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

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

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

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

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