This subject has been debated, but I want facts. Mr. Fry, car tom weigh in?
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Ground Radials - Insulated or Uninsulated?
- Thread starter druidhillsradio
- Start date
Bare copper makes direct ohmic contact with the soil, and so it should work better, at least in priciple (and when it is new). For insulated wire, the coupling to the soil is capacitive. For Part 15 use, where the antenna systems are very inefficient, any performance difference is not likely to be noticeable. Of course, copper buried in the soil deteriorates after a while. The rate of deterioration depends upon local conditions. The use of heavy gauge wire prolongs service life. Insulated wire lasts longer in the soil, and it would be cheaper because smaller gauge can be used.
Adding to the previous comments:
If there is no separate "lightning ground" for the transmit system, then bare wire might be preferable to better discharge the low frequency transients from nearby lightning strikes, and to reduce the accumulation of DC charges on the exposed wires of the antenna -- which if high enough may attract lightning, and produce noise in the transmitted signal.
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If there is no separate "lightning ground" for the transmit system, then bare wire might be preferable to better discharge the low frequency transients from nearby lightning strikes, and to reduce the accumulation of DC charges on the exposed wires of the antenna -- which if high enough may attract lightning, and produce noise in the transmitted signal.
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Thanks to both for your comments. Next I would ask with regard to length, should the radials be as long or slightly longer than the antenna is high? For my amateur radio vertical installation, which by the way is is ground mounted, the radials are a bit longer than the antenna is tall. The antenna is 22 feet high. so my radials are about 25 feet long for all bands 40 thru 10 meters. I am using 32 radials, but would like at least 64. As you might guess, soil conditions in central Florida are pretty poor. So for my Part 15 operation, will try to find some bare copper.
druidhillsradio said:
I happened to post something about this on another board that may help...
_____________
Quoting from a recent IEEE paper titled "Short Low and Medium Frequency Antenna Performance" by Prof. Valentino Trainotti of the Univ of Buenos Aires:
"The wave impedance is a complex magnitude, almost purely imaginary very close to the antenna, and almost a real magnitude at the distance of half-wavelength from the antenna base. ... it means that the half-wavelength radius space surrounding the antenna is part of the wave generator (oscillator). ... The earth area under this hemispherical space is a circle that is very important, because all of the conductive currents flowing on it are part of the antenna circuit, and, for this reason, it must have maximum conductivity in order to have maximum antenna efficiency."
Trainotti's paper does not restrict the importance of the half-wave radius around the radiator to vertical radiators below a certain electrical height. Some intuition may apply here, in that a half-wavelength of the radiation produced by a vertical monopole radiator of any electrical height is a constant for each applied frequency.
The Brown, Lewis and Epstein tests for their 1937 I.R.E. paper were conducted at 3 MHz in/over the sandy soil of New Jersey, which has a conductivity not greater than 4 mS/m, and probably less. Yet the field they found for verticals of about 45 to 100 degrees using 113 each 0.412-wave (free-space) radials was within several percent of the theoretical maximum possible at 0.3 miles for the applied power when using a zero-loss vertical with a zero-loss ground plane. This indicates a system radiation efficiency of around 95%, despite the poor ground conductivity at and around the test site.
BL&E showed that using fewer and shorter radials has a serious affect on antenna system efficiency, particularly for short verticals -- which conclusion is supported by Trainotti's paper.
//
The same paper by Trainotti cited by Fry shows that, for the same radial system, electrically-short antennas have lower ground loss resistance the lower the antenna height is. See Figure 18 in the paper. Consequently, shorter radials can be used for Part 15 AM systems compared to those for commercial broadcast antennas for the same ground resistance. Trainotti has contributed extensive technical data about ground screens for Part 15 AM, which can be found by searching the part15.us website.
Ermi Roos said:
To build on this statement, the 3-m monopoles used for Part 15 AM have very low radiation resistance. For every given applied power at the antenna feedpoint, it is the ratio of the radiation resistance to the sum of all resistances in the antenna system that determines the radiated EM energy. The rest of the applied power is converted to heat.
So even though shorter radials used with shorter monopoles produce the same real value of ground loss in ohms as longer radials with taller monopoles, that does not allow the two configurations to achieve the same radiation efficiency.
This is shown in the experimental work of Brown, Lewis and Epstein and documented in their 1937 I.R.E. paper. The link below leads to their graph showing that even with buried radials as long as 0.137 (free space) wavelengths, the number used is significant in determining the radiation efficiency of the antenna system-- even with monopoles electrically as short as used in Part 15 AM (6 degrees or less).
Using even shorter radials would be unlikely to improve or maintain this performance.
http://i62.photobucket.com/albums/h85/rfry-100/BLE_Fig33.gif
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As I was taught, every piece of metal that is the near field absorbs a portion of the energy.
To the extent they have electrical length that begins to have inductance, they should be tied to the common point to return the
energy to permit greater feed current into the radiator.
The more "stray" energy collected, the better.
Any marginal improvement in grounding, via ohmically ( non insulated) longer radials, more radials, AND
tying in plumbling and electrical conduits will be helpful. So is salting your ground rod(s). Add ground rods at the end of radials if
they are elctrically short. It can't hurt, especially if you're using insulated wire.
Size of signal makes no difference, except that when there's almost nothing to start with, efficiency of radiation is critical.
This doesn't mean that it will be necessarily noticeable, but should be measureable with a proper FIM.
To the extent they have electrical length that begins to have inductance, they should be tied to the common point to return the
energy to permit greater feed current into the radiator.
The more "stray" energy collected, the better.
Any marginal improvement in grounding, via ohmically ( non insulated) longer radials, more radials, AND
tying in plumbling and electrical conduits will be helpful. So is salting your ground rod(s). Add ground rods at the end of radials if
they are elctrically short. It can't hurt, especially if you're using insulated wire.
Size of signal makes no difference, except that when there's almost nothing to start with, efficiency of radiation is critical.
This doesn't mean that it will be necessarily noticeable, but should be measureable with a proper FIM.
It's always been my belief that "resonance" is key to antenna efficiency.
Quote.. Merriam-Webster.. Resonance..
"(1)a vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration period of the system."
It was also my belief that the short antenna system used or designed for a PART-15 xmtr, electrically, closely matches a half wave antenna to acheive proper, efficient feedpoint impedance per frequency. (Which is at the higher end of the AM spectrum)
Quote.. Merriam-Webster.. Impedance..
"The apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current"
The length of the ground radials would, in my mind thinking basic antenna theroy, have an effect on the efficiency of the final output and for the amount of radiated energy to be at its maximum.
Isn't the final output section of a PART-15 AM xmtr. designed to find a "resonance" point per frequency? Doesn't the xmtr. need input from the ground area at the highest possible amplitude to obtain oscillation and have the ability to reproduce itself with the least amount of effort?
I don't just see the ground plane as a source of return currents but also a source of return frequency at the highest amplitude possible, which would be achieved mostly by the ground area being somewhat tuned to the frequency being transmitted in length.
Different lengths of wire causes timing change. Any length of wire has its resonance point. If a wire is "resonante" per any given frequency, is creates a higher amplitude of induced voltage/current.
The transmitted frequency/energy flows over the ground plane. The ground plane reacts as the other half of the antenna system to feed the collected radiated frequency/energy back to the xmtr at 180 degrees out of phase to achieve a resonance point with the antenna tuning circuit. This is basic antenna theroy.
I believe that if the ground area/radials were somewhat "Tuned" by length, per frequency, a stronger return would go back to the xmtr. to help gain resonance, which would make the final amplifier more efficient. It wouldn't matter about the amount of power being transmitted. Everything must be in "time" to achieve maximum efficiency.
Even though the level of power used in a PART-15 xmtr. is low, it still takes the same amount of time to pass over any given ground area as it exits the radiator into space. The ground area is still going to react the same as with using 1 KW. output but not with the same amount of amplitude of returned current. If the ground area is close to resonance, it will return currents to the xmtr. at the highest possible amplitude and in proper phase/time.
I believe uninsluated wire would be the best. It would have conductivity to the surrounding earth to help induce return currents. And I also believe that the lenght should be somewhat resonant to the transmitted frequency per the electrical length of antenna being used.
Quote.. Merriam-Webster.. Resonance..
"(1)a vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration period of the system."
It was also my belief that the short antenna system used or designed for a PART-15 xmtr, electrically, closely matches a half wave antenna to acheive proper, efficient feedpoint impedance per frequency. (Which is at the higher end of the AM spectrum)
Quote.. Merriam-Webster.. Impedance..
"The apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current"
The length of the ground radials would, in my mind thinking basic antenna theroy, have an effect on the efficiency of the final output and for the amount of radiated energy to be at its maximum.
Isn't the final output section of a PART-15 AM xmtr. designed to find a "resonance" point per frequency? Doesn't the xmtr. need input from the ground area at the highest possible amplitude to obtain oscillation and have the ability to reproduce itself with the least amount of effort?
I don't just see the ground plane as a source of return currents but also a source of return frequency at the highest amplitude possible, which would be achieved mostly by the ground area being somewhat tuned to the frequency being transmitted in length.
Different lengths of wire causes timing change. Any length of wire has its resonance point. If a wire is "resonante" per any given frequency, is creates a higher amplitude of induced voltage/current.
The transmitted frequency/energy flows over the ground plane. The ground plane reacts as the other half of the antenna system to feed the collected radiated frequency/energy back to the xmtr at 180 degrees out of phase to achieve a resonance point with the antenna tuning circuit. This is basic antenna theroy.
I believe that if the ground area/radials were somewhat "Tuned" by length, per frequency, a stronger return would go back to the xmtr. to help gain resonance, which would make the final amplifier more efficient. It wouldn't matter about the amount of power being transmitted. Everything must be in "time" to achieve maximum efficiency.
Even though the level of power used in a PART-15 xmtr. is low, it still takes the same amount of time to pass over any given ground area as it exits the radiator into space. The ground area is still going to react the same as with using 1 KW. output but not with the same amount of amplitude of returned current. If the ground area is close to resonance, it will return currents to the xmtr. at the highest possible amplitude and in proper phase/time.
I believe uninsluated wire would be the best. It would have conductivity to the surrounding earth to help induce return currents. And I also believe that the lenght should be somewhat resonant to the transmitted frequency per the electrical length of antenna being used.
Ermi Roos said:
In that case, I would expect that the use of a capacity top hat would improve the efficiency of a Part 15 station, since it is possible to reduce the inductance required to tune the antenna to resonance very significantly. This technique has been used effectively by 160m DX'ers.
A capacitive hat would not only reduce the loading coil inductance required for resonance (and thus reduce the loading coil loss resistance), but it would also increase the radiation resistance of the antenna system. This would provide a considerable increase in range. It is not known, however, if the capacitive hat diameter would be added to the 3 meter antenna component length budget. I am not aware of any enforcement actions related to capacitive hats. Maybe Offgridkindaguy has more information.
You're probably right, Offgridkindaguy, but it would be interesting to learn about somebody with a capacitive hat who was inspected. The difficulty in finding out is that a lot of enforcement actions don't escalate to the level of a NOUO, and so they are not published on the FCC website.
What I would like to know is.. How many set ups out there with a good ground area and legal power and everything else "legal" in the eyes of an inspector, actually comes close to the allowable field strength levels? It must be possible to get enough efficiency to exceed this level with an extended ground wire because it happens all of the time. A hat would do the same thing!
Them 'ol boys that set the standards sure knew what they were doing. We're pretty much screwed! What's the magic set up that would "push the envelop", so to say? This seems to be what everyone is after..
Them 'ol boys that set the standards sure knew what they were doing. We're pretty much screwed! What's the magic set up that would "push the envelop", so to say? This seems to be what everyone is after..
Ermi Roos said:
If the loss resistance of a good loading coil is 5 ohms, then the above statement means that the ground loss resistance would have to be less than that.
However as many Part 15 AM setups are installed in areas where the ground conductivity is 10 mS/m or less, and referring to the quote from a post you made on another website in April, 2008 (below), it appears that the ground loss resistance would be greater than 5 ohms in those cases.
"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 data apply to the use of 180 radials, which few Part 15 AM users would install. Using a more practical number such as 20 or 30 radials would increase those losses, no doubt.
Is the 5 ohm loss assumption for a good loading coil too low a value? Does a modest ground screen have significantly lower loss resistance than 180 radials?
Could you clarify this for us please?
//
Offgridkindaguy said:
It is only part of the equation for the radiation efficiency of the antenna system. For both a naturally-resonant monopole antenna (no reactance at its feedpoint), and an electrically short radiator using a (fictional) zero-loss "loading coil" to produce no reactance at its feedpoint -- the amount of applied power that either one will radiate is still dependent on the loss resistance presented to the r-f currents returning through the earth/radial paths to the transmit system.
If that ground resistance is zero, then all of the available power is radiated. If it is infinite, then none of the available power is radiated.
As for some explanation of the functioning and performance of buried radials and ground rods, below is a clip on this topic from another website, including my response to the quoted statement:
Ermi Roos said:
Just to point out that even if such a pipe had zero ohms of loss to the earth in contact with it, that doesn't mean that it would present a zero ohm loss to the r-f earth currents returning to the antenna system from as far as 1/2-wavelength from the antenna. Those r-f currents would be greatly attenuated by earth losses before arriving at the pipe, and the net r-f ground resistance presented to the antenna system would not be zero ohms. Such r-f loss is in series with the antenna current, and therefore will reduce system radiation efficiency.
An effective, buried r-f ground needs to gather the induced r-f earth currents physically as close as possible to where they enter the earth, which is the reason that AM broadcast stations install 120 or more radials of 1/4-wave or longer each. This was all studied and documented in 1937 by Brown, Lewis and Epstein of RCA Labs, and remains the present practice of AM broadcast stations.
//
There isn't any maximum allowable field strength limit in Section 15.219, but there is a physical limit of 100,000 uV/m at 30 m in the unlikely case that 100 % of the 100 mW applied to the final stage of the transmitter is converted to radiated RF power. Because the short antenna that is allowed is very inefficient, the system efficiency for a ground-mounted transmitter is typically in the vicinity of 0.05 %, or so. This would give a field strength of about 2200 uV/m, which can give about a quarter mile of range.
As for loading coil loss resistance, 5 ohms is very unlikely because that implies a Q of about 600. Such a high Q simply does not occur in a practical Part 15 system. Overestimating the Q of a loading coil is common because computer programs for calculating the Q of coils tend to be unrealistic. Also, when mounted, the loading coil tends to lose Q because the fields from the coil pass through other conducting objects.
A ground screen of 180 radials is easily simulated by solid metal, or wire mesh, if the radius is only 1.8 m. Also, the figures cited were based on Trainotti's published data for an antenna height of .07 wavelength. (Since Trainotti's specialty is designing short antennas for licensed AM broadcasting, he considered .07 wavelengt, which is about 12 meters at 1.7 MHz, to be "short"). That was the best information that was available to me at the time I posted the data. A 3 m antenna at 1.7 MHz has a height of only .017 wavelength, which is much smaller than .07 wavelength. As I posterd earlier in this thread, the ground resistance is lower the lower the antenna height is. So, the real ground resistance is actually considerably lower than what I reported in the cited post. Also, making the radius of the ground screen 3 m, which is the height of the Part 15 AM antenna, would reduce the ground loss resistance even further.
Prof. Trainotti saw my post on part15.us at the time that I posted it, and he contacted me by e-mail. As a result of our discussions, he agreed to calculate ground resistances for 3 m antennas for various ground screen radii. This data is very useful for anyone contemplating constructing a Part 15 AM ground screen. It can be found by searching the part15.us website.
As for loading coil loss resistance, 5 ohms is very unlikely because that implies a Q of about 600. Such a high Q simply does not occur in a practical Part 15 system. Overestimating the Q of a loading coil is common because computer programs for calculating the Q of coils tend to be unrealistic. Also, when mounted, the loading coil tends to lose Q because the fields from the coil pass through other conducting objects.
A ground screen of 180 radials is easily simulated by solid metal, or wire mesh, if the radius is only 1.8 m. Also, the figures cited were based on Trainotti's published data for an antenna height of .07 wavelength. (Since Trainotti's specialty is designing short antennas for licensed AM broadcasting, he considered .07 wavelengt, which is about 12 meters at 1.7 MHz, to be "short"). That was the best information that was available to me at the time I posted the data. A 3 m antenna at 1.7 MHz has a height of only .017 wavelength, which is much smaller than .07 wavelength. As I posterd earlier in this thread, the ground resistance is lower the lower the antenna height is. So, the real ground resistance is actually considerably lower than what I reported in the cited post. Also, making the radius of the ground screen 3 m, which is the height of the Part 15 AM antenna, would reduce the ground loss resistance even further.
Prof. Trainotti saw my post on part15.us at the time that I posted it, and he contacted me by e-mail. As a result of our discussions, he agreed to calculate ground resistances for 3 m antennas for various ground screen radii. This data is very useful for anyone contemplating constructing a Part 15 AM ground screen. It can be found by searching the part15.us website.
Ermi Roos said:
A few observations that might prompt some further thought and comment...
A 3-m monopole for Part 15 AM is not intrinsically inefficient, it just has a very low value of radiation resistance. It will produce EM radiation from virtually all of the r-f current that can be made to flow on it. However that low radiation resistance means that the antenna system efficiency will be very low, because much of the available power from the transmitter is dissipated in the larger resistances of the loading coil and r-f ground.
As for the typical antenna system efficiency of a ground-mounted 3-m monopole being about 0.05%, the coil loss resistance being higher than 5 ohms, and the ground loss resistance being lower than what you reported earlier -- if we analyze a 1700 kHz Part 15 system using a 15 ohm coil loss and a 2 ohm ground loss, the antenna system radiation efficiency is about 0.7% (see link below). That is 14 X better antenna system efficiency than expected from the quotations above.
http://i62.photobucket.com/albums/h85/rfry-100/Part_15_AM_Roos_Comments.gif
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