Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G5TM, Nov 21, 2019.
Wow! Tim and I posted at the same time with the answer!
Seems like moments ago...
Experiments with AM broadcast stations using frequencies at the lower, middle, and higher end, showed that 4-radials, 6-feet above ground, produced signal strengths in par with 120-buried radials.
In terms of wavelength, 6-feet at 600 kHz equates to 2-feet at 1800 kHz.
Before my swimming pool was installed, I had 32-radials under my HyTower. Then, a lot of them were "dug up" when the swimming pool hole was undertaken. As such, I went to elevated radials that go along my wood stockade fence, along the retaining wall for the swimming pool, and along my house. There are 4-radials cut for 80-meters, 4-radials cut for 40-meters, and 4-radials that are of "random" lengths. I primarily use the HyTower on 80 / 75-meters, 40-meters, and 30-meters and the performance is much better than with the buried radials.
I also have a full sized vertical for 40-meters and 30-meters that I phase with the HyTower for gain on 40-meters. There are 4-radials for 40-meters and 30-meters that are elevated as well.
Along the stockade fence, the radials are between 2-feet and 6-feet above ground.
I've been using a portable 40m vertical for several years now.
You want 4 tuned radials up about 4 feet at the feedpoint sloping down to 2-1/2 to 3 feet toward the ends.
Some things are very often missed / not mentioned / not discussed -
1/4 wavelength long radiator with elevated radials is sometime referred to as "groudplane antenna".
The "elevated radials " IMPORTANT side effect - the "slope" of them, not the count or elevation INCREASES the feed point impedance. In general - just from word of mouth, not modeled - 45 degrees slope brings the feed point to around 50 Ohms.
Of course sometime not too practical to realize / make that.
With elevated radials, you can get almost any feedpoint resistance (at resonance) you want, from ~25 Ohms to hundreds of Ohms.
Assuming equal length, symmetrically placed radials, "resonance" is determined primarily by the total length from the tip of a radial to the tip of the vertical Monopole. The ratio of the length of a single radial to the length of the vertical monopole is the primary determinant of feed point resistance, as shown in this previous post. Post #27 and #28 in this other thread deal with this topic, too.
Varying the downward slope of radials, and varying the number of radials effects the feedpoint resistance (and resonance), but these are second order effects compared to varying the sum of the radial and monopole lengths.
This entire concept is analogous to feeding a Dipole off-center, where you can make feedpoint resistance (at resonance) anywhere from <50Ohms to thousands of Ohms. There is nothing to recommend building a Marconi antenna where the Monopole is near the same length as a Radial except that this specific example was used in Textbooks. Terman didn't have NEC... It would have taken him months to do what AutoEz can do in 60 seconds...
Read my signature... Of, Shirley has me on Ignore....
Sometimes I think Vaclav has all of QRZ on Ignore.