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Ground Systems for HF Verticals some experimental comparisons to NEC . . Rudy Severns N6LF antennasbyn6lf.com. Some typical questions on verticals. How much of ground system is it worth putting down? What will I “gain” (in dB!) by adding more radials?
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Ground Systems for HF Verticals some experimental comparisons to NEC. Rudy Severns N6LF antennasbyn6lf.com
Some typical questions on verticals • How much of ground system is it worth putting down? • What will I “gain” (in dB!) by adding more radials? • Does it matter if I lay the radials on the ground surface? • Are a few long radials useful? • Are four elevated radials really as good as lots of buried radials? • How well do “gullwing” elevated radials work?
We can use modeling or calculations to answer these questions but most people don’t have a lot confidence in mathematical exercises. • High quality field measurements on real antennas are more likely to be believed. • Over the past year I have done a series of experiments on HF verticals with different ground systems. • That is the subject of today’s talk.
Comment • Today’s talk is a snapshot of experimental work. • The talk will only cover the highlights. • A detailed summary of the test range and instrumentation along with reports on each experiment can be found on my web page: antennasbyn6lf.com . • A copy of this PowerPoint presentation will also be on the web site. • You may also see other interesting information on the web page.
What’s the purpose of the ground system? • It’s there to reduce the power absorbed by the soil close to the antenna (within a ¼-wave or so). • The ground system increases your signal by reducing the power dissipated in the soil and maximizing the radiated power. • Any practical ground system will not affect the radiation angle or far-field pattern!
Power transmission antenna 1 antenna 2 antenna equivalent circuit
Measurement schemes • The classical technique is to excite the test antenna with a known power and measure the resulting signal strength at some point in the far field (>2.5 wavelengths for 1/4-wave vertical). • This approach takes great care and good equipment to make accurate measurements.
The modern alternative is to use a vector network analyzer (VNA) in the transmission mode. This approach is capable of reliable measurements to <0.1 dB. The VNA will also give you the input impedance of the antenna at the feed-point. S21 rx antenna test antenna
The first experiment was a 160 m, ¼-wave wire vertical with two ground stakes and 4 to 64 radials. • Measurements were made with a spectrum analyzer as the receiver.
Test Results delta gain = 2.4 dB
Receiving antenna at 40’ N7MQ holding up the mast!
Network analyzers note, automatic, organic, heating system Homebrew N2PK HP3577A with S-box
Test antennas • A 1/4-wave 40m tubing vertical. • An 1/8-wave 40m tubing vertical with top loading. • An 1/8-wave 40m tubing vertical resonated with a base inductor. • A 40 m Hamstick mobile whip. • 40m SteppIR vertical
What about a few elevated radials versus a large number of surface radials?
NEC predictions • There will be a very rapid change in peak gain as we raise the base of the antenna and the radials above ground. • Lifting the radials only a few inches makes a substantial difference. • When the base of the antenna and the radials have been elevated several feet, the peak signal will be very close to that for a large number of buried radials.
Experiment 3 • I began with sixty four 33’ wire radials lying on the ground surface. • The length of the vertical was adjusted to be resonant at 7.2 MHz. • I removed the radials in the sequence 64, 32, 16, 8, 4, measuring S21 as I went. • With only 4 radials left I then raised the radials and the base of the antenna above ground incrementally measuring S21 at each height. • There were no ground stakes and the feedline was isolated with a choke.
NEC modeling predicts that four elevated radials will perform as well as 64 radials lying on the ground. • In this example, measurements show no significant difference in signal strength between 64 radials lying on the ground and 4 radials at 4’!
comment on four elevated radials • From these experiments and NEC predictions it would seem that four elevated radials are all you need. • That’s deceiving! Antennas with only a few elevated radials suffer from a number of problems: • hi-Q, radials tune the vertical • asymmetric currents in the radials leading to pattern asymmetry. • tuning and current symmetry are very sensitive to ground and mechanical variations as well as nearby conductors.
More on elevated radials • Use more than 4 elevated radials : • the Q and radial current asymmetries decrease. • tuning is less sensitive • the reactive part of the feed-point impedance changes more slowly as you add radials so you have a better SWR bandwidth. • however, the ground loss does not improve much.
Caution! • Your mileage may vary! • My soil is pretty good but for poorer soils expect more improvement with more radials. • The degree of improvement will also depend on the frequency: • soil characteristics change with frequency, • at a given distance in wavelengths the field intensity increases with frequency.
Lets do an experiment: • isolate the base of the antenna with a common mode choke (a balun). • lay out sixty four 33’ radials and adjust the vertical height to resonance (reference height). • remove all but four of the radials • Measure S21 with the reference height. • Measure S21 with the vertical shortened to re-resonate. • Measure S21 with the reference height as we shorten the radials.
The lesson here! • When you have only a few radials lying on the ground you can have much higher losses than expected! • These losses can be reduced by shortening the radial lengths, i.e. less copper = less loss.
