The AM Forum

Hi to all,

What is an excellent circuit and mechanically sound method to measure current and/or voltage and phase of current and/or voltage on open wire feeders (ladder line)?

Chuck

A pair of RF thermoammeters, one in each side.

A vacuum tube dual rectifier following a series resistance and a DC voltmeter across both sides.

Calibrate to taste.

73DG

Chuck,

Where are you coming up with these questions??  Your questions are getting more difficult with each new one.

My brain is starting to smoke.

I'll think about this one and get back.

Generally, on an antenna element the voltage and current maximums are 90 degs out of phase.  Current flows on an element, at the end of the element there is no place for the current to flow.  So, at the end the current is minimum, voltage is at a maximum.  That would be the high impedance point.

Generally 1/2 wave dipoles are fed at the center where the current is at a maximum and the voltage is at a minimum, low impedance point.  

So knowing this you can figure where current and voltage minimums and maximums occur on the feeder line.

The conditions that occur at the feed point will repeat every 1/2 wavelength down the feeders.

Not sure if any of this answers your question but it may confuse you as much as it has confuse me. :(

I'm sure you'll get some more help on this one.

Fred

1) Take the time to let everything load

2) Take the time to read the whole thing.  It ain't just amplitude, it's also phase

http://www.wz5q.net/index/shack_data/tuna.htm

a

You may not want to do all those measurements as your beliefs about the perfection of OWL may become blown away ::)
AND after all is balanced performance might suffer as the somewhat radiating feed is no longer helping the signal :o

Carl

Fred, W1VTP and all,

VK1OD states WZ5Q is flawed in measuring current.  

Fred, I understand the relationship between current and voltage that you shared with us.
I believe your statements are correct.

It would be nice, for experimenting, if I could measure the phase relationship of current and/or voltage with a flip of a switch with an open wire feed system.  

Chuck

There are a few things which are flawed in that link.

The scope calibration is referenced to the secondary link center tap which shouldn't even be there.

If one finds either a phase or current inconsistent with balanced readings how is one supposed to cure that condition? I'd sure like to see more measurements with the feedline terminals reversed before blaming the antenna itself.

One circuit configuration used on all bands is a dead giveaway there is more discovery ahead.

No wonder a few otherwise intelligent hams write off the utility of balanced feed and use cow axe.

The link, or rather the page, is flawed.  I have about 70 Mb download speed on my lunch hour but that page took way too long to load, then my web client crashed.   ???   the author or owner whoever, needs to do some page redesign--shrink the photo sizes, or divide the thing into sections each on a sub-page.   Single humongous pages and hams citing other hams are both sometimes not good.  ;) 

I'll only say that that web page used to load much better then it did this time.  Dunno what the webmaster has done (nor why he has done so) but I stand by the concept. I do not approve of making a web page that is so loaded with scripts and tags to advertisements that it is impractical to view. So, while I think the web site has some intersting and informative information, I cannot recommend viewing it at this time.  I may send an email to the author about this appropriate response from the AM fone community.  I think some of his construction hints are worthwhile and the need for monitoring the phase as well as the amplitude of each leg important.

Perhaps I'll share more about making the phase of each line is 180 degrees from each other but the concensus on this thread doesn't seem to be interested.  More some other time - perhaps. 

I must say that general comments about radiating feed lines without offering a solution does little to encourage the newbie to try open line feeders.

Al

I am not sure how deeply you want to get into this, but here is an excellent site dealing with the subject.  The resultant phase accuracy of this scheme is very good.  Peruse his site and he has an excellent tutorial on the design of current transformers as well. 

A good friend of mine passed this site along to me and its one of the best I seen. 

http://www.g3ynh.info/zdocs/bridges/appendix/a6-5.html

Joe, GMS

Al, I am interested in your thoughts on the 180 degree phase.

Please share your thoughts on this subject.  I am very interested.

Joe, thank you for the link.  I am going to read G3YNH information.

Chuck

Joe

I read the article a couple of times.  Aside from reminding me that I have some math deficiencies, I believe that  article has to do with building a phase accurate bridge for use in an unbalanced system where measurements are made that include phase data whereby one could graph a Smith chart.

I don’t think this article or the hardware would be helpful for checking the amplitude (beit voltage or current) and phase balance on an Open Wire Line (OWL) system that is the subject of this thread.

I believe that the subject in question is how we can minimize stray RF resulting from an unbalance in an OWL system where it results in a radiating feed line.

Al

We could sure use someone who does AM broadcast directional setups right about now.

All:

Using the same cores that we use to make transformers for class E rigs (the core size/shape/material is not critical):

You can easily make a sensor that can measure the current in each of the wires of the OWL at a convenient location. Refer to attachment 1. Note that, as shown,  the 50 ohm non-inductive resistor, across the secondary of each of these two current transformers, must be present whether or not you have a scope connected, and regardless of which position the DPDT switch is in. With the scope connected to one current transformer or the other (i.e. by throwing the DPDT switch), the voltage across the scope will be (1/16) x 25 ohms x the current flowing in the associated OWL wire. If you have a dual trace scope, you can eliminate the DPDT switch, and look a the current in both wires simultaneously. This will allow you to see both the individual amplitudes (which should ideally be equal), and the phase shift between the two currents (which should ideally be 180 degrees)

If you use the sensor shown in attachment 2, the voltage across the scope will be (1/16) x 25 ohms x (the sum of the two currents). If the two currents are equal in amplitude and 180 degrees out of phase (i.e. perfect balance), then the voltage at the output of the sensor will be zero.

As far as correcting imbalance goes....

Imbalance of the currents in the two wires of the OWL (i.e. not equal in amplitude and/or not 180 degrees out of phase) occurs because there is a capacitive coupling to ground from each side of the antenna. This produces a current, through the ground, that flows back to the transmitter (on the unbalanced side of the output coupler/tuner that you are using). Even if the antenna is symmetrical, and the capacitance between each side of the antennas and ground is the same, the path length (and thus the phase shift) from each side of the antenna, back through the ground, and into the unbalanced side of the output coupler/tuner, is not generally the same. In general, the currents flowing back from the two sides of the antenna, through the ground, and into the unbalanced side of the output coupler/tuner will not be equal in amplitude and will also not be 180 degrees out of phase. This will, in turn, result in an imbalance of the currents in the two wires of the OWL. [There will always be paths between the outside ground and the unbalanced side of the output coupler/tuner]

To correct that imbalance, one would have to provide a path between the unbalanced side of the output coupler/tuner, and one side of the balanced line (i.e. on the balanced side of the output coupler/tuner)... with an adjustable phase shift, and an adjustable degree of coupling. These would be adjusted to cancel out the effect of the inevitable unbalanced ground paths.

Stu

Thanks Stu!

Is there any advantage in electrically correcting imbalances in the load? I would think any imbalance would equal common mode radiation of the feeder. Would one sample point be accurate enough to implement correction?

 

Stu

I like your solution best so far - particularly in slide 1.  Many of us have dual trace scopes so the dual output (instead of the switch) would be ideal.  This is the procedure as I understand it: First, it is assumed that the scope channels are carefully calibrated in amplitude. Then with the two outputs connected to the dual scope inputs and the trigger is selected to, say, channel 1 then one could view the two traces simultaneously with correct phase information.  If the OWL is OK  the two traces would have the same amplitude and be 180 degrees phase delta in relation with each other.  Now, if one were to go to the "ADD" mode a perfectly balanced OWL would have zero display.  Any deviation would result in a display voltage on the scope.  

If the switch with one trace approach were to be used one should be careful to use a form of external triggering so that the phase delta between the two lines would be displayed accurately.  I'm not sure if a standard station monitor scope such at the HO-10 would be up to this task.

I'm still trying to figure out your solution for an imbalance - chalk that up to the several layers of bone in my head
 ;D

Al

Dave

et. al.

If the two antenna currents are not equal in amplitude and 180 degrees out of phase... then the OWL feedline will radiate. The amount of feedline radiation (i.e., the magnitude  of the E field and the magnitude of the H-field, not the radiated power level) will be proportional to sum of the two OWL wire currents.

Even if the OWL has perfectly balanced currents in its two wires  (equal in amplitude, but 180 degrees out of phase) there will still be a small amount of OWL feedline radiation at any point in your shack... whose level depends (approximately) upon the difference of: (1/the path length) from that point in the shack to each of the two individual OWL wires. Since the spacing between the two OWL wires is small, it follows that (1/path length) from any point in the shack to one of the wires is pretty much the same as (1/path length) from that point to the other wire. From algebra and trigonometry, one can shown that this residual radiation in the shack has a field strength (not power level) that rolls off roughly in proportion to: [the spacing between the 2 OWL wires/the shortest distance between that point in the shack and the OWL feedline] squared.

If the RF in the shack is not unacceptably different when running the transmitter directly into a dummy load v. running the transmitter into the OWL-fed antenna system... then there is no problem (although it still might be interesting the measure the balance, or lack thereof, between the currents in the two OWL wires).

Assuming the OWL doesn't run close to a conducting object for a long distance, balancing the currents in the two wires at the input end should result in a balanced current along the entire OWL.

Stu
 

Stu, Al, Dave and all,

Would the following tuning method work to eliminate current or voltage phase imbalance, with the open wire feed antenna system, utilizing the probing system put forth by Stu?

SETUP FOR LINK COUPLER, i.e. current feed:

Series tune configuration with one variable output capacitor symmetrically tapped to the left side of tank coil and another output variable capacitor tapped to the right side of tank coil.

1.  If needed adjust the two output capacitors independently to achieve phase balance of current or voltage.

2.  If needed adjust the two output capacitor taps on the tank coil independently to help achieve phase balance of current or voltage.

Chuck

I got a laugh when I read that.  For that to happen on 75 m. the antenna would have to be a mile away, or the ham would have to be running no more than a couple hundred milliwatts.  I suppose for testing purposes, hams with plastic radios like me, could drop the power way down and gradually increase it.


by that do you mean the unbalanced input to the link, on for example a Johnson Matchbox, or do you mean the one of the sides of the balanced output?  I assume the former because in the case of the latter, each output side is unbalanced in relation to the opposite side.

rob

Chuck, why don't you follow Dennis's advice and check first to see if you have a problem.  If you do what you have above every time you change frequency you'll be even less frequency agile than me, a hard thing to achieve  :)

  I take a lower tech approach to this topic. I have OWL from my Johnson 275 MB to my 15m loop (16'W X 8'H). I use this antenna on 20m and 40m as well. In the shack just a foot or two away from the matchox I have two tiny incandescent lamps used as feedline current indicators where each is connected to one side of the OWL over a 6" span. At 150 watts RF the lamps are at about 1/2 normal brightness.

  So on 15m the lamps show equal brightness, and on 20m only a slight imbalance is shown. But on 40m, there is a huge imbalance. So I added a tiny amount of inductance (at MB output post to OWL) to the side with the lower brilliance till I see balanced lamps. This setting is critical, and 7160 and 7290 have unique coil tap settings.

  I've pondered if current balance means 180 degrees phase difference. Well, two things are observed here. When my feeder is not balanced, my transmitter spot signal is real strong in the receiver as compared to the coax fed dipole. When the OWL feeder current is balanced, the spot signal is about the same strength in the receiver as the coax fed dipole. Also on 40m with the imbalance, my shack PC speakers pick up, and make noise when I modulate. Balance out the currents, and the PC speakers are quiet. So it seems that balanced currents mean the phase between the lines is 180 out, and therefore feedline radiation, or feedline pickup is at a minimum.

Jim
WD5JKO

[Emphsis in quote mine]  You are monitoring 1/2 of the equasion (if I can put it that way).  As Stu and others have already said, there are different ways a symetrical antenna can be out of balance regarding the phase relationship between the two lines.  Having said that, I'm betting your technique is better than NO monitoring.  And after all the bottom line is to reduce the RF radiating from the OWL to the point that you are having no problems in the shack.

But if one wants OWL radiation to be at a minimum both amplitude (beit voltage or current) and phase relationship  need to be monitored and adjusted for equal voltage (or current) and a 180 degree delta between each line

Al

More discussion = more questions : )

What condition would cause balanced current but phase relationship other than the expected 180 degree relative?

I would think relative phase follows distance in wavelengths through the feed system.

In other words what benefit is there to monitoring both relative phase and amplitude other than redundancy?

Dave

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)

I need to think a bit more about a simple example in which the currents in the OWL would be equal, but not 180 degrees out of phase.

In an extreme case, the currents in the two OWL wires would be equal in amplitude and exactly in phase ... and the ground return current coming into the unbalanced side of the tuner would be have an amplitude that is 2x the amplitude of the currents in each of the OWL wires, but which is 180 degrees out of phase with the currents in the OWL wires. This would occur if the two antenna wires were parallel, close together, and facing the same direction (i.e. the OWL just makes a right angle turn). The OWL would be acting as a vertical antenna, with both conductors carrying current in the same phase.

Stu

Some numbers on what this means relative to antenna height would be useful. As an example, what amount of imbalance would be created with the following conditions.

Freq: 3.8 MHz, dipole, center at 50 feet with one end it at 45 feet, the other at 40 feet above the ground. The dipole is connected to the tuner through 0.5 wavelength of OWL with 4 inch spacing, #12 wire. The OWL drops straight down from the feed point to a height of 10 feet above the ground. From there it takes a right turn (away from the plane of the dipole) and is straight on into the tuner, an 89.5 foot run. There are no other coupling paths or conductors in the mix.

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?

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 ;)

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.

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.

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

" 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

And measuring the common mode current on both conductors.