R-F Grounds

[rfry]

Much interest is shown about ground systems on various Part 15 boards -- what they are, and why they are important. The following comments are a bit technical, but then this is a technical subject. Hopefully the comments will have some useful bits for most readers.

Monopole radiators such as used by licensed and unlicensed transmit systems in the AM broadcast band need a good "r-f ground" in order to radiate as efficiently as possible.

The transmitter provides a source for the r-f current, and the r-f ground provides the completion of the path caused by radiation from the monopole, back into the ground side of the transmitter circuits.

A monopole antenna system can be thought of as a closed circuit where energy circulates back and forth at an r-f rate between the radiator and the r-f ground connection. The path between a monopole and an r-f ground is produced by the capacitance of the monopole to the r-f ground, across which path displacement currents flow.

Those displacement currents become conducted currents at, and just below the surface of the earth out to about 1/2-wavelength from a monopole, regardless of the monopole height in wavelengths. Once those currents enter the earth they need to be conducted back to the ground terminal of the transmitter in order to complete the path needed for r-f current to flow in the antenna system.

Soil is a rather poor (lossy) conductor of radio waves, so a system of buried radial wires often is used to provide a low-resistance path back to the transmit system in the area of the earth where those currents are highest -- within 1/2-wavelength radius of the monopole.

Carefully done physical experiments back in the 1930s determined that an r-f ground consisting of about 120 buried radials spaced 3 degrees apart around the tower base, and each radial about 0.4 wavelengths long (in free space) would produce an antenna system that radiated about 95% of the power applied to it for monopole heights of about 45 degrees or more.

Such an r-f ground is the norm for licensed AM broadcast stations, but beyond the practical range of most "Part 15" stations.

Also - it should be noted that an r-f ground consisting of one or more "ground rods" buried vertically at or near the base of a monopole, or a few buried radial wires of any length do not constitute a good r-f ground. The r-f resistance of such paths is very high to the r-f earth currents surrounding the monopole, because they are forced to travel long paths through the lossy earth from up to 1/2-wavelength away to reach those conductors.

For reference, the r-f resistance of a "broadcast type" buried radial system is 2 ohms or less, while the r-f resistance of a few buried ground rods or wires may be 50 ohms or more. This added loss makes a big difference in the percentage of available r-f energy that will be radiated by a 3-meter monopole in the AM broadcast band.

Note that the r-f grounds discussed here do not exist along the length in space of any wire or other conductor such as a tower, billboard, water tower etc that is connected to them. All of those conductors will radiate into free space as a result of the r-f current flowing along them. And by definition, an r-f ground does not and cannot radiate.

RF

 

[Tom Wells]

But isn't it true that all steps made to more effectively collect these currents by lowering RF ground resistance will be useful?
As more and more wiring, conduit, water piping, underground water piping, multiple remote ground rods, etc, become bonded to the ground system, won't all these incrementally improve the return of current to the common point?

 

[rfry]

As more and more wiring, conduit, water piping, underground water piping, multiple remote ground rods, etc, become bonded to the ground system, won't all these incrementally improve the return of current to the common point?

Conductors that are not lying on, or buried in the earth less than a foot or so, and/or are not present there within a circular area having a radius of 1/2 wavelength from the monopole will not be very effective in collecting earth currents caused by radiation from the monopole. This will be true even if those conductors eventually connect to the r-f ground terminal of the transmitter -- which may not be the case.

Wires, conduits and metallic water pipes in a structure can serve as a kind of counterpoise for a 3-meter monopole if they all have a conducting path back to the r-f ground terminal of the transmitter. But the random physical orientations of those conductors means that the r-f current they can provide back to the transmitter will be reduced compared to having those conductors in an optimum physical orientation.

The link below shows an example of a 3-meter monopole using a small, planned counterpoise. The system has no physical conductors connecting to the earth, and is modeled in an open area with no other above-ground conductors. But the fields it produces are somewhat higher than when using no counterpoise, and connecting the transmitter r-f ground terminal using a very short wire to an earth ground having an r-f resistance of 30 ohms or so (several ground rods near the antenna base).

Random combinations using counterpoise conductors in a structure along with direct connections to an earth ground via ground rods produce unpredictable results.

http://i62.photobucket.com/albums/h85/rfry-100/Monopole_with_Base_Radials.gif

RF

 

[druidhillsradio]

Rich, thanks for your logical discussion about R-F Grounds. Based on your presentation can I assume that a Rangemaster for example, mounted at or near ground level, using the 120 buried 1/4 wave radials, will out-perform a tower mounted arrangement similar to the system that got KENC radio into difficulties with the FCC?

 

[rfry]

From NEC-2D models:

• Frequency = 1.62 MHz
• Power = 35 milliwatts with Perfect Z Match to Antenna System Feedpoint Terminals
• Earth Conductivity = Perfect
• Matching Coil Loss = 10 ohms

CASE 1:
Transmitter with 3-m whip attached at top of 40' Tower, using a 40-foot conducting path from the tx chassis to a 25 ohm r-f ground connection (several ground rods at the base of the tower)

CASE 2:
Transmitter with 3-m whip attached installed with bottom of tx 4" above the earth, with a 4" conducting path from tx chassis to a 2 ohm r-f ground connection such as provided by 120 x 1/4-wave buried radials around the base of the system

Groundwave Field Intensity, µV/m

Distance (ft) CASE 1 CASE 2
500 5,006 1,063
750 3,379 716
1000 2,545 539
1250 2,040 432
1500 1,702 360
1750 1,460 309
2000 1,278 270
2250 1,136 240
2500 1,023 216
2750 930 197
3000 853 180
3250 787 167
3500 731 155
3750 682 144
4000 640 135
4250 602 127
4500 569 120
4750 539 114
5000 512 108
5250 487 103

RF

 

[druidhillsradio]

Thanks. If I am understanding correctly, the 25 ohm ground connection enhances the 40 foot wire's ability to radiate since the wire is not really at ground potential when compared to the ground mounted system. Correct? TNX.

 

[rfry]

Thanks. If I am understanding correctly, the 25 ohm ground connection enhances the 40 foot wire's ability to radiate since the wire is not really at ground potential when compared to the ground mounted system. Correct? TNX.

Not quite. Every conductor exposed to space, and carrying r-f current will radiate regardless of the series resistance in that path (as long as that is not infinite). In fact, if that resistance was zero, then even more of the available r-f current would flow along that wire rather than heating the earth, and useful radiation from the system would be maximized.

RF

 

[Don2]

I have a question that I've never received a clear answer regarding a ground system under an antenna.

Scenario.. Ground mounted Rangemaster or whatever..

Let's say I have a Rangemaster, ground mounted, with a ground area made of lengths of wire to supply return currents to the xmtr to achieve some kind of efficiency from the short radiator..

IF the length of the ground radials used in the ground area were close to resonance of the chosen frequency, would there be better current returns to the xmtr than some random length of wire in the near field? (Full Wavelength? 1/2? 7/8?.. Not just 10')

I understand that the far field currents will be very low but wouldn't a length of wire that's "close to" or "at resonance" provide a higher return amplitude in current to the xmtr than a wire not in or even close to resonance?

Just curious..

Thanks in advance..

 

[rfry]

IF the length of the ground radials used in the ground area were close to resonance of the chosen frequency, would there be better current returns to the xmtr than some random length of wire in the near field? (Full wavelength? 1/2? 7/8?.. Not just 10')

The resonance of a monopole is determined by its physical dimensions and configuration with respect to the wavelength, independent of the r-f loss in the ground system it uses.

Buried radial wires collect the r-f currents radiated by the antenna that exist in the surface of the earth in a circular area having a radius of about 1/2 wavelength (in free space) around the base of the monopole, and deliver them back to the r-f ground terminal of the transmitter. This completes the series circuit necessary for r-f current to flow on the antenna system.

Using 120 buried radials that are 1/2-wavelength long in free space collect and deliver back to the tx almost all of the r-f current in that circular area not because they are resonant, themselves* or cause the monopole to become resonant, but because of the density of those wires in that area, and the low resistance path they provide to the r-f current flowing in them. When using this ground system a monopole of about 45 degrees and more in height can radiate about 98% of the power available from the transmitter.

Extending the buried wires past 1/2 wavelength from the monopole has little affect on antenna system efficiency, because most of the energy beyond that distance is in the radiated wave above the surface of the earth.

* they're not, because radio waves travel slower along a buried wire than one in free space

RF

 

[druidhillsradio]

Rich, all that being said, based on your professional knowledge, what would your compliant Part 15 station's arrangement be assuming a Rangemaster or othe FCC Certified transmitter be. I am not trying to ambush you, but to maybe help me and others understand the physical relationships to FCC rules and a safe installation. Thanks in advance, John

 

[rfry]

Rich, ...what would your compliant Part 15 station's arrangement be assuming a Rangemaster or othe(r) FCC Certified transmitter be.

Different than that needed to provide the greatest field intensity at the greatest distance with the goal of serving the general public and/or making a profit from such an installation. My only need is to provide a short-range radio link from various audio devices to receivers in my own home, which I think is closer to the purpose that the FCC intended for the AM/FM broadcast bands when it wrote Part 15. That limited range does not need an impedance-matched antenna system with a low resistance r-f ground return, and an "elevated" mount.

Those who want maximum coverage while remaining functionally compliant with Part 15.219 could ensure that the input power to the final r-f stage of their transmitter does not exceed 100 mW, and that the radiating length of their antenna system including the entire conducting path from the transmitter r-f ground terminal/chassis to a real r-f ground (something buried in the earth) does not exceed 3 meters.

RF