Pirate AM in Marysville, OH

[rfry]

I found the there was very little radiation from a longer ground, it was basically the height that contributed to increased signal. Such a short radiator is a point source at that low frequency.

These beliefs are popular among Part 15 AM equipment suppliers and operators, but they are not supported either by theoretical physics, or decades of field experience.

Here is a quote from Radio Engineer's Handbook by F. E. Terman, 1st Editon, page 799, in the section about grounded vertical radiators:

"When the length of the grounded antenna is of the order of one-eighth wave or less, the radiation is almost exactly proportional to the cosine of the angle of elevation."

The 3-m whip portion of a Part 15 AM antenna system is much shorter than one-eighth of a wavelength even at 1700 kHz, so we can construct a short table showing its relative field in the vertical plane:

Elevation Angle = Relative Field [cos(angle)]

0 deg = 1.000 (always maximum radiation in the horizontal plane)
15 deg = 0.966
30 deg = 0.866
45 deg = 0.707
60 deg = 0.500
75 deg = 0.259
90 deg = 0.000 (always zero radiation toward the zenith, directly above the radiator)

These characteristic patterns also can be seen in paper 3 at http://rfry.org/Software%20&%20Misc%20Papers.htm (figure on page 3).

Conclusion: these electrically short antennas are far from being point sources.

Adding a long, vertical ground lead/conductor to a Part 15 AM tx+whip extends the radiating length of the antenna system. All of the r-f current that exists at the base of the elevated 3-m whip flows along that "ground" path at about the same value as at the base of the 3-m section. The flow of r-f current along the ground wire makes it radiate, just as that current makes the 3-m whip radiate. This can be seen in the figures on pages 1 and 2 of the paper referred to above.

So it is the added radiation from the ground conductor of an elevated Part 15 AM system that produces the increase in signal strength/coverage radius, not because the 3-m whip is an isolated point source, which when elevated has better line of sight paths to nearby receivers. The majority of the radiation from such systems occurs from the ground lead, not the whip. A well-designed and impartial measurement will prove this.

Some of the finer points of the engineering could be debated for decades. (I don’t enjoy that sort of thing).

What we are discussing here are not fine points that are open to debate. They are fundamental to physics, and to an understanding of how Part 15 AM systems operate.

Anyone, but especially a Part 15 AM tx manufacturer wanting to provide accurate information about the installation and operation of Part 15 AM systems should not dismiss them, and offer advice to the contrary -- which only misleads others, and may lead to FCC problems for those readers (at least).
//

 

[Hamilton]

I got the flu from the little kiddies from Sunday school.!

To be honest I find it hard to find the time to get into deep fundamental discussions on theory with folks, Fry has e-mailed me directly in the past. I work long days and am not retired. I don't mind talking and answering a few questions here and there but when I start having to schedule research time and delaying other projects it becomes difficult. The transmitter thing is actually a small part of what I do, I design products for many companies, large and small. Some radio related, some not. Especially when there is seemingly no end, One issue seems to lead to 3 others and before I know it I am spending to much time here. (have to time manage or die)
I am happy to spend what seems like useful time here but that is all. If I ever retire I’ll be back to debate whatever you want for however long you want.

 

[Hamilton]

I see what you are saying Fry but I think there are some holes there. I just don't have the time to go into responding to the pages of response you will send to anything I respond to you. However if I can have some time I will try to get something, it will take some time before I can prioritize it in.

 

[Black_Shire]

I got the flu from the little kiddies from Sunday school.!

To be honest I find it hard to find the time to get into deep fundamental discussions on theory with folks, Fry has e-mailed me directly in the past. I work long days and am not retired. I don't mind talking and answering a few questions here and there but when I start having to schedule research time and delaying other projects it becomes difficult. The transmitter thing is actually a small part of what I do, I design products for many companies, large and small. Some radio related, some not. Especially when there is seemingly no end, One issue seems to lead to 3 others and before I know it I am spending to much time here. (have to time manage or die)
I am happy to spend what seems like useful time here but that is all. If I ever retire I’ll be back to debate whatever you want for however long you want.

...And the rest of us will happily go on using your Rangemaster transmitter and the other FCC Certified Part 15 AM transmitters. In the final analysis, the FCC ID stickers on these transmitters and the way we install them are all that matters, and the FCC has the final word on the latter. If an FCC field agent comes calling and says (after examining the installation) that it's okay, then it's okay. Period.


-- Black Shire

 

[dbdigital]

Some of the finer points of the engineering could be debated for decades. (I don’t enjoy that sort of thing).

What we are discussing here are not fine points that are open to debate. They are fundamental to physics, and to an understanding of how Part 15 AM systems operate.

Anyone, but especially a Part 15 AM tx manufacturer wanting to provide accurate information about the installation and operation of Part 15 AM systems should not dismiss them, and offer advice to the contrary -- which only misleads others, and may lead to FCC problems for those readers (at least).
//

Quite frankly, this castigation of someone whose proven expertise in radio electronics is beyond question is intolerable.

And since you deem it necessary to repeat your post on another board I think it is appropriate to quote in part a reply from Neil (aka: radio8z) which raises a good question as to the true significance of the ground lead as a radiator in actual practice :

"Concerning the radiation from the ground lead, I have read a couple of reports on other boards where people claim to have shown that the field strength near the ground conductor is very small compared to that near the antenna. My take on this is there is possibly an error in the measurement technique and present it here to ask if it makes sense.

One reported experiment involved a low sensitivity field strength meter (FSM) held near the radiators giving a high reading near the antenna compared to a negligible reading near the ground. It seems to me that the voltage to ground on the ground lead would be low compared to that on the radiating antenna and that the meter being held near the conductors was responding to the capacitively coupled voltage rather than the radiated field. The current in the ground lead can cause appreciable radiation even at a low potential to ground.

If I touch the antenna of my AM tx. with a neon light, it lights: not so if I touch the ground, thus illustrating the difference in voltage but not giving any indication of the current nor the field strength."

db

 

[rfry]

And since you deem it necessary to repeat your post on another board I think it is appropriate to quote in part a reply from Neil (aka: radio8z) which raises a good question as to the true significance of the ground lead as a radiator in actual practice :

"Concerning the radiation from the ground lead, I have read a couple of reports on other boards where people claim to have shown that the field strength near the ground conductor is very small compared to that near the antenna. My take on this is there is possibly an error in the measurement technique and present it here to ask if it makes sense."

And my answer to you, Neil and other readers here is no, it doesn't make sense. This would depend on the sensitivity of the typical field strength meter (FSM) to radiated fields, and the horizontal distance from its antenna to the Part 15 AM antenna components.

The field of a radiating antenna system should not be measured very close to it, because that does not give usable results with a FSM. Such is possible only with exotic and expensive near-field probing techniques, where the complete radiation pattern can be synthesized from the near-field data measured at various locations along the antenna aperture.

A better approach for Part 15 AM would be to locate a sensitive FSM a few hundred feet horizontally distant from the Part 15 antenna system, over level ground. The meter should be tunable, to be sure that only the frequency under test is being measured. The total radiated field observed from a Part 15 AM system with a long ground lead will be higher than when the 3-m whip is mounted with its base at/very near ground, using a short conductor connected to the same r-f earth ground as used in the elevated system (and other things equal).

As the 3-m whip in the elevated Part 15 antenna system then will be virtually the same distance away from the FSM, and with no better line-of-sight path than to the earth-based system, this will conclusively show that it isn't the height of the 3-m whip that is important, it is the total length of the radiating conductors used (3-m whip plus the long conducting path from the tx chassis to r-f ground).
//

 

[Neil E.]

A couple of minor points. When I posted:

My take on this is there is possibly an error in the measurement technique and present it here to ask if it makes sense.

I was asking if my take (or understanding of the situation) as explained later in the post made sense. I suppose asking if the test method makes sense is also valid.

To clairify a bit, the finishing paragraph of my post was not quoted. It is:

Is it reasonable to question that the measurement technique described is flawed and misinterpreted and the experimenter did not measure the radiated field?

Neil

 

[dbdigital]

A couple of minor points. When I posted:

My take on this is there is possibly an error in the measurement technique and present it here to ask if it makes sense.

I was asking if my take (or understanding of the situation) as explained later in the post made sense. I suppose asking if the test method makes sense is also valid.

To clairify a bit, the finishing paragraph of my post was not quoted. It is:

Is it reasonable to question that the measurement technique described is flawed and misinterpreted and the experimenter did not measure the radiated field?

Neil

I would have gladly quoted it all but I'm not sure what is and isn't allowed on this site regarding such quotes.

But as Blackshire indicated, the final arbitor is the FCC and if they're happy with the Rangemaster and in the way it has been installed (in most cases), then that's all that matters as far as being in compliance is concerned. The rest is academic.

db

 

[rfry]

But as Blackshire indicated, the final arbitor is the FCC and if they're happy with the Rangemaster and in the way it has been installed (in most cases), then that's all that matters as far as being in compliance is concerned. The rest is academic.

And that's all well and good for those who understand the physics involved, and choose to accept the risks of an FCC inspection of a non-compliant Part 15 AM antenna system that they have consciously decided to use.

But should not the choice that users make to operate that way be based on an accurate understanding of FCC rules and the physics involved, and not on hearsay, guesswork, wishful thinking, and/or "playing the odds?" Yet that is the net result of most of street lore about these systems often seen on these sites.

Note that I have nothing against it, if so -- those decisions are up to those users. I just want them to know what they really are doing.

There is a recent, public example where a "Part 15" AM operator reportedly with an FCC-certified Part 15 AM transmitter was cited by the FCC for using a ground lead about 30 meters in length. That user reportedly stated that he was only following the concepts posted on the website of that Part 15 AM certified transmitter, and so he could not understand why he had been cited by the FCC.

Shouldn't that be a bit of wake-up to others operating that way?
//

 

[Neil E.]

db,

I hope you understand that I was not being critical of your post and I am rather flattered that you saw fit to quote me. I do wonder if all understood that I was just questioning the validity of the test method. This leads to thought about the problems that might arise from drawing conclusions based on flawed data. By adding the last sentence I meant to make this clearer.

Much discussion has been presented regarding the ground length issue and I did not address this from theory or legally, but if we are going to experiment we need to question and validate the methods before drawing conclusions based on the observations.

The tests described may well be valid but for reasons given I don't think they are. There is an old saying which floated around the lab where I did research which goes "Good data is expensive, bad data is even more expensive". Measurement techniques are not always simple and when the results deviate from predictions based on theory a careful review of the methods is needed. Most of the time an error is found but sometimes a Nobel Prize results.

Physics theory tells us that radiaton results from the acceleration of charged particles. The accelerating electrons in the antenna and ground leads radiate energy. The field produced then depends on the number of these charges (the current) and the length of the conductor (the number of charges and how their fields add). Though it takes voltage to move the charges, voltage by itself does not produce radiation. My question regarding the measurements was whether the measurement was just seeing the voltage rather than the actual field strength. Since the current delivered to the antenna must equal the current in the ground lead this same current (and therefore radiaton for a given length) can exist though the voltages to ground are different. A ground lead can radiate just as well as an intentional antenna.

This was what led me to question the tests as I did.

Neil

 

[rfry]

Also a conduit around the ground can be effective, at frequencies of 1-2 Mhz a short length of conduit is a effective (though sure, not perfect) shield.

Below is a link to NEC patterns showing the radiation from a Part 15 AM 3-m whip elevated 20 feet above the earth --first with a bare 20-foot ground lead, and then when the ground lead is centered inside a metal conduit grounded at the bottom. There is less than 1 dB difference in the peak gains of the two configurations.

Radiation from the ground wire couples into the conduit, which re-radiates nearly as much as the bare ground wire does.

http://i62.photobucket.com/albums/h85/rfry-100/ElevPart15andConduit.jpg

//

 

[dbdigital]

db,

I hope you understand that I was not being critical of your post and I am rather flattered that you saw fit to quote me. I do wonder if all understood that I was just questioning the validity of the test method. This leads to thought about the problems that might arise from drawing conclusions based on flawed data. By adding the last sentence I meant to make this clearer.

Much discussion has been presented regarding the ground length issue and I did not address this from theory or legally, but if we are going to experiment we need to question and validate the methods before drawing conclusions based on the observations.

The tests described may well be valid but for reasons given I don't think they are. There is an old saying which floated around the lab where I did research which goes "Good data is expensive, bad data is even more expensive". Measurement techniques are not always simple and when the results deviate from predictions based on theory a careful review of the methods is needed. Most of the time an error is found but sometimes a Nobel Prize results.

Physics theory tells us that radiaton results from the acceleration of charged particles. The accelerating electrons in the antenna and ground leads radiate energy. The field produced then depends on the number of these charges (the current) and the length of the conductor (the number of charges and how their fields add). Though it takes voltage to move the charges, voltage by itself does not produce radiation. My question regarding the measurements was whether the measurement was just seeing the voltage rather than the actual field strength. Since the current delivered to the antenna must equal the current in the ground lead this same current (and therefore radiaton for a given length) can exist though the voltages to ground are different. A ground lead can radiate just as well as an intentional antenna.

This was what led me to question the tests as I did.

Neil

Hi Neil,

I understood the reason for your post and your question. I just thought the example you cited seemed appropriate to the discussion.

db

 

[Tom Wells]

I have adjusted and A/B/C tested grounding configurations on my part 15 AM while having remote live radio feedback so that I can
tune up.

The grounding makes no difference in my radiated field regarding distance.
It does change reception and noise levels with in-house reception.

I have always used a L/C tuning circuit, reasonant for MW, set up with a DP/DT so you can series/parallel.
First I tune the antenna, then tune the output of the pt 15 .
Then I couple them, and retune the antenna while the tx is adjusted for low power, adjusting C for fullest mod.
Then everything back to 100 mw, and it needs no more fuss.

Does anyone else use outboard antenna tuning units?

 

[rfry]

The grounding makes no difference in my radiated field regarding distance. It does change reception and noise levels with in-house reception.

These two statements don't seem to support each, do they? If in-house reception gets better and noise level is reduced with a "ground," wouldn't that be the result of the greater field strength available then for receivers (other things equal)?
//

 

[Tom Wells]

They do seem to contradict each other, incidental "leakage" grounding does not seem more effective than intentional grounding.
But "hum fields", or other local noises are greater or lesser, depending on grounding.
Keep in mind this transmitter is at or slightly below grade level.

Distance-wise, the ground make no difference.
My neighborhood is densely packed brick buildings.

 

[Black_Shire]

They do seem to contradict each other, incidental "leakage" grounding does not seem more effective than intentional grounding.
But "hum fields", or other local noises are greater or lesser, depending on grounding.
Keep in mind this transmitter is at or slightly below grade level.

Distance-wise, the ground make no difference.
My neighborhood is densely packed brick buildings.

What kind of Part 15 AM transmitter are you using? Also, regarding the switchable series/parallel LC antenna tuning network you use, does it have provision for matching its feed point impedance to the transmitter's output impedance?

(Many Part 15 AM experimenters just use a simple tapped antenna loading coil to bring the antenna to resonance, and don't bother to make it into an L Network with a variable capacitor to match the antenna's resistive component and tune out its capacitive reactance.)


-- Black Shire

 

[rfry]

Pursuing this subject a bit more, following this paragraph is a link to an FCC Public Notice showing that the coverage expectation for a compliant, unlicensed Part 15 AM tx with 100 mW input power (per 15.219) is about the same as for a compliant, unlicensed Part 15 FM setup. Note that the field strengths that these AM/FM systems produce at a ~200 ft radius are not the same, but the "usability" of those signal strengths in typical receivers yields very roughly the same performance from AM & FM receivers in a typical, urban location. Further discussion appears following this URL:

http://www.fcc.gov/mb/audio/decdoc/scandoc/910724/1.jpg

The unlicensed, maximum AM coverage radius of about 200 feet shown in FCC Public Notice 10489 is not so different from what a strictly legal, ground-mounted Part 15.219 AM antenna and tx should provide to an average indoor receiver in a suburban metro area.

Here in round numbers is the development upon which that conclusion is based:

* The standard groundwave field produced by a 1 kW AM broadcast tx using a 1/4-wave vertical monopole with 120 radials each at least 1/4-wave long is 300 mV/m at a 1 km radius. This value is taken from the FCC website, and is supported by thousands of field measurements over many decades.

* The difference in power between 1 kW and the ~80 mW output power of a good Part 15 AM tx is 41 dB.

* The gain of a very good, ground-mounted 3-m Part 15 AM antenna system on 1700 kHz (~25 ohms total in the coil and r-f ground connection) is about 25 dB below that of the 1/4-wave monopole described above.

So the field from the Part 15 system at a 1 km radius will be about 66 dB below that of the broadcast system reference. A voltage reduction of 66 dB is a multiplier of 0.0005, which means the Part 15 AM field at 1 km will be 300 mV/m x 0.0005 = 0.150 mV/m.

At these short distances, field strength (not power density) is inversely related to the distance from the antenna (not the inverse distance squared), so we can construct this table:

Distance in km > Field Strength in mV/m
1 > 0.15
1/2 > 0.30
1/4 > 0.60
1/8 > 1.2
1/16 > 2.4

The distance of 1/16 km is 205 feet. Not too coincidentally, it takes a field of about 2 mV/m to provide a fairly noise-free AM signal to a cheap indoor radio in an urban setting.

"Part 15" systems that are capable of producing good signals to cheap indoor radios over much greater ranges than this are able to do so only by using a more efficient (elevated) antenna system including a long radiating ground conductor, more tx power output, or possibly both of those.
//

 

[Tom Wells]

They do seem to contradict each other, incidental "leakage" grounding does not seem more effective than intentional grounding.
But "hum fields", or other local noises are greater or lesser, depending on grounding.
Keep in mind this transmitter is at or slightly below grade level.

Distance-wise, the ground make no difference.
My neighborhood is densely packed brick buildings.

What kind of Part 15 AM transmitter are you using? Also, regarding the switchable series/parallel LC antenna tuning network you use, does it have provision for matching its feed point impedance to the transmitter's output impedance?

(Many Part 15 AM experimenters just use a simple tapped antenna loading coil to bring the antenna to resonance, and don't bother to make it into an L Network with a variable capacitor to match the antenna's resistive component and tune out its capacitive reactance.)


-- Black Shire

The AM part 15 transmitter is home-brew. It has an output-tuning tank with a coupliing coil for the impedance match to the antenna.
This coupling coil is in the ground leg of circuit in series with the antenna-tuning circuit, in which L is tapped, reasonated with a variable C, and the series/parallel switch permits noise noise-null tuning of the antenna.

The tuning is sharp enough to demonstrate reasonable Q, despite the short antenna.

I suspect the radiation into and from AC lines in the house makes the difference in noise with grounding.

 

[nealffischer]

He's using a standard CB whip and the transmitter is up around 40 to 50 feet in the air. The results this station is getting is very typical for a station of this nature in an area with a ground conductivity of 8 millimohs. I have a friend running a Rangemaster in upstate NY that is getting similar range because he's so close to Lake Ontario.

The range of the station in the other three directions is much shorter. This is due to the way the unit is mounted and grounded. While you can hear it ten miles in that particular direction I found it was only perhaps 3 miles in the other three directions. The transmitter is located right near downtown Marysville.

I am curious about this. If the transmitter is 40' up in the air. How is it grounded and part 15 legal? wouldn't the ground lead line then be at least 40'? Not being a jerk here... just honestly curious. I know it was looked at by the FCC and found ok. I just would like to see how to do the same thing.

Neal - Someone who is looking into this...

 

[WCWalker]

Most FCC Field Offices are satisfied with seeing the 18" ground off the bottom of the transmitter tied off on the mast or to a surge arrestor.