Bend in xmission line
 

Conventional wisdom cautions against sharp bends in open wire transmission
line. What is the practical/quantifiable effect of violating this rule?

How does the effect vary with the severity of the bend and does it vary with
the distance along the line at which the bend occurs?

Tnx es 73, AL.

Bend in xmission line
 

On Tue, 7 Mar 2006 09:15:06 -0800, "Alfred Lorona"
wrote:

Conventional wisdom cautions against sharp bends in open wire transmission
line. What is the practical/quantifiable effect of violating this rule?

How does the effect vary with the severity of the bend and does it vary with
the distance along the line at which the bend occurs?

Hi Al,

This arises from two concerns, with all probability that neither will
bother you.

1. High power presents an arcing opportunity (very high power, pulsed
100s of KW);
2. Accuracy of determining a load.

The usual response is to confine your bends to a quarter wave sweep.
Now, if it really mattered (a lot) just imagine your problems at 160M.

73's
Richard Clark, KB7QHC

Bend in xmission line
 

Alfred Lorona wrote:
"Conventional wisdom cautions against sharp bends in open wire
transmission line."

One does not want to create an impedance bump. Ed Laport shows how to
make right-angle turns. See page 396 of "Radio Antenna Engineering". It
works fine with 100 KW.

Best regards, Richard Harrison, KBB5WZI

Bend in xmission line
 

Alfred Lorona wrote:
Conventional wisdom cautions against sharp bends in open wire transmission
line. What is the practical/quantifiable effect of violating this rule?

How does the effect vary with the severity of the bend and does it vary with
the distance along the line at which the bend occurs?

Tnx es 73, AL.

Al,

I've had many real open wire lines, including one over 1000 feet long.

As Clark pointed out, at extermel high voltage levels a sharp bend can
cause corona problems. In critical systems like video it can cause
impedance bumps that might create faint ghosts in the picture. It make
cause impedance to change slightly.


Sharp bends make no noticable difference at all for amateur
applications, other than weakening the wire if it is a stiff solid wire
like copperweld.

I would never give a moments worry about it.

73 Tom

Bend in xmission line
 

Richard Harrison wrote:
Alfred Lorona wrote:
"Conventional wisdom cautions against sharp bends in open wire
transmission line."

One does not want to create an impedance bump. Ed Laport shows how to
make right-angle turns. See page 396 of "Radio Antenna Engineering". It
works fine with 100 KW.

Can you describe that, please?

Parallel-wire feedline is used with success in 432MHz moonbounce which
are very sensitive to small losses. I don't have any information on
sharp bends, but T-joints and curved bends with say a foot radius cause
no noticeable problems.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Bend in xmission line
 

"Ian White GM3SEK wrote
I don't have any information on sharp bends, but T-joints
and curved bends with say a foot radius cause no noticeable problems.
_______________

UHF broadcast transmit antenna systems often use 90 degree coaxial,
mitre-joint elbows carrying upwards of 50 kW with negligible SWR (1.02:1).

RF

Bend in xmission line
 

Ian, GM3SEK wrote:
"Can you describe that please?"

Gladly.

For low loss at 100 KW, you use large cable and wide spacings at 600
ohms. It may have been more economical to use more conductors but for
simplicity we stuck with a 2-wire balanced feed system. We had at that
particular time and location the largest congregation of high-powered
shortwave transmitters and directional antennas in the world. We
installed a crossbar switching system so that almost any transmitter
could access any antenna. We were on the air with most transmitters
operating nearly 24 hours a day, but switching antennas several times a
day according to our announced program schedule to accommodate
anticipated propagations to the target areas served.

Ed Laport as Chief Engineer of RCA International had as much experience
as anyone with commercial shortwave radio, so we relied upon him for
information and weren`t disappointed.

On page 396 of "Radio Antenna Engineering", Ed illustrates a corner made
using two transmission lines. He shows a single pole with two crossarms,
one above the other and making the horizontal angle required for the
intersection. He terminates the lines one on each crossarm. Jumpers are
used to interconnect the two lines.

Our line cables were akin to welding cable. We joined them with sleeves
and fittings attached with a hydraulic press (Nicropress). With wide
spacing the lines are tolerant of bumps and don`t tend to flashover.

Ed gives a graph of the value of the ratio of line spacing to cable
radius versus Zo. For 600 ohms the ratio is about 150.

Best regards, Richard Harrison, KB5WZI