Ground radials - questions

Some detailed information on this subject has been posted on various sites in the last month or so. Maybe search the Radio-Info Community forum for the biggest share of it.

Answers to your questions:

1. NO.

2. YES. Ground rods are there for safety to control AC/DC service faults and to direct lightning energy to terra firma. The best installation is 3-4, 8 foot long copper coated steel ground rods driven below the surface of the earth approx. 12 feet apart, all connected by the thickest or heaviest conductor possible (copper ground strap is best due to less inductance than wire.)

3. Ground radials, either elevated above ground or below the surface, work more uniformly if they cover 360 degrees around the antenna. However, understanding the ground potential of the radials is what the vertical antenna works against, any ground radials 5% longer than the actual length of the antenna is sufficient to raise the radiation efficiency of the antenna. The ground radials reduce ground losses in the antenna system.

4. The proper measurement, to determine if an antenna system is working at its design maximum, is to measure the feedpoint amperage (current) and tune the system for maximum. If you tune the feedpoint for maximum voltage, the high impedance node of the antenna would be at the wrong spot. Maximizing feedpoint current with maximum legal transmitter output power should produce maximum field strength away from the "near field" of the antenna.

A current copy of the ARRL Antenna Handbook would be a very good $35.00 investment for your station.

Below for consideration are some comments prompted by parts of the post of kk7cw.

Ground radials, either elevated above ground or below the surface, work more uniformly if they cover 360 degrees around the antenna.

Radials elevated more than a few feet above the ground, and in the horizontal plane, behave much differently than when they are buried. Dr. George Brown, the inventor of the ground-plane antenna stated that only two ~1/4-wave radials extending in opposite directions from the base of an elevated, 1/4-wave vertical radiator gave the same free-space pattern gain/shape as for a 1/2-wave dipole, and that using more than two elevated radials gave no benefit. NEC modeling also confirms this.

However, understanding the ground potential of the radials is what the vertical antenna works against, any ground radials 5% longer than the actual length of the antenna is sufficient to raise the radiation efficiency of the antenna.

The true function of ground radials is to collect the r-f currents induced into the earth by radiation from a vertical antenna. These earth currents exist out to a radius of at least 1/2 wavelength, regardless of the antenna height, and need to be delivered back to the "ground" terminal of the tx+antenna system in order to maximize radiation from the vertical radiator.

A few ground rods around the base of the antenna will provide a connection to "ground potential," but they will be a lossy means of collecting the r-f currents at distances of 1/2-wave and more from the antenna -- which have to travel that distance through a poorly conducting earth to reach them.

Field studies done in 1937, and verified in thousands of installations since prove that the radiation efficiency of even an electrically short antenna will benefit from using a large number (up to 120) of buried radials of longer length (up to ~1/2-wave) than the antenna height itself.

Those same field studies also show that if only a few buried radials are used, they may as well be "short" (maybe 0.1-wavelength), because the additional r-f current that those few radials would collect if longer is very small. But of course a ground system using only a few short, buried radials is far less effective than one with more/longer buried radials, regardless of the antenna height.
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Rich is correct regarding the prevailing science and practice for ground systems in shortwave and AM broadcasting. However, if you were to have such an enhanced ground system inspected by the FCC at your Part 15 AM radio station, you should be prepared to supply the technical reasons for operating such a system under Part 15. Simply put, when you get behind the wheel and drive a hundred mph; and then you are stopped by the police, one of the first questions you will be asked, "Did you know you were speeding?" What would your answer be?

Don't over build a Part 15 station. Several postings on this site address the Part 15 issues involved with "legal" ground systems. Unfortunately the explanation you have received so far, has either been either cryptic or overly technical. Keith Hamilton and several others, including William Walker, who have been at this game for years, will tell you to elevate the transmitter/antenna package 20-30 feet in the clear. Install a single ground path (conductor/wire) straight to a ground rod. This system has passed and will pass inspection because the FCC considers this ground system a safety or AC/DC fault ground. This ground system, realistically however, will not act solely as a "safety" ground. Because of the remainder of the transmitting system (antenna/support/feed system and ground) will radiate there by extending range some.

If you are really curious, let me give you some internet resources who will be glad to answer phone calls and discuss "real world" practices.

http://www.am1000rangemaster.com click on "technical"
http://www.WILW.com

And finally, if you really want the answer to your question, call the nearest District Office of the FCC and ask. They are the folks who will ultimately decide whether you are operating legally or not.

Quote=kk7cw:
Rich is correct regarding the prevailing science and practice for ground systems in shortwave and AM broadcasting. However, if you were to have such an enhanced ground system inspected by the FCC at your Part 15 AM radio station, you should be prepared to supply the technical reasons for operating such a system under Part 15.

The FCC does not define the electrical or mechanical configuration for the buried r-f ground used for Part 15 AM applications. It only states that the lead length used to connect to that buried ground has to be included in the 3-meter maximum radiator length as stated in 15.219(b).

And finally, if you really want the answer to your question, call the nearest District Office of the FCC and ask. They are the folks who will ultimately decide whether you are operating legally or not.

True, and here is a recent e-mail statement from the FCC about the use of elevated Part 15 AM antenna systems. A bit of the header is included to show its authenticity.

From: "Raymond Laforge" Raymond.Laforge @ fcc.gov
To: "Richard Fry"
X-OriginalArrivalTime: 07 Dec 2005 14:17:29.0078 (UTC)
FILETIME=[F49FC960:01C5FB38]
X-pstn-levels: (S:99.90000/99.90000 R:95.9108 P:95.9108 M:94.8923
C:98.7678 )
X-pstn-settings: 5 (2.0000:2.0000) s gt3 gt2 gt1 r p m c
X-pstn-addresses: from [1225/54]

This is a multi-part message in MIME format.

------_=_NextPart_001_01C5FB38.F4693A4D
Content-Type: text/plain;
charset="us-ascii"
Content-Transfer-Encoding: quoted-printable

You seek information on the legality of setting these AM transmitting systems on top of billboards and other high, metal structures. Pursuant to 47 CFR 15.219(b), the total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters. Thus, we do not except installations on higher structures that result in violation of Section 15.219.
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Rich...what about on top of a non-metallic tower, building or wood pole? My interpretation of the rule is anytime you connect a "radial" ground system with a downlead of nearly any length onto a Part 15 radio station, the operator runs the risk of being in violation. Many of the manufacturers of certified Part 15 transmitters make a clear point of a single, short downlead, straight to a ground rod. And if elevated above ground on a building or pole, an elevated ground system around 20 feet in length can be used with a very short leadt o the radial system. Metal buildings and billboards are not necessarily sufficiently"earth" grounded to constitute a safety ground. So, the FCC looks at these circumstances as just trying to sidestep the rules. Wood or concrete structures without metal roofing can be used as transmission sites for Part 15 transmitters and is done all the time, especially in commercial applications.

And by the way, thank you for the input from the FCC. Just more of the same confusing rhetoric from the policy-making folks. Yes, the regs are clear, but the practice of inspectors is inconsistent with the rule. Maybe a re-write of the tech requirements for this rule are in order?

they should just re-write the rules for 219. Keep the 3 meter limit for Radiator and coax and the 100mw input.

and just allow what ever ground that the installer wants to use. it will make the community broadcasters happy and can be slipped by w/o NAB or NPR ever getting a whiff of whats going on till it's too late.

Part 15 already contains a Rule about harmful interference to licensee's. this should be enough to counter any interference concerns.

it's already been proven real world over and over that these stations pose no interference potential, even with relaxed ground restriction.

Quote=kk7cw
...what about on top of a non-metallic tower, building or wood pole? My interpretation of the rule is anytime you connect a "radial" ground system with a downlead of nearly any length onto a Part 15 radio station, the operator runs the risk of being in violation.

The issue isn't whether or not the ground system is comprised of buried radials. The issue is the length of the conducting path leading from the tx chassis to the r-f ground plane at physical earth -- no matter what the r-f ground consists of. That conducting path to r-f ground radiates. If it is connected to a good radial ground system, more r-f current flows in that conductor, and it generates a greater r-f field to add to the 3-m section above it. A poorer r-f ground such as provided by a ground rod (or several) has higher loss to the returning earth currents, so less current flows in it, and it radiates less. But regardless of how much it radiates, it DOES radiate. That's why the FCC counts that length in the total 3-meter length allowed for Part 15 AM in 15.219(b).

Many of the manufacturers of certified Part 15 transmitters make a clear point of a single, short downlead, straight to a ground rod. And if elevated above ground on a building or pole, an elevated ground system around 20 feet in length can be used with a very short lead to the radial system.

That is the prevailing viewpoint among many commercial manufacturers of Part 15 txs (certified or not), and their users. It just isn't supported by physics. A 20 foot conducting path from an elevated tx chassis to the r-f ground plane at physical earth by itself forms a radiator over twice as long as allowed by 15.219(b).

And there is no such thing as an "elevated ground system" unless you are talking about elevated, physical radials projecting horizontally from the base of the elevated 3-m vertical, and connected to the tx chassis. In that case, no conductor is needed from the tx to the earth in order for the antenna system to radiate efficiently.

It is incorrect from the viewpoint of physics to think that r-f ground exists at the top of a 20-foot wire leading up from a buried r-f ground. That conductor is actually a part of the antenna, as developed above. Yet this is the popular concept supporting the use of elevated Part 15 AM antennas, in the belief that only the short wire from the tx chassis to the top of the 20-foot wire "counts" in the calculation of radiating length for the antenna. But that is provably incorrect -- the whole conducting path to earth counts: the "ground lead" PLUS the "ground wire" (or tower/flagpole/whatever).

Metal buildings and billboards are not necessarily sufficiently"earth" grounded to constitute a safety ground.

As in the FCC email I posted earlier in this thread, an elevated Part 15 AM installation that results in a violation of 15.219 is not acceptable to the FCC. Calling the ground lead a "safety ground" doesn't change the fact that it carries r-f current, which radiates, making that entire conducting path from the tx chassis to its connection to the r-f ground plane at the surface of the earth part of the radiating length of the antenna system -- and that radiating length cannot exceed 3 meters, altogether.

If an elevated installation of a Part 15 AM antenna plus ground lead/ground wire/safety ground or whatever you wish to call it results in operation beyond the limits contained in Part 15, then that isn't an appropriate place to install it, if your goal is to observe Part 15.
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by MRAM 1500

"If an elevated installation of a Part 15 AM antenna plus ground lead/ground wire/safety ground or whatever you wish to call it results in operation beyond the limits contained in Part 15, then that isn't an appropriate place to install it, if your goal is to observe Part 15."

In this case, beyond the limits contained in Part 15 is only referring to the 3 meter length rule and has no bearing on the range of your signal as range of signal is not defined in Part 15. Of course, when you exceed the 3 meter rule (depending upon how the Field Agent judges your installation), you are no longer operating under 15.219 rather you must comply with the 15.209 field strength.

As pointed out in previous threads on this grounding topic, you can run all the power you want and use the biggest and best antenna you want under 15.209. Just don't exceed the calculated field strength which of course, most of us can't measure. Oh, and don't interfer with any other licensed service.

mram1500:
As pointed out in previous threads on this grounding topic, you can run all the power you want and use the biggest and best antenna you want under 15.209. Just don't exceed the calculated field strength which of course, most of us can't measure.

The above sounds really good until we realize that the field strength possible from a strictly legal, ground-mounted AM system meeting 15.219(b) is many times greater than the field strength permitted by 15.209.

So the result is that you will have to run less power, and/or use a less efficient radiator under 15.209 than under 15.219(b).
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What about common mode currents through the power supply? Is the power lead to the transmitter considered part of the radiating system if it is not RF supressed? Wouldn't the power supply leads also possibly radiate? Would they be included as part of the transmitting system? And what about tuned ground systems that don't radiate? You know, a counterpoise RF ground system. Why is it that RF ground radial systems are buried when they work better on the surface of the earth? Buried, ground radials computer modeled appear more lossy. Where does the entire ground system design begin and end in the entire scope of Part 15 rules or do we have to consider other parts of CFR 47 as well? Where does the FCC term "good engineering practice" play into the grounding issues? And since the FCC doesn't necessarily regulate safety issues anymore, what section of Part 15 deals with the definitive acceptable design of radio station ground systems? In other words, how can an operator of a Part 15 AM radio station, without question, know his installation is "legal" under all circumstances? And will pass inspection "no matter what'? And at what point does the owner of one these stations have the same treatment as shopping centers, car lots and real estate agents from the FCC? Frankly it sounds way too much like NAB/NPR politics to me. But I digress.

We agree that elevated radial ground systems are beyond Part 15 design parameters. We agree the shortest path to ground is the most acceptable. We don't agree about radiating ground systems.

Some RF ground systems DO NOT radiate. Current must flow in a ground system for it to radiate. In checking ground systems for licensed AM broadcast stations, we measure the inductive current flow in each radial to determine if it is "healthy". Low or no current flow means it is operating properly. It also means the impedance in the circuit is extremely high in relationship to the radiating portion (antenna) of the system. That is essentially controlled by Ohm's Law. It is true the antenna system current return is the ground system, however even in AC circuits the neutral side carries no potential until connected to the hot side through the resistive load. Kapish? How does radiation resistance and ground loss play into all of this?
Maybe you could explain to the readers of this thread why no measureable current flows in a properly designed and constructed RF ground radial system and at the same time radiates?

by MRAM 1500 kHz

How many times have Part 15 AM stations been cited by the FCC because they were really causing interference. Reading the posted violations on the FCC site, they always nail someone for exceeding field strength limits which are 15.209. I've never seen one say that a ground wire was to long.

Your grounding system is just the ticket to let the FCC say you aren't 15.219 compliant. At that point you become a 15.209 operation and they use field strength against you. Rich, I guess that's what my point was in my previous post about power and big antennas. Not that 15.209 would have great coverage because yes, a 15.219 could easily have a greater field strength.

When you start covering the better part of a 1 or 2 mile radius, that's obviously not what the FCC had in mind for 15.219. And do they really care?

KK7CW hit the nail when he said "sounds way too much like NAB/NPR politics." You may not be causing interference but the wrong person heard your station. How else would the FCC find you if your signal is clean and clear of other stations.

By the way, does a properly designed ground counterpoise not radiate for the same reason that a top hat doesn't radiate? And if a top hat doesn't radiate, why does the top hat radius figure into the 3 meter rule? I read that somewhere.

I believe everything I read... First I write it, then I read it.

No matter what section of Part 15 you make reference as to direction to the design and operation of your radio station, if the FCC inspectors want to cite your operation they WILL find something wrong. I used to consult the FCC on station violation amelioration.
The inspectors can be pretty petty at times. I guess everybody has a bad day once in a while.

As to counterpoises and top hats, antenna systems are mechanical emulations of basic electronics components in a passive circuit (resistors, capacitors, inductors). For a little something to tweek your Part 15 thinking, consider the Isotron model 200-B antenna for Part 15 stations by The Bilal Company.

http://www.isotronantennas.com/

This antenna emulates a resonating tank circuit with reasonable efficiency. And NO ground to boot. Just mount the transmitter at the feed point and rock n roll. Talkin' about messin' wit yo mind.

No elevated ground, no ground radials, no downlead, no counterpoise and thousands of these are in use in all kinds of conditions and service all over the world. And its only 8 1/2 feet long. HMMMMMMM.

Quote=kk7cwWhat about common mode currents through the power supply? Is the power lead to the transmitter considered part of the radiating system if it is not RF supressed? Wouldn't the power supply leads also possibly radiate? Would they be included as part of the transmitting system?

Yes, all of that is true, and includes the progam audio line as well. But generally the impedance through those paths is quite a bit higher than
througn the "ground wire" from the tx chassis to a good r-f ground in the earth, so they don't have a great affect on radiation. You should consider them if you're a purist, and especially if you elect not to install the ground wire from the elevated tx chassis to a good r-f ground.

And what about tuned ground systems that don't radiate? You know, a counterpoise RF ground system. Why is it that RF ground radial systems are buried when they work better on the surface of the earth?

Most broadcast stations bury them for long-term electrical stability of the antenna system, to minimize mechanical damage to them, and to make the land above them usable for other purposes.

Buried, ground radials computer modeled appear more lossy.

Depends on their number and length. Real-world experience (and computer models) show that using 120 buried radials each at least 0.4 wavelengths long with a 1/4-wave vertical radiator produces a peak field strength that is within a percent or so of the theoretical maximum that a 1/4-wave vertical would have over an infinite, perfectly conducting ground plane.

Where does the entire ground system design begin and end in the entire scope of Part 15 rules or do we have to consider other parts of CFR 47 as well?

Part 15 does not define the design of the r-f ground. It only says in 15.219(b) that the conducting path leading to it is part of the 3-meter length allowed for a Part 15 AM radiator.

Other FCC Rules in 47 CFR define the minimum r-f ground system for licensed AM broadcast stations. They do not apply to Part 15 operations, but neither are they prohibited from Part 15 operations.

In other words, how can an operator of a Part 15 AM radio station, without question, know his installation is "legal" under all circumstances? And will pass inspection "no matter what'?

Use a transmitter and antenna system that meets the Rules set out in Part 15. An antenna meeting 15.219(b) must limit its radiating length to 3 meters or less. And as antenna physics proves, that radiating length MUST include the entire conducting path from the tx chassis to whatever comprises the r-f ground plane -- not just to a wire, tower, flagpole or billboard steel connecting to that r-f ground plane.

We agree that elevated radial ground systems are beyond Part 15 design parameters. We agree the shortest path to ground is the most acceptable. We don't agree about radiating ground systems.

I haven't written that the r-f ground, itself, radiates. I have written that a conducting path connecting a buried r-f ground to the tx chassis of an elevated Part 15 tx radiates, and it does. Physics proves it, and so does the experience of a lot of Part 15 operators who choose that mounting configuration because it does give them better "coverage," due to the added radiating length of the antenna.

Maybe you could explain to the readers of this thread why no measureable current flows in a properly designed and constructed RF ground radial system and at the same time radiates?

Your statement is incorrect as it applies to a buried r-f ground (in radial or any other form). The r-f current that flows into the vertical radiator above ground is the sum of all of the induced r-f ground currents collected by the buried r-f ground structure. When that current is flowing through an exposed, conducting path leading from the buried r-f ground to an elevated part 15 tx chassis -- that conducting path radiates. Please refer to antenna textbooks and related technical papers to verify this for yourself.
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