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.