Presentation on theme: "HF Vertical Antenna Ground Systems Some Experiments"— Presentation transcript:
1HF Vertical Antenna Ground Systems Some Experiments Rudy Severns N6LFantennasbyn6lf.com
2We’ve been using verticals for over 100 years. Is there really anything new to be said about ground systems for verticals?Yes!Little attention has been given to HF (2-30 MHz) ground systems like those used by amateurs.Soil behavior at HF is different from BC.
3Typical amateur antennas use: radials lying on the ground surface,or elevated radials,and/or small numbers of radials,short loaded verticals
4Some typical questions 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?
5That 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.
6What’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!
17Measurement 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.
18This 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
32A 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.SteppIR vertical
34Measured improvement over a single ground stake f=7.2 MHz
35Caution!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.
42Radial current distribution Radial numberRelative radial current normalized to 1 A total10.239230.25240.269
43You want a 40 m vertical for field day. Field day scenarioYou 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?
45Lets 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 resonate (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.
50small 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 to everything!This nonsense goes away as the number of radials increases!
51What about a few elevated radials versus a large number of surface radials?
55NEC 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’!
57gullwing, base at ground level ends at 48” 4 -0.36 39+j0.9 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 L5-5.19132+j22base & radials at ground level6-1.7951+j1.0four 21’ radials7-0.140-j1.264, 33’ radials
58More on elevated radials If you use more than 4 radials in an elevated system:the screen resonances and radial current asymmetries decrease.the reactive part of the feed-point impedance changes more slowly as you add radials so you have a better SWR bandwidth.the ground loss does not improve much however.
59increased 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.
60Try 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.
61All the funny business goes away with more radials! Some adviceTry to use more radials.Four is just not enough.All the funny business goes away with more radials!16 radials are a good compromise.