Are Buried Quarter-Wave Radials Still Mandated By The FCC For AM (Medium Wave) Stations ?

[Paul_WB5AGF]

Greetings; (I'm new to this group.)

Does the FCC allow the use of elevated radials for AM (Medium Wave) station antennas or are 120 quarter-wavelength buried radials still required ?

Regards;

Paul (WB5AGF)

 

[davideduardo]

Greetings; (I'm new to this group.)

Does the FCC allow the use of elevated radials for AM (Medium Wave) station antennas or are 120 quarter-wavelength buried radials still required ?

Regards;

Paul (WB5AGF)

Counterpoise systems can be used in cases where the tower is on top of a structure. Or where a structure or set of structures has been built on the property.

A good example is 1020 and 1150 in Los Angeles. The towers had a warehouse center built around them. Where there are buildings, the ground system lies across the roof. Over parking areas, the ground system is at roof level in an interlocked "net". Only at the edges of the property does the ground system go "underground".

 

[PTBoardOp94]

To the best of my understanding, elevated radials are permissible. However, if you use elevated radials they must be fenced in or otherwise inaccessible, and the antenna will receive extra scrutiny from FCC staff.

 

[davideduardo]

To the best of my understanding, elevated radials are permissible. However, if you use elevated radials they must be fenced in or otherwise inaccessible, and the antenna will receive extra scrutiny from FCC staff.

Elevated counterpoise radials are grounded, even if on a roof. There is no danger in touching them.

I've never looked for regulation data on elevated counterpoise so perhaps someone who has actually worked on one of these systems can add further comments.

 

[rfry]

... perhaps someone who has actually worked on one of these systems can add further comments.

Below is a NEC4.2 analysis showing the current/voltage distribution along the conductors of an elevated monopole antenna driven against 4 x 1/4-wave radials.

RF voltages are maximum at the open ends of each conductor, even though the common point of the radials and the base of the vertical all are connectd by Wire #6 to the ground plane beneath them.

(Antenna system conductors are nearly at Earth potential for direct current, but not alternating current — including at radio frequencies).

 

[bigdonho]

I have seen counterpoises used in two locations. One is in the Florida Keys where fluxuating tidal levels caused a change in the base impedance. The other was esentially the same thing where varying water levels in the adjacent river caused changes. In both cases the counterpoise reduced the fluxuations.

 

[davideduardo]

Below is a NEC4.2 analysis showing the current/voltage distribution along the conductors of an elevated monopole antenna driven against 4 x 1/4-wave radials.

RF voltages are maximum at the open ends of each conductor, even though the common point of the radials and the base of the vertical all are connectd by Wire #6 to the ground plane beneath them.

(Antenna system conductors are nearly at Earth potential for direct current, but not alternating current — including at radio frequencies).


View attachment 2888

Question: the systems I have seen on rooftops had radials about 8' above the roof, and at the four corners of the building there were metal "posts" holding a larger stranded copper wire surrounding the rooftop. One post was grounded to true earth by a copper cable down the side of the building. As there were different length radials based on the building being square, would there be varying currents at different points or on different radials even if they were soldered at to the surrounding common square "loop"?

 

[rfry]

... would there be varying currents at different points or on different radials even if they were soldered at to the surrounding common square "loop"?

RF currents will flow along the entire exposed lengths of all conductors connected to the "grounded" square loop setup described in the clip above. They can/may vary from each other, but each such path is a radiating part of that complete antenna system.

The electrical characteristics of "true earth" at radio frequencies do not exist anywhere along the lengths of such conducting paths not buried in the Earth.

 

[davideduardo]

RF currents will flow along the entire exposed lengths of all conductors connected to the "grounded" square loop setup described in the clip above. They can/may vary from each other, but each such path is a radiating part of that complete antenna system.

The electrical characteristics of "true earth" at radio frequencies does not exist anywhere along the lengths of such conducting paths not buried in the Earth.

Thank you. I learned something today I had not understood correctly in the past.

What if a counterpoise were not grounded itself? Would that make the tower and the ungrounded counterpoise work like a dipole? (Your answer, of course, will make me ask about radiation characteristics.)

 

[rfry]

You're welcome, David.

 

[rfry]

What if a counterpoise were not grounded itself? Would that make the tower and the ungrounded counterpoise work like a dipole? (Your answer, of course, will make me ask about radiation characteristics.)

That configuration would comprise a conventional "Ground Plane" antenna, which has a radiation pattern in free space very similar to that of a center-fed dipole in free space.

However installing it atop a building modifies that radiation pattern based on the operating frequency, the height of its radiation center above the Earth, and the effects of nearby re-radiators such as the steel framework, ductwork, conduits and wiring of a multi-story structure on which it may be mounted, and/or that are near the transmit site.

 

[pianoplayer88key]

What about stations like KFBK and KSTP (and maybe also WHO and I think I heard KNBR may be using something similar)?
Those stations (well at least KFBK and KSTP on day pattern), I believe, don't use ground radials at all, but instead use "Franklin" antennas, or big center-fed dipoles that, in the case of KFBK, are a full wavelength tall, and for KSTP, nearly so. (I'm guessing David might have touched on that type in his most recent post in the thread...)

Also a couple more thoughts in the spoiler, would be nice to know answers to them but they're not as important / relevant.

Spoiler

Could the same concept be used for shorter towers, and combined with self-supporting towers (so you wouldn't need all the land area for the guy wires or ground radials), in areas where real estate is at a premium or not readily available in large enough lots? (For example two quarter-wave elements stacked on top of each other, fed at the center - how much different would that be vs a half-wave or a quarter-wave over a ground plane?) I'm guessing there's a financial reason or maybe an NIMBY reason why I've never heard of that specific type of configuration being used, but is there a technical reason why it's not used?
(And (again, just from an engineering, not financial/NIMBY perspective), what is the most "efficient" type of antenna known to exist? As in, it produces the highest mV/m field at 1 km for 1 kW of transmitter power. Also on the "other end" of the spectrum, what about a legally-compliant 15.219 setup (if I'm remembering the rule right, I've been out of the radio hobby/etc for a bit) on, say, 540, 830, 1120, 1410 and 1700? How much difference does a full size ground plane vs a small one make?)

 

[OLDRFGUY]

I have had several AM's with counterpoise grounds. One was in Kentucky where the terrain was exposed rock (it was a 2 tower DA).

The wires were essentially just laid on the surface. They were rather vulnerable. A station in Wheeling, WV had a similar system for the same conditions. Most of it was stolen in the late 70's.

 

[RadioFan2J3]

The three tower USAGM MW site in Djibouti had a full ground system, 120 radials per tower, and because of the extreme rocky site, the radials, well, when I was last there a few years ago, were on the surface.