Ground loss for shortened vertical antenna

Yves wrote: My question is how to achieve my ground loss to 2 ohms (excellent) or less within part 15 limit?

The resistance of an r-f ground is not just its its DC resistance, but its impedance to the flow of r-f current at the frequency of interest. Part 15 puts no limit on the ohmic value of the r-f ground used, only that the length of the conductor(s) connecting it to the tx plus the length of the antenna connected to the tx cannot exceed 3 meters.

The r-f resistance of a set of radials in/on the earth is related to their physical length, and number. Careful experiments done by RCA Labs in the 1930s proved that achieving low values of r-f ground loss (2 ohms or less) for average-to-poor soil conductivity needed about 120 radials each more than 1/4-wave long, even for short vertical antennas. That is not practical for Part 15 systems, obviously, and most of them are using much worse r-f grounds -- probably 20 ohms or more, making that the lossiest part of the antenna system, by far. Most likely your set of 12 radials is not better than that.

Also it should be noted that an r-f ground does not exist at the top of any grounded conductor ("massive" wire, flagpole, billboard etc) used with an elevated Part 15 AM system. The entire length of that conductor will radiate, and r-f grounds, by definition and function, do not.

Also, is there a way to measure the ground loss of my radials?

It can be inferred from measurements made with various r-f analyzers. As the radiation resistance of Part 15 AM antennas is so small, the answer will be nearly the same as the resistance term of the impedance measured by an r-f analyzer when connected between your antenna input and the intersection (common point) of your radials.

You might find an amateur radio operator with this equipment, who would be willing to work with you on that.
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If, by chance, i have a big field here, i would install 120 radials of 1/4-wave long. I must say that i went to Dollarama store (all items at 1 dollars can) and i saw a 150' of #20 GA steel wire (or hardware wire). So considering if i broadcast on 1640 kHz (150 foot of wire needed for a 1/4-wave long *AND* a big field, i will buy 120 packs of this for $120. Maybe the steel wire is more resistive than the copper wire, but is less corrosive than copper.

What do you think?

Most of the loss in a radial ground system occurs before the r-f current enters the radials, so the fact that steel is not as good a conductor as copper is not highly serious.

But steel rusts, and a ground system with steel radials will begin to deteriorate quickly.

There are many broadcast radial systems using buried copper wire that are still in use, and "meeting spec" 30 years and more after installation.
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Okay.. i remembered that i saw some radials of an old 10 kW AM broadcast site last year... (in fact, i have unburied about 3 or 4 radials). They have used about 360 radials of 1/4-quarter wave long (one radial per degree) and it was big copper, uninsulated (i think 12 GA, maybe less) so a total of 18.5 km of copper wire!!!! Of course, these radials are green so far (corrosion with the time). I will buy some copper insulated wire because i don't want that the wire deteriorate.

Do you know if there is a difference of a ground loss between 120 and 360 radials?? I guess it would be 0 ohm for 360.

Yves

Yves wrote: Do you know if there is a difference of a ground loss between 120 and 360 radials?

Maybe around 1 ohm (for 4/10-wave long radials).

But the "Q" of a Part 15 AM antenna system will increase with reducing ground loss, and its 3-dB r-f bandwidth will decrease.

Here are some numbers for a ground loss of 2 ohms, assuming a 3-m, 1/2" OD vertical on 1500 kHz, and 2 ohms of coil and other losses.

Q= 824, r-f BW = 1.8 kHz, antenna efficiency at Fc = 2.13%

With a 25 ohm rf-ground (other things equal):

Q = 124, r-f BW = 12.1 kHz, antenna efficiency at Fc = 0.32%

So with a 2-ohm ground, you'd probably have to add some resistance to the system in order to have enough r-f bandwidth to pass your program audio -- and of course, that brings the efficiency back down.

So you may as well not spend a lot of time and money installing a "broadcast" radial ground system for Part 15 AM use.
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Thanks for your explanation... It's more clear than ever. Can you tell me how much the BW is for a commercial AM broadcast normally?

With the Cool Edit 2000 application, I try to convert a sample of MP3 file to 1.8 kHz to see how quality sound is (for Q=824). But my computer can't play at this sample. So, i tried to convert to 4 kHz and my computer was able to play at this frequency. I found that the sound wasn't clear at all compared of 12.1 kHz sample. But despite of the BW so narrow (1.8 kHz), i understand that the efficiency of antenna is much higher. But i'm not sure that my neighbors will like to listen my part 15 AM with this BW.

If you can, I would like to have the magic formula for the Q and for the r-f BW based on the ground loss, length of the antenna and the frequency.

One last thing, can you give me some numbers for broadcast on 1700 kHz with 2 ohms rf-ground, and with 25 ohms rg-ground.

Thanks in advance!!!

The bandwidths I posted are the r-f bandwidths, which consists of the upper and lower AM sidebands. The audio bandwidth at the output of an AM receiver is 1/2 of the r-f bandwidth that the receiver can pass. So you'll have to divide the bandwidths I posted by 2 to get the audio bandwidth.

The r-f bandwidth of commercial AM broadcast stations varies from about 10 kHz to maybe 20 kHz, depending on the way they set up their audio and modulation processing.

The r-f bandwidths at 1700 kHz (other things the same):

25 ohm ground = 15.5 kHz
2 ohm ground = 2.4 kHz

Receiver audio BW is 1/2 of that (at best).
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How does a full power commercial station get a 15-22khz RF bandwidth while still maintaining the required FCC antenna effeciency standards?

Thank You,

Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org

Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree

How does a full power commercial station get a 15-22khz RF bandwidth while still maintaining the required FCC antenna effeciency standards?

they don't use CB radio whips for antennas...:-)

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
http://home.earthlink.net/~wdcx

I noticed that on a near field impedance chart that a short height vertical, who's far field is defined as being Lambda/2PI. That over half of the electrical field impedance is lost in the first 10% of the radius of the near field.

In which case, I would use a ground system of 16 radials about 30 foot long, and then cover the ground around the radiator at a distance of about 10 foot radius with chicken wire fencing, all connected together and grounded to the ground system in the middle. This should be laid on top of the ground. And it reduces the ground losses in the first 10% region of the near field. This keeps the rf voltage field voltage up in this region. And so, might help the bandwidth.

I would also use a 1/2 inch copper water pipe for a radiator rather than a CB whip. The rf skin resistance of the CB whip is higher than the 3 meter high radiation resistance. The 1/2 inch copper pipe has lesser losses.

And if you want to keep the ground losses down at the feed point never end feed the base loading coil. Always ground the bottom end of the coil and ground the coax shield to that also. And tap the base loading coil for a impedance transformation. And well, a impedance matching network really should be used to fine tune the match to the coil tap.

Technically, a capacitance hat does not add electrical wavelength length to the antenna. And since a hat folds back downwards and has a circular wire connecting all the ends. No length is added to the radiator. Of course views will vary. Pro and Con on the definition of a 3 meter monopole.

Prof. Valentin Trainotti has recently published a paper about calculating the ground loss of a ground plane of a vertical MW antenna in the December, 2007 issue of the IEEE Antennas and Propagation Magazine. The name of the article is "Accurate Evaluation of Magnetic and Electric Field Losses in Ground Systems." The "accurate evaluation" mentioned in the title is possible by a reader of Prof. Trainotti's article only after a lot of labor. The integrals in Trainotti's results cannot be evaluated in closed form, but must be numerically calculated. A young doctorate student is named as the co-author of the article. I don't know for sure, but knowing about how things are usually done, I would guess that the graduate student did the computer programming necessary for the numerical calculations, but Trainotti developed the general theory of the problem in mathematical form.

When I learned about Trainotti's recent article, I saw that the subject matter relates directly to Part 15 AM. I saw this to be an opportunity to ask Prof. Trainotti to apply his powerful intellect to the subject of Part 15 AM. I asked him to provide a numerical value of the ground resistance of the gound plane described in the post by Dan Jackson. Prof. Trainotti's response was disappointing. He said, in effect, that the whole point of Part 15 regulations, and similar regulations in other countries, is to limit coverage area. So, the problem of poor range with Part 15 AM is mostly regulatory, and not technical. Consequently, Prof. Trainotti was disinclined to help with the technical aspects of Part 15 AM.

I think that Prof. Trainotti is only partially right. There can be no doubt that the FCC intends to limit the range of Part 15 stations. In the case of TIS stations, which use a lot more power than Part 15 stations, the FCC requires that the TIS station must be outside the .5 mV/m contour of any licensed broadcast stations on the same frequency.

I had failed to point out to Prof. Trainotti how poor the state of the art of Part 15 AM actually is. In a recent NOUO, a supposedly Part 15 station was cited for using excessive input power to the final stage, even though The FCC's field strength reading indicated that that the effective radiated power was only about 22 uW. 22 uW, and more, could have been obtained from a perfectly legal Part 15 AM installation. After all, 22 uW represents an overall system efficiency of only .022% in a legal system.

Although Prof. Trainotti does not wish to directly contribute to the technology of Part 15 AM, the information in his articles can be very useful. He supplies a great deal of information about the resistance of ground planes. Anybody is free to use the information contained in his published material.

Here is the information Trainotti has published about a ground plane with 180 radials, 1.8 m in radius, operating at 1.7 MHz:

For seawater, with a conductivity of 5 S/m, and a relative permittivity of 80, the ground resistance is .4 ohms.

For wet soil, with a conductivity of 30 mS/m, and a relative permittivity of 20, the ground resistance is 5 ohms.

For normal soil, with a conductivity of 10 ms/m, and a relative permittivity of 10, the ground resistance is 10 ohms.

For dry soil, with a conductivity of 1 mS/m, and a relative permittivity of 4, the ground resistance is 35 ohms.

These results, while disappointing for licensed operators, look pretty encouraging for Part 15 AM operators. Dan Jackson's ground plane should give pretty good results, also.

The 180 radials specified by Trainotti are pretty dense, but the radius of the ground plane is less than 6 feet. So, laying out a solid metal radius of about six feet radius should give about equivalent results. For regulatory reasons, I think that solid metal is better than radials. This is because the FCC could possibly include the total length of the radials toward the allowable length of the ground lead.

Remember that the total radius of the ground plane is about a half wavelength, although, in this example, the metal portion of the ground plane reaches a radius of only about six feet.

Trainotti said that, because of the low radiation resistance of the antenna, as much ground plane and top loading as possible should be used. My guess is that the FCC will not tolerate a very big ground plane, and, especially, not a lot of top loading. So, I think that a ground plane and top loading should be used with moderation.

Prof. Valentin Trainotti has recently published a paper about calculating the ground loss of a ground plane of a vertical MW antenna in the December, 2007 issue of the IEEE Antennas and Propagation Magazine. The name of the article is "Accurate Evaluation of Magnetic and Electric Field Losses in Ground Systems." The "accurate evaluation" mentioned in the title is possible by a reader of Prof. Trainotti's article only after a lot of labor. The integrals in Trainotti's results cannot be evaluated in closed form, but must be numerically calculated. A young doctorate student is named as the co-author of the article. I don't know for sure, but knowing about how things are usually done, I would guess that the graduate student did the computer programming necessary for the numerical calculations, but Trainotti developed the general theory of the problem in mathematical form.

When I learned about Trainotti's recent article, I saw that the subject matter relates directly to Part 15 AM. I saw this to be an opportunity to ask Prof. Trainotti to apply his powerful intellect to the subject of Part 15 AM. I asked him to provide a numerical value of the ground resistance of the gound plane described in the post by Dan Jackson. Prof. Trainotti's response was disappointing. He said, in effect, that the whole point of Part 15 regulations, and similar regulations in other countries, is to limit coverage area. So, the problem of poor range with Part 15 AM is mostly regulatory, and not technical. Consequently, Prof. Trainotti was disinclined to help with the technical aspects of Part 15 AM.

I think that Prof. Trainotti is only partially right. There can be no doubt that the FCC intends to limit the range of Part 15 stations. In the case of TIS stations, which use a lot more power than Part 15 stations, the FCC requires that the TIS station must be outside the .5 mV/m contour of any licensed broadcast stations on the same frequency.

I had failed to point out to Prof. Trainotti how poor the state of the art of Part 15 AM actually is. In a recent NOUO, a supposedly Part 15 station was cited for using excessive input power to the final stage, even though The FCC's field strength reading indicated that that the effective radiated power was only about 22 uW. 22 uW, and more, could have been obtained from a perfectly legal Part 15 AM installation. After all, 22 uW represents an overall system efficiency of only .022% in a legal system.

Although Prof. Trainotti does not wish to directly contribute to the technology of Part 15 AM, the information in his articles can be very useful. He supplies a great deal of information about the resistance of ground planes. Anybody is free to use the information contained in his published material.

Here is the information Trainotti has published about a ground plane with 180 radials, 1.8 m in radius, operating at 1.7 MHz:

For seawater, with a conductivity of 5 S/m, and a relative permittivity of 80, the ground resistance is .4 ohms.

For wet soil, with a conductivity of 30 mS/m, and a relative permittivity of 20, the ground resistance is 5 ohms.

For normal soil, with a conductivity of 10 ms/m, and a relative permittivity of 10, the ground resistance is 10 ohms.

For dry soil, with a conductivity of 1 mS/m, and a relative permittivity of 4, the ground resistance is 35 ohms.

These results, while disappointing for licensed operators, look pretty encouraging for Part 15 AM operators. Dan Jackson's ground plane should give pretty good results, also.

The 180 radials specified by Trainotti are pretty dense, but the radius of the ground plane is less than 6 feet. So, laying out a solid metal radius of about six feet radius should give about equivalent results. For regulatory reasons, I think that solid metal is better than radials. This is because the FCC could possibly include the total length of the radials toward the allowable length of the ground lead.

Remember that the total radius of the ground plane is about a half wavelength, although, in this example, the metal portion of the ground plane reaches a radius of only about six feet.

Trainotti said that, because of the low radiation resistance of the antenna, as much ground plane and top loading as possible should be used. My guess is that the FCC will not tolerate a very big ground plane, and, especially, not a lot of top loading. So, I think that a ground plane and top loading should be used with moderation.

Here is the information Trainotti has published about a ground plane with 180 radials, 1.8 m in radius, operating at 1.7 MHz: ...

What was the height of that monopole, Ermi?

Calculated earth currents within 1.2 wavelengths from a monopole antenna are given in GENERAL CONSIDERATIONS OF TOWER ANTENNAS FOR BROADCAST USE (Gihring and Brown of RCA). They show that the ground currents within about 1/2 wavelength of a monopole get smaller as the monopole height ranges from about 90 to 230 degrees.

So a radial system for 1.7 MHz that was only 1.8 meters in radius would not be very useful unless the monopole was very short.
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Here is the information Trainotti has published about a ground plane with 180 radials, 1.8 m in radius, operating at 1.7 MHz: ...

What was the height of that monopole, Ermi?

Calculated earth currents within 1.2 wavelengths from a monopole antenna are given in GENERAL CONSIDERATIONS OF TOWER ANTENNAS FOR BROADCAST USE (Gihring and Brown of RCA). They show that the ground currents within about 1/2 wavelength of a monopole get smaller as the monopole height ranges from about 90 to 230 degrees.

So a radial system for 1.7 MHz that was only 1.8 meters in radius would not be very useful unless the monopole was very short.
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