Chokes for Part 15 AM "Ground Conductors"

[rfry]

A "Part 15 AM" transmitter and 3-meter whip installed atop a tower, mast, flagpole, roof mount, billboard etc, and using a long, conducting path to a functional r-f ground (something buried in the earth, typically) will have a significantly larger coverage area than if that system is mounted a few inches above the earth, other things equal.

This is shown in the analysis at http://i62.photobucket.com/albums/h85/rfry-100/150_microvolt_per_meterRadius_Part_.gif .

Some may suggest that using an optional choke, or another, undefined, optional circuit installed at the connection of the transmitter r-f ground terminal to the ground lead + ground conductor will prevent significant radiation from the entire length (height) of that conducting path to a functional r-f ground.

But whatever the circuital and electrical characteristics of such an option (inductive choke, tuned r-f filter, etc), and if it reduces the field radiated by the transmit system even by 20 dB, which is a reasonable value for such devices to be fairly effective, then the transmitter may as well be mounted with its ground terminal just a few inches above the earth -- as shown in the link above.

Such systems mounted within a few inches of the earth not only will be functionally compliant with Part 15.219(b), but when they are connected to a good r-f ground with no choke/filter, they will tend to be better protected from being damaged by nearby lighting strikes than when elevated while using such chokes/filters.

Installing such systems a few inches above the earth also will reduce installation cost, and the effort required to "tune" (optimize) such systems.

RF

 

[druidhillsradio]

Could you re-model assuming an output of 75 milliwatts, but keep all other parameters the same? Also, I am not sure I agree with your analysis about ground mounting versus mounting high up. Asuming the 20 dB of attenuation in the ground path, the transmitter with a 3 meter antenna would still benefit from being high and free of nearby obstructions, not to mention the reduced probability of theft.

 

[rfry]

... I am not sure I agree with your analysis about ground mounting versus mounting high up. Asuming the 20 dB of attenuation in the ground path, the transmitter with a 3 meter antenna would still benefit from being high and free of nearby obstructions ...

Apparently your above statements assume that ...

1. all of the radiation from an elevated "Part 15" AM setup using a long, conducting path from the ground terminal on the transmitter chassis to a functional r-f ground (something buried in the earth, typically) is produced only by the 3-meter whip attached to the transmitter, and

2. useful propagation in the MF spectrum is similar to that in the spectrum of VHF frequencies and above (which basically is line of sight).

Sorry, but both of those assumptions are invalid -- as is shown in engineering textbooks going back many decades, and can be confirmed by objective, real-world tests.

Note that the Numerical Electromagnetics Code models used to produce the coverage circles in my analysis all used paths over level terrain, with no obstructors/reflectors such as buildings, overhead wires etc. The only contributor to the differences seen for the four configurations in my analysis was the radiation efficiency of those four, different antenna systems.

In each case that was dependent on the length of the radiating "ground" conductor, and the amount of r-f current flowing along it.

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[druidhillsradio]

Maybe I was not clear. I am assuming that a "long radiating ground" is NOT used.

 

[rfry]

I am assuming that a "long radiating ground" is NOT used.

If a "Part 15" AM transmitter and attached, base fed, ~3-meter whip was elevated 10 meters above the earth and had no conductors connected to it other than the 3-m whip, then the feedpoint impedance of the system on 1700 kHz would be on the order of 0.1 -j 26,000 ohms.

It is unlikely that such an antenna could be resonated by any practical matching network having reasonably useful values of series resistance and operating Q, and that this system would have much worse performance than if it was mounted with its base on/near the earth, and used a few ground rods for an r-f ground.
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[Goat Rodeo Cowboy]

Asuming the 20 dB of attenuation in the ground path, the transmitter with a 3 meter antenna would still benefit from being high and free of nearby obstructions, not to mention the reduced probability of theft.

I don't think the longer waves of signal in the AM band pays any attention to what you think of as "nearby obstructions". If you have a metallic structure near an AM station such as a water tower, a construction crane or maybe a 10 story or taller building with a conductive structure then the directionality of the pattern gets distorted.

Some of the finest AM transmitter sites are located in bottom-land near streams. The soil in those locations is often more accepting of the ground-wave component. A few orange trees or live-oak trees or farm houses do not appear as obstructions to AM band waves.

Here is an engineering theory question for you: Are obstructions near the transmitter any more of a problem that obstructions near the receiver? If I put a Part 15 AM whip antenna in my back yard, and you live half a mile from me, are the shrubs, trees, yard barn for my lawn mower and my ornamental trees doing any more harm to your ability to receive my station than are the shrubs, trees, gazebo and kids playhouse in your own yard?

Is there any research that shows that if I will put my transmitter in the middle of a 5-acre hay field with no trees, no obstructions, and you have your receiver across the road from your house in a similar 5-acre cleared meadow that a stronger signal will arrive than in a typical suburban or small town setting? (I would assume the interference noises and static might be less in your receiving meadow, but will the measured incoming signal actually be stronger?)

How can you have a lengthy ground connection from an elevated whip antenna that is NOT a radiating ground?

 

[druidhillsradio]

I noticed that neither of you made an argument about reduced probability of theft.

You asked,"How can you have a lengthy ground connection from an elevated whip antenna that is NOT a radiating ground?"


I never said you could. I just asked a question. Mr. Fry answered it.

With regards to AM Broadcast Transmitter Operation, you cannot compare a full blown 1/4 wave installation to a 3 meter whip running 50 milliwatts. Simply moving a hand or a piece of metal near the antenna, detunes it enough to reduce the signal. I can assure you, standing next to a 205 foot tower, running 1KW has no effect on the transmitter signal. But moving a 3 meter Rangemaster away from trees has a big effect on the received signal.

 

[rfry]

I noticed that neither of you made an argument about reduced probability of theft.

Just to note that outdoor configurations are not necessary to install/use a functionally compliant, unlicensed AM system permitted either by Part 15.209 or Part 15.219.

Those concerned about the possibility of the theft (and lightning damage) of such systems when installed outdoors could simply elect to install them in a more protected area, such as indoors.
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[rfry]

It has been observed elsewhere that the result of installing an r-f choke between the ground terminal of an elevated Part 15 AM transmitter and the connection to its "ground lead" can be eliminated simply by adjusting the amount of reactance used between the final r-f amplifier stage of the transmitter and the 3-meter whip, until the antenna system is resonant.

However doing so means that the r-f choke in the ground lead is doing practically nothing to reduce radiation from the long, conducting path to a functional r-f ground -- which is the purpose of installing the choke in the first place.

It will be interesting to learn what any commercial suppliers of such chokes provide in the way of installation and setup instructions, and the final operational effect of those chokes on the radiation efficiency of elevated Part 15 AM systems.

If such chokes do not provide a significant amount (20 dB or more?) of reduction in radiation from an elevated Part 15 AM setup using a long, radiating ground conductor, then there is little point in using them.
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[druidhillsradio]

Mr Fry said: "Those concerned about the possibility of the theft (and lightning damage) of such systems when installed outdoors could simply elect to install them in a more protected area, such as indoors."

I said: You are obviously not married, or you would have never maid that suggestion. ;D

 

[WCWalker]

Mr Fry said: "Those concerned about the possibility of the theft (and lightning damage) of such systems when installed outdoors could simply elect to install them in a more protected area, such as indoors."

I said: You are obviously not married, or you would have never maid that suggestion. ;D

Even a simple indoor installation may not completely alleviate potential problems with lightning damage. The best way to solve this problem is to use only battery power to power the transmitter and then bury it and the antenna in the ground underneath the basement of a home.

 

[rfry]

The best way to solve this problem is to use only battery power to power the transmitter and then bury it and the antenna in the ground underneath the basement of a home.

Hyperbole noted.

Readers of this thread (perhaps with at least one exception) hopefully will recognize that there is a large range of performance difference between the "buried" configuration suggested in the paste above, one installed indoors but above/near the surface of the earth, and an outdoor, elevated Part 15 AM setup using a long, radiating "ground" conductor connecting it to a functional r-f ground.
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[Tom Wells]

Here's another good question in kind: If a transmitter or a section of feedline is "underground", below grade level in a basement, etc, does the underground, (impossible to radiate) section of feedline still count as feedline, or not until it emerges above ground?
I'm imagining a feedline sloping down from the 3m stick at about 45 degrees into the basement.

If it's 4 feet over from the house, the transmitter needs to be 3 ft deep, and the feedline is now 5 feet, which leaves about 3 feet for the antenna.

If it didn't, then all that would really count and radiate would be the 3m exposed.
I'd even bet the lack of "cancelling", radiation off the feedline back to the actual RF common point, would improve the pattern
and overall coverage.

Rich, do the part 15 rf models specify any conditions on feedline lengths and/or relationships/vectors of feedline vs radiator, and earth ground relationship?

Do fully buried sections of feedline count since they can't radiate?
Has the FCC ever issued an opinion on this?
Wouldn't feedline burial for pt 15 AMs answer many problems in antenna location?
Lots of questions, I know...

 

[rfry]

If a transmitter or a section of feedline is "underground", below grade level in a basement, etc, does the underground, (impossible to radiate) section of feedline still count as feedline, or not until it emerges above ground?

There are two ways to respond to such questions: one based on physics, and one from the viewpoint of the FCC -- which of course, no one can accurately predict.

Physics shows that a conductor carrying r-f current will radiate even though it is installed in a space below grade level, such as in a basement. Exactly how that radiation will propagate above grade level into the area surrounding the basement would take some serious study/experimentation, but that radiation is not insignificant.

A case in point here are the two towers of AM radio station WTAD installed on piers on a flood plain near the Mississippi River. A wide bluff nearby where most of the city of license lies rises about 130 feet above this flood plain. This means that most of the radiation from WTAD occurs from below grade level for listeners there. WTAD's signal in the city and well beyond it is consistent with FCC values for their ERP, frequency, and the ground conductivity.

Rich, do the part 15 rf models specify any conditions on feedline lengths and/or relationships/vectors of feedline vs radiator, and earth ground relationship?

The models I have made use NEC-2, which does not support the use of buried conductors. NEC-4 does, but it costs considerably more for the license required to use it. But this is not an issue because NEC-2 models can accurately show the parameters of antennas above and in contact with a perfect earth, which can then be used with the FCC's propagation curves to show the performance of the antenna system for those conditions.

NEC-2 calculates and shows the earth-ground relationship for the net radiation from all of the above-ground antenna conductors (ground wire, feedline, and 3-m whip).

Do fully buried sections of feedline count since they can't radiate?

This is more of an FCC question, but any conductor carrying r-f current will radiate, buried or not. It is possible to locate the buried radial wires of a broadcast type r-f ground system by using a field strength meter with its antenna a few inches above the earth, over the buried wire(s).

Has the FCC ever issued an opinion on this?

Not to my knowledge.

Wouldn't feedline burial forpt 15 AMs answer many problems in antenna location?

It might if the FCC accepted it. But a single conductor feedline will radiate along the entire length from the transmitter to the 3-meter whip. If part of that conductor is installed in PVC tubing (for example), and buried then radiation from that buried section may not be very useful, but it is still there.

Even a coaxial cable between the r-f output of the transmitter and the base of the 3-meter whip will radiate from the r-f current flowing on the outside of its outer conductor (shield), unless some means is provided to prevent it. In fact this is the configuration that gave the 2nd-largest coverage area in the comparison at http://i62.photobucket.com/albums/h85/rfry-100/150_microvolt_per_meterRadius_Part_.gif .

So if the 3-meter whip is installed on an elevated mount, and even if the mount is a non-conductor, most likely the radiating length of the whip plus feedline will exceed the 3-meter limit, not even counting the length of whatever conductor is connected from the indoor transmitter chassis to a functional r-f ground.

Short questions sometimes need long answers (sorry).
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[druidhillsradio]

Do you have any new graphs? That stuff is getting stale. ;D

 

[rfry]

Do you have any new graphs? That stuff is getting stale. ;D

With respect -- that graphic is not stale to those who haven't yet seen it, and/or to those who do not yet understand or (publicly) accept what it shows.

Hopefully re-posting the link to that graphic, with additional comments, will benefit both of those groups of readers.

RF

 

[rfry]

PhilB : I'll keep my comments to your post impersonal. Given that the FCC does cite for the use of long, radiating "grounds" at least on occasion, it might be important to some to understand the reasons why -- particularly if dozens of other posters including even some manufacturers of low power AM transmitters have tended to support their use, and to object to the posting of accurate technical information on the subject.

Would you not agree that both sides of the issue should be publicly available, so that Part 15 AM users can make better decisions about how they install and operate their systems?

RF

 

[Ermi Roos]

The radiation resistance of Ken Cartwright's main antenna is about 8.3 ohms. At ground level, the radiation resistance is about 0.11 ohms. The ratio of radiation resistances is the ratio of radiated powers, which is 8.3/.11 = 75.5. The field strength ratio is SQRT(75.5) = 8.7. The measured field strength reported in the NOUO is 4,000 uV/m at 137 m, which is equivalent to 18,300 uV/m at 30 meters. Dividing by 8.7 gives the expected field strength when the transmitter is mounted at ground level: 18,300/8.7 = 2100 uV/m. This field strength is typical for Part 15 AM transmitters mounted at ground level. I don't know how this would compare to Ken Cartwright's field strength with the ground lead of his elevated transmitter disconnected. The FCC inspector made measurements comparing the field strength with the ground lead disconnected, and with Hamilton's ground lead filter installed. These measurements have not been published.

The FCC measured the field stregths with and without the ground lead connected during the Liberty 1640 inspection. The 20 foot ground lead would give a radiation resistance of about 2.4 ohms. The ratio of the elevated radiation resistance to the ground level radiation resistance is 2.5/.11 = 22.7. The field strength ratio should be SQRT(22.7) = 4.8. The field strength with the ground lead connected was reported in the NOUO to be 73,800 uV/m at 30 m. The field strength at ground level should be 73,800/4.8 = 15,400 uV/m at 30 m. The field strength with the ground lead disconnected was measured to be 19,700 uV/m at 30 m, which is 2.28 dB higher than the expected field strength at ground level; giving a slight field strength advantage to elevating the transmitter with the ground lead disconnected compared to mounting the transmitter at ground level.

Since data is not available, it is not known how much of an advantage there is to elevating the transmitter with the ground lead disconnected, compared to mounting the transmitter at ground level, in the Ken Cartwright installation. Since the elevation of the Ken Cartwright transmitter is twice as high as the elevation in the Liberty 1640 installation, Ken Cartwright should have a greater advantage compared to a ground level transmitter than Liberty 1640.

 

[rfry]

Since data is not available, it is not known how much of an advantage there is to elevating the transmitter with the ground lead disconnected, compared to mounting the transmitter at ground level, in the Ken Cartwright installation.

Just to note that even if the ground lead and all other conducting paths in the mounting and use of Ken Cartwright's elevated "Part 15" AM installation are removed or insulated from his elevated transmitter+whip to the top of his grounded metal tower, the other conductors associated with the DC power and audio input connections to that elevated transmitter, if not adequately choked/filtered at the transmitter chassis, could result in a radiating structure for an unlicensed system that was functionally non-compliant with Part 15.219(b).

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[rfry]

The use of chokes/filters on the "lightning ground," power and audio conductors of an elevated Part 15 AM transmitter have been proposed to make such an installation functionally compliant with Part 15.219(b). That also would require an insulated mount for the transmitter to any type of conducting structure used to support the transmitter and 3-m whip (mast, flagpole, roof tripod, billboard steel etc).

The expected performance of this configuration can be developed using Numerical Electromagnetics Code (NEC) by analyzing a base-fed, 3-meter whip at some elevation above the earth.

A simple NEC model was developed to do this, with these parameters:

• 1620 kHz
• 3-meter whip, 1/4" diameter
• Base of the whip located 40 feet above the earth
• Earth conductivity = 5 mS/m, Dielectric Constant = 15
• No conductors attached to the transmitter other than the 3-m whip, which is the equivalent of using perfect filters on the ground, power and audio conductors, and an insulated mount for the transmitter + whip

The feedpoint impedance of the whip for those conditions is about 0.14 -j 19,000 ohms.

That amount of capacitive reactance would require about 1,800 microhenries of inductance to resonate the whip. The loss in such an inductor would be relatively large, and its Q would be relatively high.

Even though such an elevated whip could be resonated, its VSWR bandwidth would be very small due to the high Q of the inductor. It is likely that the audio response above 1 kHz would be noticeably and unacceptably reduced at the output of an AM receiver tuned to such a system.

The antenna system also would be very critical to tune/match, and it would be difficult to keep it matched.

Given all of the above, and that the signal coverage of such an installation would be approximately the same as if it was installed with the transmitter at earth level using an unfiltered ground conductor, such "filtered" configurations might not be the precedent-setting advance that has been expected.

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