HI, Grant! A couple of things to think of.
First, as reported in the IEEE paper mentioned earlier, increasing the number of elevated radials to more than four , yields a rather limited benefit, This work was done by professional antenna engineers with plenty of equipment and resources. The point of the investigation was the development of a viable replacement of degraded buried radial systems under broadcast towers with elevated radials and did involve measurement of radiated field intensity. As for our 160 antenna with elevated radials - think of it as a ground-plane antenna! In the case of the inverted-L the vertical monopole element is simply bent at a convenient height to take advantage of available supports. In the case of the tee, this can be very convenient for supporting the upper end of the shortened monopole. Additionally, in the case of the TEE any residual horizontally polarized radiation from the flat-top of the TEE will be pretty much cancelled by the equal and opposite currents flowing in opposite directions in the flat-top wires. Unless you are very space limited, the major advantages of RESONANT radials are that the current maxima in the radials will occur at the antenna feed-point. That allows the driving-point impedance to approach pure real in a well-behaved manner - being something less than 70 ohms. (More like 35-50 ohms if you have a good radial system. Short radials introduce a reactive component into the antenna feed that needs to be dealt with. I expect that using many short radials acts more like capacitive coupling to the underlying soil and its losses to provide and "image" for the vertical monopole. Dig, if you wish, but there is nothing "magic" about dirt! And it can be a bit lossy! I think you are on a good track! I'll be interested to see how it turns out! I'm very much in favor of your elevated resonant radial approach! It has worked very well for me! The closest I ever came with a "direct ground "system" was in another location where I had a very tall pine on dam (that I owned) of as good-sized lake! I drove an 8-foot copper ground rod at the edge of the lake, at the base of the inverted-L, and using my canoe, I ran several quarter-wave radials out into the water. Worked pretty well! - With a fairly well behaved driving-point impedance! I could see the drop in the driving-point impedance when I ran the radials out into the water. (Turned out that some copper sulfate had been added to the lake to kill off some vegetation! I expect that helped!) Since then, I've done quite well with elevated resonant radials and an inverted L - Until a hurrlcane tilted the tall oak that was supporting the far end of the inverted L and we had to have it taken down.) My "elevated radials ran around the perimeter of my lot and had 90 degree bends in them 60-70 feet from the antenna feed point! Good luck!! "If you build it, they will come"! :) Regards, Charlie, K4OTV -----Original Message----- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Grant Saviers Sent: Thursday, December 13, 2012 5:28 PM To: Dennis W0JX Cc: topband@contesting.com Subject: Re: Topband: Elevated Radials Questions Thanks for the comments and pointers. The land around the antenna is mixed grass and forested islands so on the ground radials would be partially buried and partially on the surface. Digging through the trees and clearing the brush is not something I want to do. Also, based on prior experience with verticals on metal roofs, I'm a real fan of elevated radials. I am relying on the credibility of the N6LF QEX series for how well/not well elevated radials will work (Mar - June 2012). I realize this work was all analysis with EZNEC PRO, but it seems to be the similar to results of others I've read. Googling "K5IU elevated radials" I did find the 2008 N6LF article which has the experimental data as well. His analysis shows there isn't much difference in losses with more than 4 radials between 0.15 and 0.27 wavelengths long. I've heard conventional wisdom is to tune radials for resonance, but the analysis for 4 or more radials elevated > than a couple of feet seems to indicate it is a lot of work for little benefit. I also found the 2005 thread "tuning elevated radials" on this reflector quite informative. One thing that stands out is that I may be better off with more than 7 shorter than 130' radials. Grant KZ1W On 12/13/2012 12:06 PM, Dennis W0JX wrote: > Grant, you should consider putting in an additional 23 radials and put the radial system on or in the ground. This will eliminate any possible detuning by the big metal building and interaction with the RX 4 square. You said that your vertical T will go up to 85 feet. However, by elevating the radials 10 feet, your effective vertical distance is 75 feet which will allow you to shorten the top hat wires a bit. As an alternate, you could put down 1/8 wavelength radials on the ground but more of them and have a good system too. > > If you must go with an elevated radial system, I recommend that you read the articles by Dick Weber, K5IU, who strongly advocated elevated radials shorter or longer than 1/4 wavelength. If shorter, then the radials are loaded with a small coil. If longer, then they are tuned out with a capacitor. W5UN uses shortened elevated radials on his 160 meter 4 square with great results. They are about 70% of a quarter-wave in length. > > 73, Dennis W0JX/8 > Milan OH > _______________________________________________ > Topband reflector - topband@contesting.com > _______________________________________________ Topband reflector - topband@contesting.com _______________________________________________ Topband reflector - topband@contesting.com