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Author Topic: Open feeders impedance  (Read 3205 times)
9H1FQ
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Posts: 144




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« on: January 07, 2012, 03:47:41 AM »

How can I determine the impedance of a home made open feeder/ladder line ?
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In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
G8HQP
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Posts: 123




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« Reply #1 on: January 07, 2012, 04:12:52 AM »

Measure the diameter and spacing of the wires. Plug values into the formula for twin wire transmission line. If necessary, correct for insulation permittivity.

Alternatively, measure using an RF impedance meter.
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9H1FQ
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Posts: 144




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« Reply #2 on: January 07, 2012, 07:30:55 AM »

What I mean is not theoretical but a practical set up , using a signal generator etc ( I do not have a z meter ) just like when we check coax with a gdo !
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
W5DXP
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Posts: 3578


WWW

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« Reply #3 on: January 07, 2012, 07:46:36 AM »

Could you tell us exactly why you need to know the Z0 of the line?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WX7G
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Posts: 6032




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« Reply #4 on: January 07, 2012, 08:05:11 AM »

Use this formula:

Z = 276LOG(2D/d), where D is the center-to-center spacing of the wires and d is the wire diameter. Note that units (mm, cm, inches) do not matter as long as D and d use the same unit.

LOG is the logarithm to base 10.
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WB6BYU
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Posts: 13231




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« Reply #5 on: January 07, 2012, 09:17:12 AM »

When checking coax with a GDO you are more often finding the velocity factor
of the line, not the impedance.  You can do that the same way with open wire line
if you wish, but usually the velocity factor is close enough to 1.0 that an estimate
of 0.97 or so is good enough for most purposes.

If you want to measure the impedance of the line, probably the simplest method is
to attach a quarter wavelength to  the output of an SWR meter and see what
value of resistor placed across the far end gives the lowest SWR to 50 ohms.
Then the square root of 50 * the resistor value gives you the line impedance.
(For example 4.7K would be about 485 ohms.)  This may require an SWR meter that
works at low power levels if you only have 1/4 watt resistors to try:  the types
designed for CB use usually frequency-sensitive and I'd driven them to full scale with
a signal generator on 2m.  Or you can build a 50 ohm Wheatstone bridge (or other
impedances for that matter) and drive it with your signal generator.

Another method would be to use the formula that the reactance of a shorted
length of lossless feedline less than 1/4 wave long is Zo times the tangent of the l
ength (in wavelengths.) So, for example, such a length of shorted line could be
connected in parallel with a dummy load and resonated with a capacitor:  at
resonance this will give a 1 : 1 SWR (degrading on either side) and you can
then calculate the inductance (and hence the line impedance) knowing the value
of the capacitor connected across it.
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KE3WD
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Posts: 5689




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« Reply #6 on: January 07, 2012, 12:23:55 PM »

If you want to measure the impedance of the line, probably the simplest method is
to attach a quarter wavelength to  the output of an SWR meter and see what
value of resistor placed across the far end gives the lowest SWR to 50 ohms.
Then the square root of 50 * the resistor value gives you the line impedance.
(For example 4.7K would be about 485 ohms.)  This may require an SWR meter that
works at low power levels if you only have 1/4 watt resistors to try:  the types
designed for CB use usually frequency-sensitive and I'd driven them to full scale with
a signal generator on 2m.  Or you can build a 50 ohm Wheatstone bridge (or other
impedances for that matter) and drive it with your signal generator.

Hold yer cards, we have a winner!  

I like to use a Pot instead of a fixed resistor, with low power and quickly dial in the SWR exactly, then measure the Pot resistance with the ohmmeter. 


73
« Last Edit: January 07, 2012, 12:26:50 PM by KE3WD » Logged
WB6BYU
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Posts: 13231




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« Reply #7 on: January 07, 2012, 01:58:36 PM »

As long as it isn't a wire-wound pot, of course.
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KE3WD
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Posts: 5689




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« Reply #8 on: January 07, 2012, 05:34:10 PM »

Good point. 

Carbon, folks.

Wirewound would be inductive...



73
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G3RZP
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Posts: 4543




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« Reply #9 on: January 08, 2012, 03:41:26 AM »

Since open wire line is used at HF, the impedance isn't critical, so calculation is good enough. If it's for a matching stub, you'll need, in practice, to trim it anyway. In any case, it will depend on the environment - if it's close to metal guttering or the like, there'll be an impedance bump there.
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WX7G
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Posts: 6032




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« Reply #10 on: January 08, 2012, 06:57:01 AM »

If you want to measure the impedance with an antenna analyzer such as the MFJ-259 take a convenient length, perhaps 8 feet. Connect the analyzer to one end, suspend the open line in the air, and leave the far end open.

Dial the analyzer upwards in frequency until you find the lowest frequency that shows a short. An 8' length will show this at about 30 MHz and the line length is 1/4 wavelength. Now dial to exactly half that frequency and read the reactance. The line is 1/8 wavelength. The reactance at that frequency is equal to the characteristic impedance of the line.
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AA5WG
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Posts: 496




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« Reply #11 on: January 08, 2012, 09:10:06 PM »

OM:
Here is a nice on line calculator for open wire feeders.  Make sure you use radius and not diameter measurements.

http://www.smrcc.org.uk/tools/OpenWire.htm

73,
Chukck
« Last Edit: January 11, 2012, 08:11:15 AM by AA5WG » Logged
AA5WG
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Posts: 496




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« Reply #12 on: January 11, 2012, 08:11:45 AM »

Above response has been updated.
Chuck
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G3TXQ
Member

Posts: 1512




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« Reply #13 on: January 11, 2012, 12:38:49 PM »

Here is a nice on line calculator for open wire feeders.  Make sure you use radius and not diameter measurements.

http://www.smrcc.org.uk/tools/OpenWire.htm
Don't try to use it for close-spaced line - equivalent to Zo values below about 150 Ohms: it uses unstated approximations which cause gross errors at close spacings.

Steve G3TXQ
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