Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old November 2nd 03, 12:21 AM
KS7J
 
Posts: n/a
Default Methods for ladder line feed on rotatable antenna?

Please reply to the list or directly to me: shack AT tomochka DOT com.

Hello antenna mavens,

Searching high and low today for examples of feeding a rotating
antenna with ladder/window/open feed line. My idea would be to use a
standoff near the tower top (say, 3-4' perpendicular to the tower) and
drape a large loop of window line from the feedpoint to this standoff,
then route the remainder of the feed line down and away from the tower
using standoffs as necessary.

Anyone have experience with this sort of setup or can you suggest a
better one? Is there a rule of thumb for how large the loop should be
to accomodate a 450 degree turn?

I would imagine using a stranded conductor feed line would be
preferable given the amount of flex that may be inherent. How would
one stiffen the overall draped loop though to prevent it being blown
near metal in a breeze?

TIA es 73.

Please reply to the list or directly to me: shack AT tomochka DOT com.

- Casey

  #2   Report Post  
Old November 2nd 03, 01:59 AM
'Doc
 
Posts: n/a
Default



Casey,
'Slip-rings' is one way. Several of the older
handbooks have pictures of them.
'Doc
  #3   Report Post  
Old November 2nd 03, 04:11 AM
Reg Edwards
 
Posts: n/a
Default

How would
one stiffen the overall draped loop though to prevent it being blown
near metal in a breeze?

========================

What's the matter with metal, steel, aluminium ?

Are they not good, low-loss conductors?

No harm in running, if you must, balanced lines near to metal structures for
a few inches.


  #4   Report Post  
Old November 2nd 03, 09:02 PM
N3UMH
 
Posts: n/a
Default

"Reg Edwards" wrote in message ...
How would
one stiffen the overall draped loop though to prevent it being blown
near metal in a breeze?

========================

What's the matter with metal, steel, aluminium ?

Are they not good, low-loss conductors?

No harm in running, if you must, balanced lines near to metal structures for
a few inches.


I have seen a method which involves inductive coupling between a pair
of single turn loops. I guess the idea is that you have a loop on the
tower and a loop on the mast (possibly both self resonant to present
the proper impedance to the ladder line). You make them coplanar and
close together so that they couple to each other, and connect the
ladder line to their terminals.

This, of course, only works for monobanders...

73,

Dan, N3UMH
  #5   Report Post  
Old November 3rd 03, 12:54 AM
Reg Edwards
 
Posts: n/a
Default



--
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================
"N3UMH" wrote in message
om...
"Reg Edwards" wrote in message

...
How would
one stiffen the overall draped loop though to prevent it being blown
near metal in a breeze?

========================

What's the matter with metal, steel, aluminium ?

Are they not good, low-loss conductors?

No harm in running, if you must, balanced lines near to metal structures

for
a few inches.


I have seen a method which involves inductive coupling between a pair
of single turn loops. I guess the idea is that you have a loop on the
tower and a loop on the mast (possibly both self resonant to present
the proper impedance to the ladder line). You make them coplanar and
close together so that they couple to each other, and connect the
ladder line to their terminals.

This, of course, only works for monobanders...

73,

Dan, N3UMH


======================

Dan, you are quite right of course. I am incited to go further.

I was concerned with the long-lived popular misconception, plagiarised by
old wives like Yuri who have never thought about it, that running balanced
lines very near to metal structures is the worst possible thing to do
because it is 'lossy'. But nobody ever states how much more lossy it is.
(Lord Kelvin.)

In fact, if you MUST run a balanced line, even wide-open-wire stuff, near to
an expanse of some material then the metals copper and aluminium are the
best materials to run it near to. No significant extra loss occurs in the
line - the only effect at HF is a slight reduction in line Zo due to an
increase in line capacitance the length of line involved. There's no more
radiation than what occurs from the line when it is isolated in space.
Perhaps less. And as Cecil has observed, nobody knows what the Zo of
nominal 450-ohm ladder-line actually is within +/- 15 percent anyway,
although it has a serious effect on the performace of G5RV's and similar
(hopefully no tuner) multiband antennas.

The small currents induced in the adjacent 'foreign' metal run TRANSVERSELY
on the metal surface over a distance equal to the line conductor spacing and
are harmless. The other old-wives story that the line should be twisted may
in fact spoil the effect and do more harm than good. It also spoils the
cosmetics.

The usual example is when bringing an open-wire or ladder-line into the
shack. No harm occurs when bringing the line wires together for a few
inches and drawing them through a single oversize hole drilled in a steel or
aluminium window frame. A liitle more increase in insulation thicknes is
advisable for high power transmitters.

It is pointless to convert to a pair of short coaxial lines drawn through
TWO spaced holes. That causes a much larger lump of capacitance across the
line although even then, at HF, it will be harmless - equivalent to an
unnoticeable readjustment of a tuner capacitor setting.

The problem needs more thought when bringing an open-wire line through a
timber, brickwork or concrete wall. Those materials ARE lossy (dielectric
loss) and should be avoided. They may heat up when on continuous high power
with standing waves located at a voltage (not current) maximum.
----
A few more icons demolished, eh Roy? ;o)
Reg, G4FGQ




  #6   Report Post  
Old November 3rd 03, 01:27 AM
Reg Edwards
 
Posts: n/a
Default

"N3UMH" wrote -
"Reg Edwards" wrote -
one stiffen the overall draped loop though to prevent it being blown
near metal in a breeze?

========================

What's the matter with metal, steel, aluminium ?

Are they not good, low-loss conductors?

No harm in running, if you must, balanced lines near to metal

structures
for
a few inches.


I have seen a method which involves inductive coupling between a pair
of single turn loops. I guess the idea is that you have a loop on the
tower and a loop on the mast (possibly both self resonant to present
the proper impedance to the ladder line). You make them coplanar and
close together so that they couple to each other, and connect the
ladder line to their terminals.

This, of course, only works for monobanders...

73,

Dan, N3UMH


======================

Dan, you are quite right of course. I am incited to go further.

I was concerned with the long-lived popular misconception, plagiarised by
old wives like Yuri who have never thought about it, that running balanced
lines very near to metal structures is the worst possible thing to do
because it is 'lossy'. But nobody ever states how much more lossy it is.
(Lord Kelvin.)

In fact, if you MUST run a balanced line, even wide-open-wire stuff, near

to
an expanse of some material then the metals copper and aluminium are the
best materials to run it near to. No significant extra loss occurs in the
line - the only effect at HF is a slight reduction in line Zo due to an
increase in line capacitance the length of line involved. There's no more
radiation than what occurs from the line when it is isolated in space.
Perhaps less. And as Cecil has observed, nobody knows what the Zo of
nominal 450-ohm ladder-line actually is within +/- 15 percent anyway,
although it has a serious effect on the performace of G5RV's and similar
(hopefully no tuner) multiband antennas.

The small currents induced in the adjacent 'foreign' metal run

TRANSVERSELY
on the metal surface over a distance equal to the line conductor spacing

and
are harmless. The other old-wives story that the line should be twisted

may
in fact spoil the effect and do more harm than good. It also spoils the
cosmetics.

The usual example is when bringing an open-wire or ladder-line into the
shack. No harm occurs when bringing the line wires together for a few
inches and drawing them through a single oversize hole drilled in a steel

or
aluminium window frame. A liitle more increase in insulation thicknes is
advisable for high power transmitters.

It is pointless to convert to a pair of short coaxial lines drawn through
TWO spaced holes. That causes a much larger lump of capacitance across the
line although even then, at HF, it will be harmless - equivalent to an
unnoticeable readjustment of a tuner capacitor setting.

The problem needs more thought when bringing an open-wire line through a
timber, brickwork or concrete wall. Those materials ARE lossy (dielectric
loss) and should be avoided. They may heat up when on continuous high

power
with standing waves located at a voltage (not current) maximum.
----
A few more icons demolished, eh Roy? ;o)
Reg, G4FGQ



  #7   Report Post  
Old November 3rd 03, 06:21 PM
KS7J
 
Posts: n/a
Default

Reg,

Thanks for the demolition work! HI. I guess I was less concerned with
losses than detuning of the antenna system by an inadvertent
reconfiguration of the feedline should it come close to metal or
itself. Not so much per usage/session at the rig, but in the case of
doing a large turn of the antenna or in a high breeze situation that
would necessitate re-tuning between QSOs or mid-QSO. Or, at the very
least not distract me with large variances of SWR whilst I'm
transmitting. Still haven't looked into the slip ring method, but it
seems to me a good stiff ladder line or twinlead shouldn't pose too
great a problem in a rotary situation.

73,

Casey, KS7J
  #8   Report Post  
Old November 3rd 03, 08:24 PM
Tarmo Tammaru
 
Posts: n/a
Default

Casey,

What kind of directional antenna do you have, that you need to feed it with
450 Ohm line?

Tam/WB2TT


  #9   Report Post  
Old November 4th 03, 06:03 AM
KS7J
 
Posts: n/a
Default

"Tarmo Tammaru" wrote in message ...
Casey,

What kind of directional antenna do you have, that you need to feed it with
450 Ohm line?

Tam/WB2TT


Well, I don't *need* to feed it with ladder line, I *want* to feed it
with it so that I can use it multi-band. It is armchair fantasy at the
moment, no antenna yet exists, but it would be essentially a 40/30 2
el. yagi (it's a bit more complicated than that, but I'm still
designing). My very preliminary modeling indicates that at resonance
you get the typical yagi directional pattern. Driving it on other
bands, even with high SWR, will yield anything from normal dipole to
meaningful gain in one direction depending on band. So, at the least I
could have a rotatable dipole on the non-resonant bands and a bit of
F/B on one or two with some luck.

73,

Casey
  #10   Report Post  
Old November 4th 03, 03:31 PM
Tarmo Tammaru
 
Posts: n/a
Default

You might want to check on a Smith chart to see how bad things get if you
patched in about 3 - 4 feet of coax. I know this has been done before. You
could also make up a short piece of 100 Ohm balanced line from 2 pieces of
RG8, and put 4:1 baluns on each end of that.

Tam/WB2TT
"KS7J" wrote in message
om...
"Tarmo Tammaru" wrote in message

...
Casey,

What kind of directional antenna do you have, that you need to feed it

with
450 Ohm line?

Tam/WB2TT


Well, I don't *need* to feed it with ladder line, I *want* to feed it
with it so that I can use it multi-band. It is armchair fantasy at the
moment, no antenna yet exists, but it would be essentially a 40/30 2
el. yagi (it's a bit more complicated than that, but I'm still
designing). My very preliminary modeling indicates that at resonance
you get the typical yagi directional pattern. Driving it on other
bands, even with high SWR, will yield anything from normal dipole to
meaningful gain in one direction depending on band. So, at the least I
could have a rotatable dipole on the non-resonant bands and a bit of
F/B on one or two with some luck.

73,

Casey





Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Inverted ground plane antenna: compared with normal GP and low dipole. Serge Stroobandt, ON4BAA Antenna 8 February 24th 11 10:22 PM
QST Article: An Easy to Build, Dual-Band Collinear Antenna Serge Stroobandt, ON4BAA Antenna 12 October 16th 03 07:44 PM
70 ohm dipole to 50 ohm feed line question Tom Sedlack Antenna 10 October 6th 03 01:24 AM
50 Ohms "Real Resistive" impedance a Misnomer? Dr. Slick Antenna 255 July 29th 03 11:24 PM


All times are GMT +1. The time now is 01:19 PM.

Powered by vBulletin® Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright 2004-2019 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017