Presentation on theme: "Ground Systems for HF Verticals some experimental comparisons to NEC."— Presentation transcript:
1 Ground Systems for HF Verticals some experimental comparisons to NEC. Rudy Severns N6LFantennasbyn6lf.com
2 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?
3 That is the subject of today’s talk. 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.
4 Today’s talk is a snapshot of experimental work. CommentToday’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.
5 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!
6 Power transmissionantenna 1antenna 2antenna equivalentcircuit
7 Measurement schemesThe 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.
8 This approach is capable of reliable measurements to <0.1 dB. 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.test antennarx antenna
22 A 1/4-wave 40m tubing vertical. Test antennasA 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
26 Lifting the radials only a few inches makes a substantial difference. NEC predictionsThere 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.
27 I began with sixty four 33’ wire radials lying on the ground surface. Experiment 3I 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.
30 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’!
33 Variations in elevated radials configurationnumber|S21|[dB]Zi[Ohms]h=33.5’139+j6.3base & 4 radialselevated 48”2-0.4736+j6.2base at ground levelradial ends at 48”3-0.6529-j11gullwing, base at ground levelends at 48”4-0.3639+j0.9base & radials at 48”four 17.5’ radials, 2.2 uH L
34 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 verticalasymmetric 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.
35 More on elevated radials Use more than 4 elevated radials :the Q and radial current asymmetries decrease.tuning is less sensitivethe 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.
36 Some experiments with radials lying on the ground surface
37 Measured improvement over a single ground stake f=7.2 MHz
38 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.
47 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 radialsMeasure 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.
50 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.
51 Practical example: Field day scenario You want a 40 m vertical for field day.¼-wave = 33’. So you start with about 33’ of aluminum tubing for the radiator and four 33’ wire radials.You erect this, with the radials lying on the ground and it’s resonant well below the band!What to do?Nothing, use a tuner and move on,Shorten vertical until it’s resonant,add more radialsor, shorten the radials until the antenna is resonant.Which is best?
52 Direct measurement of several options Do nothing: G= 0 dBShorten height: G=-0.8 dBShorten radials: G=+3.5 dBUse 16 radials: G=+4 dBUse 64 radials: G=+5.9 dB
54 small variations in radial layout, coupling to other conductors, An observationWhen you have only four radials the test results are always a bit squirrelly:small variations in radial layout,coupling to other conductors,like the feed-line,all effect the measurements making close repeatability difficult between experiments.The whole system is very sensitive!This nonsense goes away as the number of radials increases!
55 increased loss with longer radials SummarySparse radial screens (less than 16 radials) can have a number of problems:increased loss with longer radialsunequal current distributions between radials.system resonance shifts.A few long radials can be worse than shorter ones.screen resonances can alter the radiation pattern as the radials begin to radiate substantially.
56 Try to use at least 8 radials but 16 is better. Summary continuedTry to use at least 8 radials but 16 is better.The more radials you use, the longer they can be.A number of 1/8-wave radials will be better than half that number of ¼-wave radials. At least until you have 32 or more radials.In elevated systems:try to use at least 8 radialsyou can use radials shorter than ¼-wave and either re-resonate with a small L or make the vertical taller or add some top loading.the “gullwing” geometry can work.