Radial Ground Systems

Oh, Ohoo.. Another "What If".. 😕

Does the diameter of the ground radials make a difference? Or, Does the type of wire make any difference?

I find that electric fence wire is cheap. Most is steel and is galvanized and comes in long lengths.. (1/4 mi,1/2mi.) It would last for a good time in the ground I believe..

So.. What if a ground area was made with this type of wire (120 radials at least .4 wavelength) with a center loaded antenna at 3 meters above the ground. Would it be safe to say that this configuration would be about the best possible? (I have a couple of open acres to play in) 8)

So.. What if a ground area was made with this type of wire (120 radials at least .4 wavelength) with a center loaded antenna at 3 meters above the ground.

If you follow the Part 15 Rules option B which it says:
"The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.",

It means that you can't install radials more than 3 meters long. You can, however, install radials as many as you want.

Regards.

Does the diameter of the ground radials make a difference? Or, Does the type of wire make any difference? I find that electric fence wire is cheap. Most is steel and is galvanized and comes in long lengths.. (1/4 mi,1/2mi.) It would last for a good time in the ground I believe..

Here is a link that may help: http://www.bencher.com/pdfs/00361ZZV.pdf . This paper is written for amateur radio operations, and not everything in it applies to Part 15 AM -- but there is some good basic information there.

Soft-drawn copper is certainly easier to work with, as making good mechanical and electrical bonds between galvanized steel wires is difficult.
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If you follow the Part 15 Rules option B which it says:
"The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters." It means that you can't install radials more than 3 meters long. You can, however, install radials as many as you want.

Probably the FCC and its Canadian counterpart would not view a [u:28b998dbdd]buried[/u:28b998dbdd] radial ground system, or any other [u:28b998dbdd]buried[/u:28b998dbdd] ground conductors as part of the radiating portion of the antenna subject to the 3-meter limit.

Buried radials do not themselves radiate in any way to improve groundwave field strength. They only provide a low resistance path to Earth potential that is used as a reference ground for the 3-meter-long radiator system defined by the Rules.

This low-resistance path to true Earth potential reduces antenna system losses, and THAT is why distant field strength improves when using a good radial ground system.
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I believe this is in reference to the "Ground Lead" from the xmtr to an earth ground. This could be to the mount of the xmtr which should be as short as possible to stay within the "Legal" antenna length total.. Combined with the antenna..

Let's imagine a 6 meter high "Mount" made with whatever (Tower, Pipe) that is "Grounded" to the earth by whatever means. (Ground Rod, "Radials"..hehe) I didn't see anywhere that the "Ground" couldn't be improved. And the fact that the "Radials" would be buried.. out of sight, out of mind, correct? If an 8' ground rod was in plain sight at the base of the "Mount", who would know..

I understand that the field agent may frown on ground radials but I believe they would just tell ya to shut it down..

Let's imagine a 6 meter high "Mount" made with whatever (Tower, Pipe) that is "Grounded" to the earth by whatever means.

A Part 15 AM antenna as defined in the Rules is a vertical radiator somewhat less than 3 meters long, its connecting wire to the r-f output connector of the Part 15 AM tx, and the "ground lead," if used. The Rules say that the sum of those lengths cannot exceed 3 meters.

The Rules do not define what constitutes a "ground lead," or what constitutes ground. This is where a lot of confusion exists.

From a classic r-f system analysis viewpoint, a ground lead must include all conductors that lead from the r-f ground connection of the tx (normally its chassis) to a connecting point on/at the surface of the Earth.

Such conductors can consist of one or more of the following:

- a dedicated wire (insulated or uninsulated) leading from the tx chassis to a ground rod, ground screen, ground radials, cold water pipe, etc.
- a short, small gauge wire connected to the tx chassis, leading to a longer, larger gauge wire connecting to/at physical Earth. Both wires are part of the "ground lead."
- a short, small gauge wire connected to the tx chassis, leading to a grounded, metallic tower. Both the wire and the tower length are part of the "ground lead."
- a path through the shields of cables carrying program audio to the Part 15 tx.
- a path through the power supply connections and circuits for the Part 15 tx.

So the electronic reality is that a 6-meter-high mounting structure with a Part 15 AM tx and a ~ 3-meter radiator on top of it, and having one or more conductive paths of any kind leading from the tx chassis to physical Earth does not, in fact, meet the Rules for Part 15 AM station antenna systems.

Please don't shoot the messenger ❗
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Hello,

I've never experiiment this before. But, is it a good idea to have a big 6-meter diameter copper plate? I don't know if this object exists... but i guess it's very expensive to build this kind of plate.

Regards

Dag-nabbit, Rich.. You sure makin' this Tuff.. 😥

I understand what you are saying. If the field agent wanted to be technical about things, I'll bet there are many that could be busted..

In these terms, this means that the antenna needs to be "On the Ground" to be legal. What a bummer! There's more loss to deal with.. 👿

Rich,

If you have some time, can you comment on this recent post from the Community Radio forum. I am particularly wondering about the statement "buried ground radials need not be longer than the antenna is tall, since a shortened, loaded vertical antenna has a more compact near field".

BEGIN QUOTE---
Also, our buried ground radials need not be longer than the antenna is tall, since a shortened, loaded vertical antenna has a more compact near field. A 6 meter wide shallow-buried square or circle of copper sheet (or 120 three-meter long buried bare copper wires) work better than fewer longer radials.

Buried ground radials don't have to be resonant. As this University of Hawaii Ham Radio web page http://www.chem.hawaii.edu/uham/radials.html explains:

"Ground radials need not be resonant...Ground radials do not need to be much longer than the antenna is tall. A shortened antenna with loading coils will have a more compact "near field" where the majority of the antenna field is. The ground needs only reach out as far as the near field extends. Field intensity drops off with the square of the distance from the base of the antenna...The ground around a vertical monopole type antenna can be viewed as strings of series connected resistors fanning out from the base. The purpose of the radials can be viewed as attempting to short circuit as many of these resistors near the base as possible. This is especially critical very close to the base where RF field density is highest, and its importance drops off quickly beyond 1/8th wavelength from the base of any vertical antenna, where the RF field density per unit area goes down sharply."

"It is important not to confuse this application with elevated ground planes. We are talking about radials that supplement the return of ground currents to the base of the antenna, especially in the near field. They work "in parallel" with the existing earth ground surface to supplement it. Elevated radials are a resonant element and serve a decoupling function and establish a completely artificial ground. They should be resonant, quarter wave wires, but still in fair numbers, probably more than the four usually seen, for best results."

The web page also describes interesting portable set-ups (which could be used for Part 15 AM remote broadcasts) in which they use large metal gluing clamps to connect ground leads to fire hydrants, irrigation pipes, backflow preventers, and large water mains ("These masses of metal act as giant counterpoises as well as being metalic paths for ground currents to supplement soil conductivity. Plus since they are buried in the ground, they have good ground contact over a very large surface area for many wavelengths.") -- Jason
END QUOTE---

Can you comment on this recent post from the Community Radio forum. I am particularly wondering about the statement "buried ground radials need not be longer than the antenna is tall, since a shortened, loaded vertical antenna has a more compact near field". (etc)

The performance of radial ground systems with MW vertical radiators was measured by Dr. George Brown and others (all of RCA Labs) in a landmark study in 1937. The conclusions of the study were adopted by the FCC and many other regulatory agencies, and have been proven accurate in thousands of data measurements ever since.

This study measured the base impedance values of vertical radiators of five heights ranging from 22 to 99 degrees, and their corresponding field strengths when using buried radials of varying numbers and lengths.

Both the base impedance of a vertical radiator referenced to a perfect, infinite ground plane, and the field strength produced by that radiator at a given distance per watt of radiated power can be calculated accurately using appropriate equations.

The study showed for radiator heights of 22 to 99 degrees that a ground system of at least 113 buried radials, each at least 0.41 wavelength long produced measured base impedances and field strengths that were within a few percent of their calculated values for a "zero ohm" ground connection (a perfect connection to a perfect Earth). The field strength from radiators shorter than about 5 degrees falls rapidly from the theoretical maximum, as even this elaborate ground system has losses greater than the natural radiation resistance of these short radiators.

They further found that as the number of radials is decreased, so does their effective length. So radials can be shortened when fewer of them are used. But antenna systems with fewer, shorter radials don't perform as well -- ground system losses rise, which means more tx power is lost in heating the ground, rather than being radiated as a radio wave.

Here is a significant quote from this study: "When the radial wires were 45 feet long, the measured (base) resistance was practically independent of the number of wires. Evidently, most of the earth loss occured in regions beyond the periphery of the ground system."

This study demonstrates that the effectiveness of a radial ground system is independent of the electrical height of the radiator -- which differs from the statements in the Hawaii Ham Radio webpage.
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Question about the 3 meter limit - obviously the radiator is factored in, and the TX is usually directly connected to the radiator which eliminates the transmission line, BUT.... there's something that's confusing me....

is the ground lead legally whatever is connecting the transmitter to physical earth ground, OR, can you have the ground lead stop somewhere well above the physical earth ground (like, for example, almost high enough that you'd have to check with the FAA (even though I'm several miles from an airport, for example) to make sure planes wouldn't be taking your antenna down)?

For example... assuming you had no loading coil and your antenna and ground lead was straight vertical, should you make sure the tip of your antenna is no more than 3 meters plus the thickness of the transmitter box (assuming the ground lead is mounted on the bottom and the antenna is mounted on the top) above physical earth ground, OR, could you put the tip of your antenna about 164', 156'9", 152'10", or 146'6" (about 1/4 wavelength for several various frequencies I'm considering) above the ground, use a 114" antenna, a 4" ground lead to a metal (or something) stake, then the remainder of the distance to physical ground is that stake, then you bury your radial system at physical ground? Would that stake be counted as the ground lead, or would it only be the 4" piece of wire leading from the TX to that stake?

Question about the 3 meter limit ... is the ground lead legally whatever is connecting the transmitter to physical earth ground ...

Yes. That "ground lead" can consist of one or more conductors of any form (wire, tower, metal pole or whatever) that are electrically bonded together so as to provide a continuous conducting path from the ground terminal of the Part 15 tx to a connection at/on the physical earth.

OR, could you put the tip of your antenna about 164', 156'9", 152'10", or 146'6" (etc)

Presumably you would want your Part 15 tx and antenna to have a conducting path leading down the tower to a physical earth ground, which means that the sum of that ground lead and 3-meter Part 15 radiator would greatly exceed the 3 meter limit. Also -- those tall supporting towers may not function properly for their primary use if they are grounded by a Part 15 installation.

The total length of the Part 15 conducting path to a physical earth ground really is part of the radiating portion of the antenna system. If that total, composite "ground lead" conductor length is longer than the "3-meter antenna" portion, it will radiate more r-f field than the 3-meter section itself.
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Hi Everyone,

I will have to disagree about the tower, mast and grounding radials being a consideration in the part 15 antenna specifications. The rules specify ground "lead" and not the grounding system. As proof of that assumption take a look at the commercial and toy products certified by the FCC. Many of them include an alligator clip to connect to ground and the directions even suggest using a cold water pipe or electrical ground of the house or business. Clearly that would suggest that the transmitter designs were accepted with the knowledge that the ground lead would be connected to something much larger. Also I feel if that were a legitimate issue there would be restrictions as to what the ground lead of these transmitters could be connected to.

I think the rules specify a ground lead because in many cases measuring a ground system would be impractical. For instance, how would you measure a solid sheet ground (or metal roof) or a mesh? Comparing the radius of a mesh or solid ground to similar length radials would not be fair to those using radials as the combined length of the radials would be much longer than the radius of a mesh or solid, yet not have anywhere near the performance.

I believe that if the FCC was truly concerned about ground systems, they would have written rules that were precise and detailed. A ground lead connects to ground. It does't say that the lead is the entire grouding system. Their biggest concern about part 15 would be interference to licensed stations. For those of us using legal power and being diligent in selecting a frequency, there's no chance of that.

I will have to disagree about the tower, mast and grounding radials being a consideration in the part 15 antenna specifications. The rules specify ground "lead" and not the grounding system.

You are not disagreeing with me, though, because I did not state that the ground radials were part of the 3-meter limit on antenna length. Here is a paste of my statement from earlier in this thread:

"Probably the FCC and its Canadian counterpart would [b:2e84867163]not[/b:2e84867163] view a buried radial ground system, or any other buried ground conductors as part of the radiating portion of the antenna subject to the 3-meter limit.

Buried radials do not themselves radiate in any way to improve groundwave field strength. They only provide a low resistance path to Earth potential that is used as a reference ground for the 3-meter-long radiator system defined by the Rules."

This low-resistance path to true Earth potential reduces antenna system losses, and THAT is why distant field strength improves when using a good radial ground system. "
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