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Ladder Line Current and Phase Measurment




 
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Author Topic: Ladder Line Current and Phase Measurment  (Read 12730 times)
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Steve - K4HX
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« Reply #25 on: May 11, 2013, 04:29:47 PM »

I tried to model the conditions in my previous post. I didn't have the exact set up. The OWL only included the vertical portion (from 10 feet above the ground up to the feed point at 50 feet). Two sources were placed at the end of the OWL wires (one in each wire). I was able to model different amounts of imbalance and compare the radiation patterns to a "perfect" dipole with zero feed imbalance or any other feedline radiation. I looked at the changes in the vertical radiation since that's where the most variation occurred (makes sense since the OWL section is 40 feet vertical and imbalance would cause it to radiate).

Small imbalances in amplitude (while maintaining perfect phase balance) didn't cause much increase in the vertical radiation (10-20% difference between the two OWL wires). With a 2:1 ratio of current between the two wires, the vertical field was increased about 2 dB. The same amount of increase in vertical radiation was produced with a 20 degree phase imbalance (but perfect amplitude imbalance). With a 2:1 amplitude imbalance and a 20 degree phase imbalance, the vertical radiation increased well less than another dB - mostly just made the pattern more lopsided. I have no idea if such an imbalance would/could happen in the real world.

In any of the above cases, the horizontal and the total field patterns broadside to the plane of the dipole were almost unchanged. The nulls off the ends of the dipole began to fill in (especially at low elevation angles) as imbalance increased.

The above is pretty limited data but it would seem, even a fairly serious amount of imbalance may not make much difference in your signal as received at the other end of the QSO. The near field effects will probably be much different and RFI or increased noise pickup on receive would be symptoms.

Thoughts?
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KM1H
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« Reply #26 on: May 12, 2013, 08:57:58 AM »

Steve, I agree that your installation likely has very little if any radiation at least caused by the OWL installation.

Other feeds that are close to trees and structures or go at a 45 degree angle or less from the antenna are more prone to radiation; Ive seen a few using window line actually running over branches and/or running a few inches away from house siding and trim using the old TV type standoffs. That may actually be beneficial for transmitting as a pattern fill as mentioned or somewhat transform the antenna into a top loaded T.
Trying to model all that is far beyond my ability Wink
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Steve - K4HX
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« Reply #27 on: May 12, 2013, 11:47:08 AM »

Yes, I think that in the real world, the far field pattern would be even more distorted. And the far field pattern doesn't tell us much about what's happening in the near field and the coupling to power lines, telephone lines, etc. Modeling this would be much more useful. If I ever figure out how to do it, I'll give it s shot.

BTW, the antenna described below is one I made up to represent a "typical" installation for discussion purposes. Currently, all my antennas are coax fed.
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W3RSW
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Rick & "Roosevelt"


« Reply #28 on: May 12, 2013, 01:17:06 PM »

Quote
Attached is an example of a configuration that would disturb the balance of the currents in the OWL:

The right side of the antenna and the left side of the antenna return currents to the unbalanced side of the tuner which are:

Not necessarily equal in amplitude (because C1 and C2 do not have the same values)

Are phase shifted with respect to the OWL input currents (because the round trip path length from the tuner to the end of the OWL, along each end of the antenna, and back through ground is a significant fraction of a wavelength... or more)

But he mentioned in the loop case that the lower frequencies showed out of balance lamps, not higher freqs.
 
I think I'd look at the lamp 6 inch spacing pickoff, maybe change that to a couple of feet for 40. Maybe change the pickoff point that's originally about one foot out from the MB.  Might be that there's no "imbalance" at all on 40, but that since the antenna is only a 15 meter loop that one of the lamps is at a current null from a wave going all the way around, so to speak.

If some results are seen there then make a larger loop for 40.  The small inductor has sufficient phase change that the lamps now 'seem' to indicate balance when in fact it was fine in the first place.

What is the ERP variance at the far field with and without the small inductor? I'll bet none.   

Yeah, model it.
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RICK  *W3RSW*
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« Reply #29 on: May 13, 2013, 01:28:17 PM »

K5UJ and all,

I want to build a very good system to monitor the voltage or current  phase when using ladder line with the link antenna coupler.  A balanced antenna system in terms of voltage and current is desired.  

I could then experiment with the antenna by changing the capacitance to ground of one or both sides of the doublet.  Or, I could introduce very sappy tree limbs or wire or what ever to one side of the antenna to upset the balanced phase of voltage or current.

Having made the above changes, to upset the antenna, I would experiment with different link coupler settings to see it they actually can make a difference in balancing the antenna system back to normal/balance.

When using light bulbs, at times, the bulbs would show an imbalance with one light bright and the other not very bright.  Making adjustment to the link coupler did help but having a designed current/voltage phase monitoring circuit should work better in terms of response than the light bulb approach.

Chuck
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #30 on: May 14, 2013, 12:17:57 PM »

" A balanced antenna system in terms of voltage and current is desired"

It is the imbalance of the currents (i.e. currents that are not equal in amplitude and 180 degrees out of phase) in the wires of the OWL (at a given position along the length of the OWL) that affects the near field (RFI) and far field (antenna radiation pattern) of the combination of the antenna and the OWL feed line.

For most intents and purposes, the concept of "balanced OWL voltage" is meaningless.

Voltage is always defined between two points.

Even if the OWL and the associated antenna system is perfectly balanced, the will be a voltage from one wire of the OWL to the other. The amplitude of this voltage will vary with the position along the OWL at which it is measured. Unless the SWR is very high (essentially infinite) the amplitude of this voltage will not be zero anywhere along the OWL.

Measuring the voltage from one wire of the OWL to a third reference point ("ground"), and then from the other wire of the OWL to that same reference point, will result in meaningless measurement outcome because of the voltage induced by the time varying magnetic fields (from both OWL wires) into the (wires of the) loop formed by the measurement circuit.

Measuring the voltage from one point along one of the OWL wires to another nearby point (a tiny fraction of a wavelength away) along the same OWL wire is actually measuring the voltage induced around the loop (circuit) that corresponds to the measurement apparatus. This voltage is induced by the time varying magnetic field passing through that loop ... and the time varying magnetic field passing through that loop is produced by the currents in each of the two OWL wires at that location along the OWL. Since the loop is closer to one OWL wire than the other, more of the time varying magnetic field of the closer OWL wire will be coupled into the loop. Therefore, one will be making a very crude measurement of the amplitude of the current in the closer OWL wire by observing a brightness of a small light bulb that is in series with the measurement loop.

Therefore, one should focus on measuring the balance of the currents in the two wires of the OWL at a convenient point along the length of the OWL.

Stu
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
Steve - K4HX
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« Reply #31 on: May 14, 2013, 10:48:00 PM »

And measuring the common mode current on both conductors.
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