Ground radials comment


Date: Wed May 15 1996 - 22:11:52 EDT

I've seen a few posts on burying radials and thought I would add my comments
on ground and roof mounted radials.

Generally, roof-mounted verticals work better. They are higher above ground
and other RF-absorbers such as nearby buildings. Low-angle DX, and local
groundwave and direct contacts are improved. Not as many radials may be
required for an efficient roof installation.

Some commercial vertical antennas employ only two or four radials. This is
inefficient, but pushes the resistance closer to 50 ohms allowing the user to
see a lower SWR. A 1/4^ vertical over a good groundplane is closer to 30
ohms; the SWR is a tad higher (no importance) and the efficiency is much

Some ARRL and other groundplane texts suggest that it is better to have many
short radials all of the same length than the typical two or four 1/4^
"resonant" radials of decades-old antenna texts. Eight or more radials, all
the same length, 1/8^ or longer on the lowest frequency (3.5 MHz?) should be
adequate for a roof-mounted trap vertical. Fifteen or twenty radials would
probably be adequate in a ground-mounted antenna.

Some authors of old amateur antenna texts seems to be confused on the
differences between lightning or AC ground, HF grounds, and VLF grounds. HF
ground radial systems work the best if above the soil, and almost as well if
buried a few inches below the soil (though more radials may be needed).
 Deeply buried radials and ground rods are not needed (and counterproductive)
at HF. Save the ground rods for lightning grounds.

I've installed trap verticals on the ground and on roofs. Rather than use
two or four 1/4^ radials per band, I think I have had more efficient antenna
systems by using multiple radials, all the same length, arranged as the
spokes in a wheel. Since aesthetics are important in many areas, I use
insulated wire for roof-mounted radials. The radials are stapled or attached
to the roof, rather than suspended over the roof with bulky hardware and
insulators. I choose a wire color which blends in with the roof. Radials
may be symmetricallyzig-zagged or spiralled if there are problems getting
them to fit a smaller roof. I have encountered no resonance or tuning
problems with this efficient radial system and trap verticals. I install all
of the radial system first, connect the antenna base and after it is in place
I tune each section of the trap antenna.

73, Jay

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