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.

"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.

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

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?

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

"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