Presentation on theme: "Ground Systems for HF Verticals some experimental comparisons to NEC. Rudy Severns N6LF antennasbyn6lf.com."— Presentation transcript:
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 dont 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 todays talk.
Comment Todays 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.
Whats the purpose of the ground system? –Its 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 equivalent circuit antenna 1 antenna 2
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
Some experimental results
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
A new antenna test range on 40 m
Antenna under test
Test antenna with sliding height base
Adding radials to the base
Elevated radials close-up
Loop receiving antenna
Receiving antenna at 40 N7MQ holding up the mast!
Network analyzers HP3577A with S-box Homebrew N2PK note, automatic, organic, heating system
Inside the N2PK VNA
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
1/8-wave, top-loaded, 40 m vertical
What about a few elevated radials versus a large number of surface radials?
NEC modeling prediction
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.
4-64 radials lying on ground surface 5.8 dB
4 radials raised above ground 5.9 dB
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!
Some more elevated radial experiments
Gullwing radials a la N6BV
Variations in elevated radials configuration number |S21| [dB] Zi [Ohms] configuration h=33.5 1039+j6.3base & 4 radials elevated 48 2-0.4736+j6.2base at ground level radial ends at 48 3-0.6529-j11gullwing, base at ground level ends at 48 4-0.3639+j0.9base & radials at 48 four 17.5 radials, 2.2 uH L
comment on four elevated radials From these experiments and NEC predictions it would seem that four elevated radials are all you need. Thats 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.
Some experiments with radials lying on the ground surface
Measured improvement over a single ground stake f=7.2 MHz
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.
Measured base impedances
Antenna resonance versus radial number
Radial current for different heights
A current sensor
Radial current measurements
Measured current distribution on a radial
Radial current distribution Radial numberRelative radial current normalized to 1 A total 10.239 2 30.252 40.269
NEC modeling prediction
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.
Effect of shorting radials, constant height
Radial current distribution
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.
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 its resonant well below the band! What to do? –Nothing, use a tuner and move on, –Shorten vertical until its resonant, –add more radials –or, shorten the radials until the antenna is resonant. Which is best?
Direct measurement of several options Do nothing: G= 0 dB Shorten height: G=-0.8 dB Shorten radials: G=+3.5 dB Use 16 radials: G=+4 dB Use 64 radials: G=+5.9 dB
An observation When 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!
Summary Sparse radial screens (less than 16 radials) can have a number of problems: –increased loss with longer radials –unequal 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.
Summary continued Try 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 radials –you 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.