Re: Topband: Fw: Shortened Radial Experiments
Actually, I don't believe the Brown et al experiments showed this conclusion at all. Their extensive experiments showed an optimal radial length of about 3/8 wavelength (actually 0.41). This, with ON4UN's typical soil Vf of 0.5, would yield an rf length of 3/4 wavelength. This 3/4 wavelength has the same properties as a quarter wavelength, just for taller vertical antennas. My write-up was specifically about shortened radials for 160 meter inverted L antennas with 50' vertical sections, and the resonant radials are 48 to 49 feet, so the fit is good. I clearly stated that if I was to use a taller vertical, I would use a 3/4 length radial (modified by Vf, of course). If you wish to upgrade your thinking to today, try using the EZNEC model for a simple VHF ground plane antenna (vertical antenna with 4 ground radials). Then change the radial length to something other than 1/4 wavelength or some odd multiple of it. I don't think you'll like the resultsOf course, the VHF ground plane is in free space. If you lower it to the ground, the radial lengths would be affected by Vf would have to be shortened (just like lowering a dipole requires pruning). If you are thinking that a buried radial wouldn't be the same anymore, I would point out that the soil penetration depth of rf at 1.8 MHz is considered to be 30 to 50 feet, so the radials are very visible to the vertical. Recalling Jim Brown's posting yesterday of Rudy Severn's excellent recent work, the current maximum in a radial occurs at 0.25 wavelength from it's open end loss will be minimized when that current maximum is at the feedpoint. Brian K8BHZ -Original Message- From: Richard Fry Sent: Friday, December 19, 2014 7:18 PM To: topband@contesting.com Subject: Topband: Fw: Shortened Radial Experiments RE: Brian Mattson's post of Friday, 19 Dec 2014 12:23:52 -0500 The velocity of propagation in the MF and HF bands along radial conductors that lie on, or are buried several inches in the earth is inconsequential. What DOES matter is the free space wavelength, and the number of those radial conductors. This was shown in the real-world experiments of Brown, Lewis Epstein of RCA Laboratories, published in the Proceedings of the Institute of Radio Engineers in 1937. Those BLE findings established the requirements for such radial systems required of AM broadcast stations subject to FCC jurisdiction, to the present day. R. Fry, CPBE _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Fw: Shortened Radial Experiments
Recalling Jim Brown's posting yesterday of Rudy Severn's excellent recent work, the current maximum in a radial occurs at 0.25 wavelength from it's open end loss will be minimized when that current maximum is at the feedpoint. _ For consideration: The first link below is from a recent paper of Valentino Trainotti, a highly-regarded antenna engineering professor at the University of Buenos Aires. He shows that it is the quality of the ground plane within 1/2 of a free-space wavelength surrounding the base of a monopole (regardless of its electrical height) that is important in its operation. He states that the conductivity of the ground plane within a 1/2-wave radius of a monopole is part of the wave generator, and its r-f loss must be low in order to maximize the radiation efficiency of the antenna system. No requirement or effect is stated for the v.p. along any buried radial wires used in that boundary area. RF currents entering the earth from radiation by the monopole out to a radius of 1/2-wavelength need to travel back to the ground terminal of the antenna system in order for the monopole to radiate. The conductivity of the ground plane in that region needs to be minimized, no matter what the presence of the earth around buried radial wires does to their v.p. So the wires ideally should extend out to 1/2 of a free space wavelength, no matter what electrical length they may have due to their v.p. when buried. http://s20.postimg.org/xviw4hrot/Trainotti_Clip.jpg Here is a quote from the 1937 BLE paper on ground systems. Note especially the last sentence there. \\ Brown, Lewis, and Epstein: Ground Systems (p. 757) These losses are due to conduction of earth currents through a high resistance earth and to dielectric losses in the base insulator of the antenna. We shall next consider the earth currents flowing toward the antenna. The earth currents are set up in the following manner. Displace- ment currents leave the antenna, flow through space, and finally flow into the earth where they become conduction currents. If the earth is homogeneous, the skin effect phenomena keep the current concentrated near the surface of the earth as it flows back to the antenna along radial lines. Where there are radial ground wires present, the earth current consists of two components, part of which flows in the earth itself and the remainder of which flows in the buried wires. As the current flows in toward the antenna, it is continually added to by more displacement currents flowing into the earth. It is not necessarily true that the earth currents will increase because of this additional displacement current, since all the various components differ in phase. // Below are two clips from the BLE paper showing the distribution of r-f current in the earth around the base of monopoles of heights from 22 degrees to 99 degrees, and the current distribution along buried radial wires of 0.41 wavelength (in free space). Note that current is not maximized at a distance of 1/4-wavelength (free space) from the open ends of those buried radials. It is maximized near the common point of those radials, adjacent to the base of the monopole. http://s20.postimg.org/qji1coyul/Earth_Currents_near_Monopole.gif http://s20.postimg.org/zfstgmpgt/Currents_in_Radials.gif R. Fry, CPBE _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Fw: Shortened Radial Experiments
That might mean a lot at only 4 ohms radiation resistance 73 Peter -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of k8...@hughes.net From the antenna measurements, you can see that doubling the amount of copper ( labor!) resulted in only 2 ohms improvement from 16 to 32 radials. _ Topband Reflector Archives - http://www.contesting.com/_topband
