Way back in about 2006 member Yves Roy calculated a very interesting chart showing a table of values for designing a loading coil for use with a 3-meter AM antenna under Part 15.
http://www7.brinkster.com/yvesroy/10ft-antenna.asp
Way back in about 2006 member Yves Roy calculated a very interesting chart showing a table of values for designing a loading coil for use with a 3-meter AM antenna under Part 15.
http://www7.brinkster.com/yvesroy/10ft-antenna.asp
Thanks Carl for letting some
Thanks Carl for letting some users of Part 15 AM know. 🙂
Have a great day.
Unusual Coils
Yes of course Mr. Yves Roy, I appreciate your contribution and will use the chart as a point of reference in the specialized coil experimentation happening here.
My Indoor Antenna Project includes the construction of coils for indoor use.
The first one is a success in that it loads an AMT3000 to its antenna, and is a triangular shaped coil placed in a horizontal direction at the base of a vertical antenna.
As the project becomes documented there will be links, photos, and technical details.
I am going to design each coil in such a way that it can be plugged and unplugged, so that each coil can be compared to the other.
Well, i feel some kind of
Well, i feel some kind of excitement when i just read your message. I would like to see some photos and technical details on your side once they will become available. I haven’t used my am transmitter (AM25) since 2007 but who knows, i will use it again later in my life.
Sorry for my english, because i speak usually in french here in my province of Québec (Canada). 🙂
Regards
Yves
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Starter Picture
That’s unexpectedly good news Mr. Roy that you might get back to transmitting.
I do have a starting photograph which I took just after building the coil-form for my triangular AM coil. Keep in mind this is for indoor use only, as the wood materials would be useless outdoors in the weather.
http://kdxradio.com/lph_files/TriCoil_Form.JPG
RE: Loading Coils for Part 15 AM
FWIW,I looked over Msr Roy’s chart back a few years and decided a few pertinent attributes could be made to improve real-world performance as I learned more about loading coils from other group members.
Here are a few:
1) Almost anything that can be done, even if minimal, within reasonable construction ability, to reduce resistance and/or increase ERP is worth the effort.
2) Frequencies higher in the band have shorter wavelengths and so require fewer turns and equivalent wire lengths=less wire resistance.
3) Similarly, larger wire gauges also offer less resistance. IMO, 22awg wire is at the minimum wire mass/length. I use 16awg.
4) Close-wound lacquered wire wound on standard white PVC pipe can introduce eddies that interfere with signal to the pole section of the antenna. I use white PVC Drain Pipe which has thinner walls for better simulation of an air-coil form, and insulated wire, which separates the wires in the windings. True, that also makes it thicker, but also adds strength when the weather coating is applied, while providing less chance of eddies developing in the coil.
BTW, black ABS pipe has metallic particles in the pigment, making it a less efficient air coil form.
5) Skin effect can be a factor. For a monopole, a larger diameter metal pipe in the radiator section can increase skin effect and signal strength. Of course, efficiency gain is going to diminish at some point, particularly with cost. I have used 3/4″ i.d. copper.
In order to lower costs, another approach I want to try out is a cage monopole, which essentially can emulate dramatically more skin effect without the expense of solid radiator materials. I’m also interested in seeing what it will do to aperture effect.
The loading coil would be at bottom.
Skin Effect
The term “skin effect” applies to the reality that r-f current is confined by physical principles to flow on the outermost layers of a conductor, whether or not the conductor has a solid cross section.
The depth from the outer surface of a circular copper conductor carrying most of such r-f current at a frequency 1700 kHz is about 50 micrometers, or about 0.002 inches.
This, and economics are the reasons that many/most coaxial conductors rated for high-power broadcast applications at MW frequencies and above use a hollow-core center conductor.
Continuing on Coils
I appreciate this recent input on this subject, and have been pouring over not only previous postings on this website, but other reference sources, including Terman’s 1943 Radio Engineering Handbook which goes into elaborate detail on these subjects.
Terman confirms what has been said, for example by saying (p.73) “Losses in air-cored coils at RF — Factors Contributing to Coil Losses (include) 1) skin effect; 2) proximity effect; 3) losses in dielectrics in coil vicinity; 4)eddy-current losses in neighboring metal objects.”
Terman mentions that Litz wire gives higher Q in the lower RF region, including the medium waves, and gives the size and number of strands in litz for best results ranges from 3-strands of No. 40 or 5-strands of No. 41 wire for very compact coils.
A very interesting thing was said in the book around page 730, —
“Theory indicates that direction of rotation of polarization should be left-handed in the Southern Hemisphere and right-handed in the Northern Hemisphere as a result of the earth’s magnetic field in the two hemispheres.”
Granted, when he made that statement the subject was “electromagnetic propagation”, and no longer coils per se, but I do wonder if the turn direction at a coil’s input falls under this same umbrella. I will run a test later tonight using a spectrum analyzer.
Here’s a link to Terman’s book, although some pages are missing (?)
http://www.scribd.com/doc/23847007/Radio-Engineers-Handbook-1943-Terman
Hemispheric Polarity Matters
This is right now an opinion sprinkled with a small amount of experimental evidence.
Does it matter whether the transmitter drives a coil with clockwise windings or counter-clockwise windings?
I now believe it does matter.
By feeding the RF output of my AMT3000 into the clockwise end of the loading coil I read 66dBm on the spectrum analyzer.
By flipping the coil around so the RF is fed into the counter-clockwise end the reading drops to 65dBm.
With part 15 power levels every drop is important!
Coils, etc.
Carl,
That is a very nice summary of antenna systems and it is good to see it compiled in one place. In item 5 you mentioned increasing the diameter of the radiator increases the skin effect. Should that read “decreases the skin effect” since the larger diameter gives a larger cross section area for the current which reduces the resistance to RF? The effect of the skin effect is reduced due to lower R.
The question of the earth’s magnetic field is interesting but another question regards flipping the coil around. If the coil is wound clockwise and fed from the bottom and the coil is turned end for end and bottom fed the winding is still wound clockwise is it not?
For the coil I would think that the superposition principle would apply for the earth’s field and the RF magnetic field in the coil and it wouldn’t matter unless there is some non-linearity to the magnetic fields. They should just add. Thoughts?
Neil
Brain Illusion
Radio8Z posted —
“If the coil is wound clockwise and fed from the bottom and the coil is turned end for end and bottom fed the winding is still wound clockwise is it not?”
So it would appear, but this is a case of a brain illusion.
Consider the hands of a clock mounted on a sheet of glass. From the normal observers view of the front of the clock the hands turn clockwise.
But if you walk around to the back of the clock and observe the hands through the glass, the hands will be turning in a counter-clockwise direction.
Therefore the same would necessarily be true of a coil, despite the convincing brain illusion that makes it appear otherwise.
Coil Topology
Maybe we are talking about two different things but I retrieved a spring from my junk pile and viewed from the bottom it is wound clockwise. If I flip it end over end it is still wound clockwise viewed from the bottom. As long as the view is top or bottom and not changed the direction of the winding doesn’t change.
By the way, I wonder what they did before the invention of the clock when they needed to describe rotation direction.
Neil
One Different Thing
No, Neil, we are not talking about two different things. There is only one different thing, and that is my own belief that the far end of a coil would be a mirror image of the near end.
As you know, items viewed in a mirror show a complete swapping of left and right, although oddly, mirrors do not flip the top and bottom, another brain illusion.
Since my experiment didn’t prove what I thought it did, I will here modify my finding to reflect what really took place in retrospect:
A loading coil wound clockwise produced a spectrum analyzer reading of 65dBm, and when the same loading coil was flipped to feed the other clockwise-wound end, it produced a reading of 66dBm.
Conclusion? Flipping a coil increases output by 1dBm.
To conduct the experiment I thought I was conducting, I must wind an identical coil counter-clockwise for comparison.
On the Same Page
OK, I think we are in agreement especially since you propose winding another coil.
Your 65dBm and 66dBm readings raise a question. 65 dBm represents a power of 3162 watts! Are you sure you are measuring dBm?
The uncertainty of digital meters include +/_1 least significant digit which would account for the difference between 65 and 66 being due to digital uncertainty and perhaps not a difference in the signal strength.
Neil
Current Flow Along a Coil
Just to note that the direction of the r-f current flowing along a helically-wound coil depends on the physical construction of that coil.
As Neil pointed out, when a coil is swapped end for end and with other conditions equal, the r-f current flowing on it still travels with same rotation sense as before the swap, when considered from the same location in physical space.
But in any case this is irrelevant, as the only practical result of that coil is the inductive reactance it produces — which is independent of the circular direction (“CW/CCW”) used for the conductor in that coil.
Measuring Details
The dBm measurements are taken from the screen of a spectrum analyzer, which receives its signal from an antenna 13″ in length and located 10-feet from the test antenna. The screen is a grid-pattern of vertical and horizontal graticules, with the manual stating that the horizontal graticule lines, with the bottom line at zero, each mark 10dBm with the top line being the top edge of the screen representing 80dBm.
The transmitter is an AMT3000, which has never reached above 3kW of output power.
I believe that a paranormal change in the law of physics has taken place, as prior to yesterday the far end of a coil was wound in the reverse direction compared to the near end.
This can only mean that the earth’s magnetic polarity is shifting exactly as Alex Jones has warned.
Spectrum Analyzer Comments
Carl Blare on February 10, 2013 – 20:01: The dBm measurements are taken from the screen of a spectrum analyzer, which receives its signal from an antenna 13″ in length and located 10-feet from the test antenna. (etc)
Carl – A few thoughts about the use/capabilities of your AT5005 spectrum analyzer.
The specifications for it (below) show that the maximum safe input power applied with zero input attenuation is +10 dBm, which is 10 mW.
From looking at a picture of an AT5005 showing the display of a spectrum, there appears to be no electronic calibration of amplitudes shown in the display, itself.
With zero input attenuation, a +10 dBm signal at the input will produce a display with a peak amplitude that is 3 dB below the top horizontal line of the display. That would be 1-1/2 minor divisions on the vertical axis.
A signal peak that just reached the horizontal line at the vertical center of the display would indicate an input power 40 dB below the +13 dBm full scale value, or -27 dBm, which is 0.002 mW. Etc.
But all of this applies to conducted (closed circuit) signals in a 50 ohm environment. The calibration of the display in dBm will not be accurate when the r-f input source is an antenna, unless its “antenna factor” is known at the measured frequency, and its feedpoint impedance is matched to the 50 ohm value needed by the spectrum analyzer.
The analyzer may show a display when connected to an uncalibrated antenna, and the displayed values may be useful for relative measurements. But it isn’t possible to make accurate measurements of those received signals, in absolute terms (dBm).
Hopefully this information about the AT5005 is accurate, even though I don’t have one myself.
_____________
Company: Shenzhen Atten Electronics Co., Ltd.
Product Name: Spectrum Analyzer
Model No: AT5005
SPECIFICATIONS:
Frequency range: 0.15 to 500MHz
Center frequency display accuracy: +/-100kHz
Marker accuracy: 0.1% span +100kHz
Resolution of frequency display: 100k(4.5digit LED)
Frequency scanwidth accuracy: +/-10%
Frequency stablility: Better than 150kHz/hour
If Bandwidth (-3dB): 400kHz and 20kHz
Video-Filter (ON): 4kHz
Sweep rate: 43Hz
Amplitude range: -100dBm to +13dBm
Screen display range: 80dB(10dB/div. )
Reference level: -27dBm to +13dBm (in 10dB steps)
Reference level accuracy: +/-2dB
Average noise level: -90dBm(20kHz Bandwidth)
Distortion: <-55dBc of 2nd and 3rd harmonic Third order intermod: -70dBc (two signals>3MHz apart)
Sensitivity: Better than -90dBm
Log scale fidelity: +/-2dB (without attn. ) 250MHz
Input attenuator accuracy: +/-1dB/10dB
Max. Input level: +10dBm, +/-25V DC (0dB attn. ) +20dBm (40dB attn. )
Frequency scanwidth: 100kHz/div. To 50MHz/div, in 1-2-5 steps and oHz/div. (Zero Scan)
Spectrum Analysis
Rich offered:
“The calibration of the display in dBm will not be accurate when the r-f input source is an antenna, unless its “antenna factor” is known at the measured frequency, and its feedpoint impedance is matched to the 50 ohm value needed by the spectrum analyzer.”
I agree with and have generally understood this to be the situation with this analyzer, even though the antenna and its short extension cable is that which came with the equipment.
Therefore readings are of a sketchy and imprecise nature, not accurate by laboratory standards.
The value to the part 15 engineer of the AT5005 Spectrum Analyzer is to make relative observations by comparing a self-produced signal to other radio station signals, which vary in amplitude based not only on their power but also by their distance, and by providing detail as to the increase or decrease of signal output brought by changes in the antenna system of an experimental transmitter.
MORE COMMENT (edit)
Brought to mind is the fact that the reading of the spectrum screen, even knowing it will be a wildly general estimation, is still made via the dBm unit of measurement, with an unknown accuracy factor.
In the more scientific realm an accuracy factor can be given with an error scale, e.g. +/- 10dBm or whatever it happens to be.
But in the part 15 application where an error factor is unknown, we need some kind of symbol to indicate “unknown error factor”, perhaps “~error” or “+/- infinity”.
Hard Time
Carl,
I am not trying to give you a hard time about this but am trying to understand what it is that you are measuring. Could it be dBuV or dB relative to some preset reference?
The reason for questioning this is that you are about to invest some time and effort in your coil experiments which can yield some great information for the body of knowledge on this topic and the measurements need to be understandable and repeatable.
The spectrum analyzer I once used displayed dB relative to an adjustable reference and thus the readings were relative. I used 0dB = 1 mV or 1 uV depending on the signal strength and by doing so the measurements were repeatable and could be compared to other similar measurements. One of the demos I did for my class was to connect a short wire to the input and scan the FM band. For this the reference was in the uV range.
Neil
Reading the Scope
Right away I admit being very new to the task of interpreting any spectrum analyzer, let alone this ATTEN AT-5005, which I believe is somewhat scaled down without features found on more elaborate instruments.
The only instructions available are the Chinese-to-English translation manual, which seems to jump around and skip some sought-for information.
For example, though included, the 13″ telescoping antenna is not described as to impedance nor any other useful technical facts.
As far as I can tell at this time my only points of comparative reference are the signals seen from licensed stations, which appear taller or shorter based on their geographic proximity.
The simple benefit to my experiments with antennas and coils is a report of “more or less” on the scale, whatever terms we use.
The manual says the scale is to be read in dBm, but there may be more that the manual does not put into words.
In a way it’s like using a shoe to scrape ice from the windshield, but it’s the only tool available.
Further Discussion RE: Analysis
Upon thinking about this present ongoing experimentation being done with loading coils, my intention is to discover whether particular coils work better than other coils. Seeing higher or lower comparative peaks on the scope answers that basic question.
It has not (so far) been my intention to provide precise, meaningful electronic measurements in terms of any particular units of measurement.
I only used the term “dBm” because the manual for the scope attributed dBm as the working term for attributing each horizontal graticule as “10dBm”.
As I said elsewhere, we need a simpler language for running the most basic tests where the only objective is to find incremental improvements.
So far as I know the most common term for the unknown is “X”.
I am looking for more of “X”.
Experiments
Carl,
There is value in being able to report “I did this and the signal was up/down” and you are on the right track to make this happen.
There is no need to get to balled up over units, etc. and I certainly don’t want to pull you off track.
Do you recall the rough start I had when logging the field strength? Questions about my receive antenna, receiver S meter readings, old weather worn coax, and others came up. Once I settled on a system which I believe gives reliable relative readings I became able to compare data and draw some conclusions.
If there is something in your measuring setup which is causing problems you will find out soon enough so don’t try to anticipate all that might happen. Just do what you can to make the readings repeatable, in other words you get the same readings for the same transmitter and antenna time after time. Once the bugs are understood and worked with good conclusions are possible.
Neil
Blind with Sight
Yes, as to experimenting, one must try to manage a stable “known” environment within which to make observations.
Much of RF electronics is invisible and thus amounts to “working blind”, while a few instruments provide windowed sight into certain traces of what is taking place.
I think a sensitive experimenter considers the chance that “something not realized” may be taking place out of sight. Like when a possible ghost passed near Tha Dood’s antenna in Poca, West Virginia, causing his signal to shift around exactly as it would if a living person had gotten too near the antenna.
Part 15 is comfortable for the weekend experimenter because we are not seeking the Nobel Prize nor struggling for tenure at some university. We cannot be fired for our dabblings, although marriage is another matter and don’t forget Vantine’s Day.
Going Over Past Ground
At a certain point in this thread we raised a question as to whether clockwise or counter-clockwise winding of coils is by any measure different in the result produced, and to this member rich replied:
“the only practical result of that coil is the inductive reactance it produces — which is independent of the circular direction (“CW/CCW”) used for the conductor in that coil.”
This is said with the air of authority, but unaccompanied by any of the source references characteristic of this member. Therefore we take it to be an opinion from belief, and not, so far as we can tell so far, a proven reality.
Unconvinced as we remain, work shall proceed on winding an identical coil to that used for the first test, but wound in counter-clockwise direction.
Further test results will be shared in this same thread.
Authority
Inductance can be defined as L = v/(di/dt) as well as by similar equations. There is no mechanical parameter in this equation and a device which has the property described by the equation (such as an inductor) will have this property set by mechanical factors such as dimensions, number of turns, and permeability and equations can be derived with these terms but I have never seen the direction of the winding as being a factor.
By all means try the two coil directions and see what happens.
Neil
This, and economics are the
This, and economics are the reasons that many/most coaxial conductors rated for high-power broadcast applications at MW frequencies and above use a hollow-core center conductor.
As do high tension powerlines…