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 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.

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.

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.

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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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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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.

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

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