is sometimes replaced by
gas or air to make "foamed" dielectric cable, or by other devices such
as plastic disks or a helically wound plastic string. This reduces the
effective dielectric constant of the cable, which if the dimensions
remained the same, would raise the characteristic impedance. It also
increases the velocity factor. In those cables, the characteristic
impedance is lowered to its nominal value by increasing the diameter of
the center conductor. That is, for a given cable outside diameter and
Z0, a cable with more air and less plastic will have a larger center
conductor. The larger conductor reduces the I^2 * R loss by decreasing
the R. So foam dielectric cable and others having a high velocity factor
have lower loss than solid dielectric cables with the same OD because
the center conductor is larger.

At a frequency of about 1 - 10 GHz or so, dielectric loss begins to
dominate, and different relationships exist.

The equations describing the relationships among dielectric constant,
velocity, impedance, and loss are simple and can be found in a great
number of texts. I'm sure they can also be easily found on the web.

Roy Lewallen, W7EL

Hal Rosser wrote:
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.

VF, low-loss line, high-impedence line - relationship
 

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.

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.

That makes perfect sense. Just like for power lines - higher voltage means
less loss over the same line. I need to try to use ohms law a little more
often. I've wound impedence matching transformers myself - without even
thinking about the fact that I was also increasing (or decreasing -
depending on the flow) voltage.

thanks

"Roy Lewallen" wrote in message
...
The loss has nothing to do with the speed of travel, except that the
effective dielectric constant has a direct effect on speed and an
indirect effect on loss.

At frequencies from at least HF well into the UHF range or higher, the
loss in transmission lines having decent insulation (e.g., PE or PTFE)
is almost all due to conductor loss rather than dielectric loss. Higher
impedance line has lower loss simply because for a given amount of power
being conveyed, the current is lower. Therefore, the conductor I^2 * R
loss (which is nearly the total loss) is lower.

If you introduce a dielectric material (other than air) between
conductors, the characteristic impedance drops and the velocity factor
increases, due to the same effect. Only in that way are they related in
a ladder line.

In a coax cable, some of the plastic insulation is sometimes replaced by
gas or air to make "foamed" dielectric cable, or by other devices such
as plastic disks or a helically wound plastic string. This reduces the
effective dielectric constant of the cable, which if the dimensions
remained the same, would raise the characteristic impedance. It also
increases the velocity factor. In those cables, the characteristic
impedance is lowered to its nominal value by increasing the diameter of
the center conductor. That is, for a given cable outside diameter and
Z0, a cable with more air and less plastic will have a larger center
conductor. The larger conductor reduces the I^2 * R loss by decreasing
the R. So foam dielectric cable and others having a high velocity factor
have lower loss than solid dielectric cables with the same OD because
the center conductor is larger.

At a frequency of about 1 - 10 GHz or so, dielectric loss begins to
dominate, and different relationships exist.

The equations describing the relationships among dielectric constant,
velocity, impedance, and loss are simple and can be found in a great
number of texts. I'm sure they can also be easily found on the web.

Roy Lewallen, W7EL

Hal Rosser wrote:
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.

On Mon, 04 Apr 2005 17:35:37 -0700, Roy Lewallen took
the words right out of my mouth:

The loss has nothing to do with the speed of travel, except that the
effective dielectric constant has a direct effect on speed and an
indirect effect on loss.

At frequencies from at least HF well into the UHF range or higher, the
loss in transmission lines having decent insulation (e.g., PE or PTFE)
is almost all due to conductor loss rather than dielectric loss. Higher
impedance line has lower loss simply because for a given amount of power
being conveyed, the current is lower. Therefore, the conductor I^2 * R
loss (which is nearly the total loss) is lower.

If you introduce a dielectric material (other than air) between
conductors, the characteristic impedance drops and the velocity factor
increases, due to the same effect. Only in that way are they related in
a ladder line.

In a coax cable, some of the plastic insulation is sometimes replaced by
gas or air to make "foamed" dielectric cable, or by other devices such
as plastic disks or a helically wound plastic string. This reduces the
effective dielectric constant of the cable, which if the dimensions
remained the same, would raise the characteristic impedance. It also
increases the velocity factor. In those cables, the characteristic
impedance is lowered to its nominal value by increasing the diameter of
the center conductor. That is, for a given cable outside diameter and
Z0, a cable with more air and less plastic will have a larger center
conductor. The larger conductor reduces the I^2 * R loss by decreasing
the R. So foam dielectric cable and others having a high velocity factor
have lower loss than solid dielectric cables with the same OD because
the center conductor is larger.

At a frequency of about 1 - 10 GHz or so, dielectric loss begins to
dominate, and different relationships exist.

The equations describing the relationships among dielectric constant,
velocity, impedance, and loss are simple and can be found in a great
number of texts. I'm sure they can also be easily found on the web.

One old wives' tale (*not* attributed to Roy) is that ladderline has
lower loss than coax (given as a blanket statement). Therefore,
laderline is "good" and coax is "bad."

However, compare something like Andrew LDF4-50 to Wireman 554 and you
find that the "lossy" coax has a loss of 0.48 dB/100' @ 50 MHz and the
"low-loss" ladderline has a loss of 0.41 dB under the same conditions.

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

Wes Stewart wrote:
However, compare something like Andrew LDF4-50 to Wireman 554 and you
find that the "lossy" coax has a loss of 0.48 dB/100' @ 50 MHz and the
"low-loss" ladderline has a loss of 0.41 dB under the same conditions.

Hi Wes, let's say I'm trying to choose between the two.
Wireman 554 is about 25 cents/foot. How much did you say the
Andrew LDF4-50 costs? :-) (LMR-1700 is about 8 bucks/foot.)

Here's another way to look at things for multi-band non-
resonant antenna lengths. The feedpoint impedance for
that type antenna may vary from a low of about 50 ohms
to a high of about 7500 ohms. To minimize SWR for all
conditions, Z0 should equal the square root of those
two values or 612 ohms. Given 600 ohm open-wire line,
the SWR shouldn't go much above 13:1 for the open-wire
line but may go as high as 150:1 for the coax. I don't
know about you, but I would rather run with a maximum
SWR of 13:1 rather than a maximum SWR of 150:1.
--
73, Cecil http://www.qsl.net/w5dxp

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Roy Lewallen wrote:
....
At frequencies from at least HF well into the UHF range or higher,
the
loss in transmission lines having decent insulation (e.g., PE or
PTFE)
is almost all due to conductor loss rather than dielectric loss.
Higher
impedance line has lower loss simply because for a given amount of
power
being conveyed, the current is lower. Therefore, the conductor I^2 *
R
loss (which is nearly the total loss) is lower.

If you introduce a dielectric material (other than air) between
conductors, the characteristic impedance drops and the velocity
factor
increases, due to the same effect. Only in that way are they related
in
a ladder line.

Here's a slightly different way to look at the same thing Roy has said.
For a given coaxial cable outer conductor diameter, assuming smooth
copper conductors, there's a particular ratio of D/d (outer to inner
conductor diameters) that gives you the lowest loss. So long as
there's negligible loss in the dielectric, that D/d is independent of
what dielectric you put in there. But since putting in a dielectric
lowers the impedance, the loss goes up as a result of higher current
for a given power level.

You can put numbers on it pretty easily. Assuming no dielectric loss,
the attenuation of the line in dB per unit length is inversely
proportional to the line impedance: dB/100ft = 4.34*Rt/Zo, where Rt is
the total RF resistance of the wires. But Zo is inversely proportional
to the square root of the relative dielectric constant of the
dielectric in the line. Putting the two together, for a given
conductor configuration (D and d in coax), if there's no loss in the
dielectric itself and only loss in the resistance of the wires, the
loss in dB/unit length is proportional to the square root of the net
effective dielectric constant around the line. Since the velocity
factor is inversely proportional to the square root of the same net
effective dielectric constant, then for a given configuration of
conductors, the loss is indeed dependent on the velocity factor, even
with no power dissipated in the dielectric itself. This is true for
coax and open wire line in equal measure. But beware that you are more
likely to have dielectric loss in open-wire line for a variety of
reasons...

For lossless dielectric and a fixed conductor configuration (coaxial,
two-wire, or other TEM line with fixed conductor sizes and spacings),
varying just the dielectric, then,

dB/unit length = k1/v.f. = k2/Zo = k3*sqrt(net effective dielectric
constant)

where k1, k2 and k3 are proportionality constants depending on the
conductor configuration.

Cheers,
Tom

On Tue, 05 Apr 2005 11:28:46 -0500, Cecil Moore
wrote:

|Wes Stewart wrote:
| However, compare something like Andrew LDF4-50 to Wireman 554 and you
| find that the "lossy" coax has a loss of 0.48 dB/100' @ 50 MHz and the
| "low-loss" ladderline has a loss of 0.41 dB under the same conditions.
|
|Hi Wes, let's say I'm trying to choose between the two.
|Wireman 554 is about 25 cents/foot. How much did you say the
|Andrew LDF4-50 costs? :-) (LMR-1700 is about 8 bucks/foot.)

Typically I buy it at hamfests for $1.00/ft. I have about a dozen
short lengths that I bought just for the connectors for $20.00. I
also have "in stock" a 110' length of LDF5-50 (7/8") that I paid a guy
in San Diego $200 for and a friend brought home to me for free. I've
been saving this for a new EME antenna....someday.

|
|Here's another way to look at things for multi-band non-
|resonant antenna lengths. The feedpoint impedance for
|that type antenna may vary from a low of about 50 ohms
|to a high of about 7500 ohms. To minimize SWR for all
|conditions, Z0 should equal the square root of those
|two values or 612 ohms. Given 600 ohm open-wire line,
|the SWR shouldn't go much above 13:1 for the open-wire
|line but may go as high as 150:1 for the coax. I don't
|know about you, but I would rather run with a maximum
|SWR of 13:1 rather than a maximum SWR of 150:1.

Who's talking about multiband non-resonant antennas? I prefer to
operate with SWR = 2.0. I can bury my line, strap it to the tower,
run it through a hole in the block wall without heartbreak and the
only concern I have with rain is that we don't get enough.

Furthermore, I don't have concerns with breakage or degradation from
UV and the only tuner I need is the one built into the plate circuit
of my Drake L-4B.

Wes Stewart wrote:
|Here's another way to look at things for multi-band non-
|resonant antenna lengths. The feedpoint impedance for
|that type antenna may vary from a low of about 50 ohms
|to a high of about 7500 ohms. To minimize SWR for all
|conditions, Z0 should equal the square root of those
|two values or 612 ohms. Given 600 ohm open-wire line,
|the SWR shouldn't go much above 13:1 for the open-wire
|line but may go as high as 150:1 for the coax. I don't
|know about you, but I would rather run with a maximum
|SWR of 13:1 rather than a maximum SWR of 150:1.

Who's talking about multiband non-resonant antennas?

Usually, anyone considering ladder-line for the feed
system. If the antenna is single-frequency with a 50
ohm feedpoint, there's not much of a reason to even
consider ladder-line except for very long runs.
--
73, Cecil http://www.qsl.net/w5dxp

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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.

Roy Lewallen, W7EL

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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On Tue, 05 Apr 2005 21:55:35 -0500, Cecil Moore
wrote:

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.

Isn't that a differential-mode choke?

One old wives' tale (*not* attributed to Roy) is that ladderline has
lower loss than coax (given as a blanket statement). Therefore,
laderline is "good" and coax is "bad."

However, compare something like Andrew LDF4-50 to Wireman 554 and you
find that the "lossy" coax has a loss of 0.48 dB/100' @ 50 MHz and the
"low-loss" ladderline has a loss of 0.41 dB under the same conditions.

If they make a coax as low-loss as ladder line, I'll concede you that - but
then:
Could we agree that ladderline (or window line - or twinlead) has these
characteristics:
1. Ladderline (or twinlead or windowline) costs less than an equal length of
low-loss coax .
2. The weight of the ladder line would probably be much less than the weight
of an equal length of low-loss coax.
---Well, sir - that sells it for me. I'm a cheapskate and I don't like the
coax loading down the dipole and stretching it from all that weight. AND I
like to play around with something other than the 50-ohm ho-hum stuff.
Ham-nerd is a good word, I think.

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 ?

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

Wes Stewart wrote:
On Tue, 05 Apr 2005 21:55:35 -0500, Cecil Moore
wrote:

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.

Isn't that a differential-mode choke?


You can make a very good HF common-mode choke by deliberately resonating
the inductance of a coil of coax with its self-capacitance... so it
seems to follow that a resonant UHF common-mode choke can be made by
tying the coax into exactly the right knot.

Don't know if it's of any practical use, but it isn't a completely April
Fool idea.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

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.

"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

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

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

"Wes Stewart" wrote

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

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

I don't see line length in the formula.

What do they say about line lengths less than 20 times wire spacing
for small spacings?
----
Reg.

Wes Stewart wrote:
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

Reg Edwards wrote:

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.

You've told us about radiation from the connections to the generator and
the termination.

Now tell us about radiation from the line.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Reg Edwards wrote:

"Cecil Moore" asks -
How much does an infinitessimally short
line radiate? :-)

Cec, you took the bait.

We probably need an adjective to describe line
radiation from a line that isn't there. How
about "phantom radiation"? You know, like
phantom pain from a leg that isn't there? :-)
--
73, Cecil http://www.qsl.net/w5dxp


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On Thu, 7 Apr 2005 23:27:29 +0000 (UTC), "Reg Edwards"
wrote:


"Wes Stewart" wrote

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

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

I don't see line length in the formula.


That's because for the condition of length 20 * spacing it drops
out.

What do they say about line lengths less than 20 times wire spacing
for small spacings?

They say a whole bunch of things in a complicated formula full of
cosine integrals, etc. Too complicated to express here in plain
ASCII.

I'll try to scan it to pdf and post is somewhere.

Wes

On Fri, 08 Apr 2005 06:05:49 -0700, Wes Stewart
wrote:


I'll try to scan it to pdf and post is somewhere.

http://www.qsl.net/n7ws/Sterba_Openwire.pdf

Wes Stewart wrote:
http://www.qsl.net/n7ws/Sterba_Openwire.pdf

There seems to be a dotted line for feedline
radiation going to zero as feedline length
goes to zero. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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You've told us about radiation from the connections to the generator
and
the termination.

Now tell us about radiation from the line.

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

Ian, you are falling into the same sort of trap as old wives who
imagine most radiation comes from the middle 1/3rd of a dipole because
that's where most of the current is.

It is self-misleading to consider the various parts of a radiating
system to be separate components which are capable of radiating
independently of each other. They can't. A system's behaviour must
be treated as a whole.

We have already discussed that the power radiated from a generator +
twin-line + load is a constant and is independent of line length.

Total power radiated is equal to that radiated from a wire having a
length equal to line spacing with a radiation resistance appropriate
to that length. The location of the radiator, insofar as the
far-field is concerned, can be considered to be at the load. The
current which flows in the radiator is the same as that flowing in a
matched load. And the load current is independent of line length.

Mathematically, the only way for the total power radiated to remain
constant and independent of line length is for zero radiation from the
line.

In summary, the system as a whole BEHAVES as if there is NO radiation
from the line itself - only from fictitious very short monopoles (or
dipoles?) at its ends.
----
Reg, G4FGQ