Cecil Moore wrote:
Jeff wrote:
There is an apocryphal story that 50 ohms started out as a common impedance
for coax because that happened to be the number that came out using common
British copper pipe sizes.

I vaguely remember something about 50 ohms being good
for transmitting and 73 ohms being good for receiving.
--
73, Cecil http://www.w5dxp.com

:-) It shouldn't be vague. It should be crystal-clear. If lowest
loss is important and you're going to use coax with smooth conductors
and the same metal for inner and outer conductors, and you have a fixed
outer conductor diameter, you want the ratio of the inside of the outer
conductor to inner conductor diameters to be 3.59:1. That assumes
negligible dielectric loss. It's not difficult to find the ratio for
other cases, if you know the ratio of RF resistivities of the inner and
outer conductors and the dielectric loss. The loss doesn't increase
very quickly as you get away from that ratio some, but that's the ratio
for lowest loss. If you have air dielectric, that 3.59:1 ratio gives
you 76.7 ohms. If you have solid polyethylene dielectric, it gives you
about 50.6 ohms. Foam dielectric would give you roughly 60 ohms.

There are different conductor diameter ratios for maximum voltage
handling (assuming uniform dielectric breakdown rating and a fixed
outer conductor size; you want a conductor diameter ratio that
minimizes the maximum voltage gradient, i.e., the gradient next to the
center conductor) and maximum power handling (assuming the line is
voltage-limited, which generally only is the case for very low duty
cycle, like radar pulses). If the line is thermally limited (almost
always the case for typical ham installations), lowest attenuation will
give you very close to the highest power handling...details depend on
how well the center conductor can get rid of heat.

Cheers,
Tom

When I was experimenting on 440 MHz I made several 1/4 wave matching
sections out of 1/2 inch copper pipe. PL259s on the ends with hose
clamps on the connectors. By varying the inner conductor it was pretty
simply to construct the impedance needed.

The reason for the need was that when you stuff a military surplus
cavity to raise the frequency you also booger up (technical term) the
impedance of the system. An appropriate quarter wave section would go
a long ways toward matching 50 ohm line to antenna.

It would take a lot of motivation to construct your own hardline.

On Tue, 12 Sep 2006 07:46:18 -0700, Bill Turner
wrote:

Hardline is good stuff, right? Does anyone make their own out of
copper water pipe?

Seems doable, but have never heard of anyone doing it.

This is what happens when I have too much time on my hands. :-)
John Ferrell W8CCW

considering that 1" copper water pipe, 10 feet long is around $30, I
would say no.

Scott
N0EDV

Bill Turner wrote:

Hardline is good stuff, right? Does anyone make their own out of
copper water pipe?

Seems doable, but have never heard of anyone doing it.

This is what happens when I have too much time on my hands. :-)

Now copper pipe WAVEGUIDE might be a different story!

Scott
N0EDV

K7ITM wrote:

At about $25 for ten feet of 3/4" copper pipe, plus lots of time to put
it together, why bother?
now.

ORIGINAL MESSAGE:

On Tue, 12 Sep 2006 12:04:30 -0400, "Jimmie D"
wrote:

After I realisesd I would have to keep it pressurised ti keep out
the water I pulled it out and replaced it with LMR 900.

------------ REPLY FOLLOWS ------------

I thought about that. Perhaps a small aquarium air pump would do the
job. Just a guess.

Bill, W6WRT

What some do for pressurizeing coax is (like
Andrew), run an aquarium pump , thru a canister,
that is filled with dissecant (moisture
absorbing), and then to the coax . Also, can
use compressed nitrogen, or another thing,
would be canister of air conditioner rechargeing
material (used to be cheap, but now,??),
as info, Jim NN7K

Bill Turner wrote:
ORIGINAL MESSAGE:

On Tue, 12 Sep 2006 12:04:30 -0400, "Jimmie D"
wrote:

After I realisesd I would have to keep it pressurised ti keep out
the water I pulled it out and replaced it with LMR 900.

------------ REPLY FOLLOWS ------------

I thought about that. Perhaps a small aquarium air pump would do the
job. Just a guess.

Bill, W6WRT

Howdy:
Might want to check out this site, and download "feedline.exe". The
icon has "single 4 hardlines" on it. Small program calculates dimensions
for building hardline using wire, beads, and conduit.
VE3SQB ANTENNA DESIGN PROGRAMS:
http://www.ve3sqb.com/

--

SeeYaa Harbin Osteen KG6URO

When American Citizens with dual citizenship pledges allegiance
to the flag, to which flag do they pledge allegiance too?

-
"John Ferrell" wrote in message ...
When I was experimenting on 440 MHz I made several 1/4 wave matching
sections out of 1/2 inch copper pipe. PL259s on the ends with hose
clamps on the connectors. By varying the inner conductor it was pretty
simply to construct the impedance needed.

The reason for the need was that when you stuff a military surplus
cavity to raise the frequency you also booger up (technical term) the
impedance of the system. An appropriate quarter wave section would go
a long ways toward matching 50 ohm line to antenna.

It would take a lot of motivation to construct your own hardline.

On Tue, 12 Sep 2006 07:46:18 -0700, Bill Turner
wrote:

Hardline is good stuff, right? Does anyone make their own out of
copper water pipe?

Seems doable, but have never heard of anyone doing it.

This is what happens when I have too much time on my hands. :-)
John Ferrell W8CCW