Vertical Monopoles using a Counterpoise vs. Buried Radials

[R. Fry]

Hello, all -

The exchange below was posted on another board, but it might be interesting here as well (discussion invited).

The kickoff was provided by a description at this link: http://www.kiss-ssb.com/testimonials.html

RF (ex-K8CBZ, General; license not renewed after 1967 due to other pursuits in electronics)
__________________

The bigger the counterpoise in the form of the number of radials will always be more efficient than 1 radial in collecting antenna return currents.

The wires in an elevated counterpoise do not collect returning antenna currents. The source for the total amount of r-f current that flows on them is the return side of the transmission line. An elevated counterpoise needs to have only a few resonant wires, compared to using buried radials (see the NEC analysis I linked to earlier).

Buried radial wires are not resonant. The v.p. of those conductors when buried is far slower than when they are in free space. They do serve to collect the r-f current flowing in the ground plane. Those ground currents are produced by the displacement currents flowing across the capacitance of the monopole to the earth, and for efficient radiation need to be returned to the transmit system with minimal loss. The paper linked below was made for a "Part 15" board, but should help illustrate this point.

Those r-f currents are present on and somewhat below the surface of the earth from the base of the monopole to a radius of about 1/2 of a free-space wavelength. To collect them rather efficiently the buried radial system needs to use about 120 evenly-spaced conductors, each at least 1/4-wave long in free space wavelength. This was proven experimentally by Dr. George Brown of RCA Labs back in 1937, and that finding has been used by the FCC ever since as a baseline for the performance of AM broadcast stations.

Dr. Brown also invented the ground plane antenna, which is a vertical monopole driven against 3 or 4 elevated quarter-wave conductors -- which form a counterpoise for the vertical section.

A vertical monopole using a counterpoise does not require or use the r-f current flowing in the earth (or sea water) around the antenna site to achieve efficient radiation. The radiation it "launches" is independent of the earth, but reflections of that radiation from the earth will affect its far-field radiation pattern.

While a system of 120 x 1/4-wave buried radials and an elevated counterpoise permit essentially the same radiation pattern and gain from the monopole section, they do it in different ways.

Probably the "600 feet" of wire that bothers some in this thread is a bit of marketspeak, in that the total length of all the series-loaded conductors for the various bands might add up to that length if all those conductors had the physical length to be self-resonant (guessing).

http://i62.photobucket.com/albums/h85/r ... iagram.gif

RF

[K9EZ]

I ALWAYS enjoy your post Richard!

Couple of questions.....
In an elevated counterpoise, if you had ferrite beads or a choke at the feedpoint what would happen?

You say "but reflections of that radiation from the earth will affect its far-field radiation pattern." How is it effected?

This is all very key to me right now as I am working on different thoughts of a very portable antenna system for Ham Radio. Many people use a small dipole called a Buddy Pole, but it is only 8 feet up thereby sending the signal straight up. I was then thinking since I may not get much height, a vertical would be better. But would a vertical dipole be even better? No ground plane needed. Or how about a vertical dipole with a horizontal ground side like an L. Yes I know I need to get in NEC to look at it, but have been on the road a lot.

[R. Fry]

I ALWAYS enjoy your post Richard!

Thanks for saying so.

Couple of questions.....In an elevated counterpoise, if you had ferrite beads or a choke at the feedpoint what would happen?

If no r-f current could pass from the transmission line to the elevated wires then there would be no radiation.

You say "but reflections of that radiation from the earth will affect its far-field radiation pattern." How is it effected?

If the lower end of a vertical, base-fed monopole with horizontal counterpoise wires attached is located close to the surface of the earth (say within 5 physical degrees as a guess), then the reflection of the radiation from the vertical off the earth will add to the radiation from the vertical very similar to when it is installed with its base barely off the earth and uses 120 x 1/4-wave buried radials. In the case of a perfect, flat, infinite ground plane that reflection adds 3 dB to the peak directivity (which lies in the h-plane) of the radiation from the vertical itself.

Many people use a small dipole called a Buddy Pole, but it is only 8 feet up thereby sending the signal straight up. I was then thinking since I may not get much height, a vertical would be better. But would a vertical dipole be even better? No ground plane needed. Or how about a vertical dipole with a horizontal ground side like an L.

A vertical dipole close to the earth would have better low-angle radiation than a horizontal dipole close to the earth. Don't know about adding a horizontal wire to the bottom arm of a vertical dipole - would have to model it -- but suspect it would not be very useful.

[Sam Buca]

I ALWAYS enjoy your post Richard!

True that. I don't always reply to the posts, but I do read them and occasionally will go back and reference it.

[K9EZ]

Richard,

I belong to a ham radio group (Potomac Valley Radio Club - PVRC) that puts on webinars for hams all over the world. Do you think we might be able to entice you to present some of this type of information in a webinar? I can tell you we usually get people from all over the world, and your topic(s) would have a huge gathering!

[R. Fry]

... Do you think we might be able to entice you to present some of this type of information in a webinar?

Kent - could be a possibility, but so far I have never even participated in a webinar, let alone been the focus of one. I tried to find some on-line recorded samples of a "live" interactive webinar, but really didn't see anything in the 1st two Google pages. Does anyone have a link to something like that?

I tend to produce better results when I have time to research answers I don't know off the top of my head, and maybe prepare/post graphics to illustrate points I want to make. My Photobucket "album" has a few hundred such at links that I refer to again and again in various posts.

Anybody with any comments about this is welcome to email me: rfry at adams dot net .