Shortened Verticals: experiments
In a mailing list, ham radio operator Tom W8JIT described a
controlled experiment that he performed with a vertical antenna
built in an open field. He compared the field strengths produced
by a set of ground radials suspended 8 feet above the earth with
the same set of radials sitting on the surface of the soil.
Interestingly, the elevated system produced better results when
Shortened Verticals: experiments
In a mailing list, ham radio operator Tom W8JIT described a
controlled experiment that he performed with a vertical antenna
built in an open field. He compared the field strengths produced
by a set of ground radials suspended 8 feet above the earth with
the same set of radials sitting on the surface of the soil.
Interestingly, the elevated system produced better results when
there were only 4 radials. As the number of radials increased,
the differences between the elevated and ground-level systems decreased.
"First I added four radials, and measured FS [field strength] at
just over one mile and recorded that value. Then I went back, lowered
the radials and antenna, and pegged the bare wire to the dirt with little hooks."I repeated this with 8, 16, and 60 radials. Here’s what I measured. The
percentage and dB reading is referenced to the highest FS I measured, NOT to
a theoretically perfect system:"
| 8 foot high counterpoise | ground mounted radials | |||||
| 4 wires | 37 % | -4.3 dB | 20.1 mV | 28 % | -5.5 dB | 17.5 mV |
| 8 wires | 58 % | -2.38 dB | 25 mV | 53 % | -2.73 dB | 24 mV |
| 16 wires | 86 % | -.63 dB | 28.7 mV | 74 % | -1.3 dB | 28.4 mV |
| 60 wires | 96 % | -.18 dB | 32.5 mV | 100 % | 0 dB | 33 mV |
Data in the table below is extracted from the article "Ground
Systems as a Factor in Antenna Efficiency" by G.H. Brown, R.F. Lewis,
and J. Epstein, in Proceedings of the Institute of Radio Engineers,
vol.25, no.6, June 1937, pages 753-787. This is one of the most thorough
scientific experiments to date. The table gives actual field strength
measurements in dB referenced to a theoretical ideal vertical antenna.
| radial length | number of radials | 1/4-wave vertical | 1/8-wave vertical | 1/16-wave vertical |
| .14-wave | 2 | -4.2 | -6.3 | -10.6 |
| " | 15 | -2.3 | -3.6 | -5.5 |
| " | 60 | -2.1 | -3.1 | -4.8 |
| .27-wave | 2 | -4.2 | -6.1 | -10.7 |
| " | 15 | -2.1 | -2.4 | -4.7 |
| " | 60 | -0.8 | -1.2 | -2.7 |
| .41-wave | 2 | -3.9 | -5.6 | -10.0 |
| " | 15 | -1.8 | -2.2 | -3.8 |
| " | 60 | -0.5 | -1.0 | -2.3 |
This experiment showed that 120 radials nearly 1/2 wavelength long,
combined with a 1/4 wave vertical radiator, came very close to the
performance of the ideal 1/4-wave vertical antenna over perfect ground.
Shorter vertical radiators, shorter ground radials, and fewer radials all
resulted in lower field strengths at the monitoring points.