Topband: Fw: Shortened Radial Experiments

[k8bhz at hughes.net]

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 
results....Of 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 at 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 BL&E findings established the requirements for such radial systems
required of AM broadcast stations subject to FCC jurisdiction, to the
present day.

R. Fry, CPBE

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