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Low Frequency Transmit Antennas Using Simple Formulas You Already Know

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Low Frequency Transmit Antennas Using Simple Formulas You Already Know Vienna Wireless Society Low Frequency Transmit Antennas Using Simple Formulas You Already Know For Amateur 2200 Meter (135.7-137.8 kHz) band VWS 12 April 2019 George Lemaster WB5OYP VWS 4/2019 WB5OYP Vienna Wireless Society FCC Rules for 2200 Meter band: 135.7 to 137.8 kHz 1 Watt EIRP (5 W on 472-479 kHz) Maximum transmitter output 1500 Watts 60 Meter (~197 ft) height limit AGL for both 2200 and 630 Meter antenna Available to General and higher VWS 4/2019 WB5OYP Vienna Wireless Society LF Receiver antennas (Topic for another day) E-Field Probe (‘active’ antenna) Active LF antenna – QST Sep 2001 p31 Loop antenna Air loop or ferrite loaded loop Beverage antenna VWS 4/2019 WB5OYP Vienna Wireless Society Short Transmitting Antennas: For 136 kHz Low Frequency band you need vertical polarization Wavelength = 2200 meters ¼ wavelength 550 meters Amateur antenna for LF might be about 10 to 30 meters vertical height (limit 60 meters above ground level) 10 Meters = 0.0045 wavelength 30 Meters = 0.0136 wavelength VWS 4/2019 WB5OYP Vienna Wireless Society Short Transmit Antennas example: WWVB Ft. Collins CO 60 kHz Parameters South Antenna North Antenna 60 kHz (5000 meter wavelength) Radiation Resistance (Ohms) 0.46 0.46 Antenna Gross Resistance (Ohms) 0.80 0.91 Antenna Radiation Efficiency 57.5% 50.6% Antenna Base Reactance (Ohms) -114.9 -112.9 Antenna Downlead Inductance (microheneries) 208.8 208.0 Measured antenna parameters at 60 kHz for both the north (WWVL) and south (WWVB) antennas. 122 meter downlead (appx) is ~ 0.0244 Wavelength From: ‘WWVB Improvements’, https://tf.nist.gov/general/pdf/1406.pdf VWS 4/2019 WB5OYP Vienna Wireless Society Short Transmit Antennas examples: ‘Triatic’ SAQ Grimeton Sweden 17.2 kHz VLF Antenna Patent 1,360,167 Filed 1917 http://www.pa3hcm.nl/?p=1232 RCA Engineer Ernst Alexanderson ‘Multiple Tuned’ Antenna All downleads radiate as one vertical antenna VWS 4/2019 WB5OYP Vienna Wireless Society Short Transmit Antennas examples: Southern Avionics Beaumont, TX www.southernavionics.com Commercial Non Directional Beacon Antenna 30 Ft. low power symmetrical T, towers 90 meters apart. 25 ft vertical radiator with 2 wire top hat ‘Static Capacitance’ =869 pf, 12 radials typical, 190-535 kHz VWS 4/2019 WB5OYP Vienna Wireless Society Topics: LF transmit antenna design •Many characteristics can be treated as lumped elements • You need to know: • Effective Height • Static capacitance • With these parameters you can predict much about the antenna performance and look for ways to optimize your design VWS 4/2019 WB5OYP Vienna Wireless Society Antenna Antenna R Loss in Inductance Capacitance antenna structure 1 X 2FL X L C 2FC Radiation Resistance R Loss in Ground system Equivalent Circuit of Short antenna VWS 4/2019 WB5OYP Vienna Wireless Society Concept of Effective Height: Vertical Monopole @ ¼ wavelength Current Distribution (approx) Physical Height Input current value Ground VWS 4/2019 WB5OYP Vienna Wireless Society Very Short antenna ~ 100 ft at 136 kHz Mean current ~= Physical Height / 2 Physical Height Input current value Ground VWS 4/2019 WB5OYP Vienna Wireless Society Concept of Effective Height: If the input current at the antenna base were constant over the vertical height, the Effective Height would be equal to the physical height. The objective of our LF antenna design is to achieve maximum radiation in the vertical plane. To do this we need to optimize the current through the vertical radiating element. The goal is to optimize the Effective Height, not just make the antenna taller. VWS 4/2019 WB5OYP Vienna Wireless Society Very Short antenna ~ 100 ft at 136 kHz Linear current over the physical height of the antenna would be the best you could do to maximize Physical radiation Height (maximum effective height) Input current value Ground VWS 4/2019 WB5OYP Vienna Wireless Society If we can make the average current in the vertical portion of the antenna higher without increasing the base current, we can increase the vertical radiated power. With top loading, the average current is increased along the vertical direction, increasing the ‘Effective Height’ VWS 4/2019 WB5OYP Vienna Wireless Society Radiation Resistance Calculate Radiation Resistance of the antenna: For this type of short vertical antenna (< 0.1 wavelength) 2 2 HEFFECTIVE RRADIATION 160 Notice the Radiation Resistance increases as the Square of the Effective Height This means that you should try to increase it and not do things that decrease it 2 The antenna radiated power is just power law, P = I Rrad VWS 4/2019 WB5OYP Vienna Wireless Society Current Distribution (approx) Very Short Top Loaded Physical Antenna Height ~ 100 ft at 136 kHz Input current value Ground Effective Height > Physical Height / 2 VWS 4/2019 WB5OYP Vienna Wireless Society Top Loading adds capacitance to the antenna. This is beneficial because increasing the capacitance lowers the inductance of the loading coil required to resonate the antenna at the base feedpoint. By reducing the amount of inductance required to resonate the antenna there will not need to be as many turns on the loading coil. This reduces the R loss of the loading coil and reduces another loss resistance in the antenna circuit, increasing efficiency. As we will see this increased capacitance also helps increase the bandwidth of the antenna. VWS 4/2019 WB5OYP Vienna Wireless Society “Umbrella” Top Loading to add capacitance You need guy wires anyway so connect them at the top Insulated section Topload wires are not necessarily the same length, equal spaced, etc. Sloping wires carry current downward somewhat reducing the Effective Height Ease of construction and increased capacitance generally outweigh loss of Effective Ht. Trade off bandwidth versus power with umbrella characteristics length. VWS 4/2019 WB5OYP Vienna Wireless Society How to Decrease Effective Height VWS 4/2019 WB5OYP Vienna Wireless Society R Loss in Antenna Antenna Antenna Antenna antenna capacitance Loading Loading inductance structure 1 Coil X Coil 2FL X L C 2FC inductance Loss Radiation Resistance Resistance X L RLOSS QL R Loss in Ground system Equivalent Circuit of Short antenna with Loading Coil VWS 4/2019 WB5OYP Vienna Wireless Society Example Capacitance Estimates: Vertical tower ~ 5 pf per foot (estimates vary 3-5 pf) 100 ft X 5 pf = 500 pf Topload wire, 4 wires 60 ft long attached at tower top Single wire ~ 1.5 pf per foot 4 wires X 60 ft X 1.5 pf = 360 pf Total Capacitance = 500 pf + 360 pf = 860 pf Single wire Capacitance can be increased by, for example, 2 parallel wires separated by 1 ft. VWS 4/2019 WB5OYP Vienna Wireless Society Capacitive Reactance of the antenna No top load 100 ft. X 5 pf = 500 pf C = 500 pf 1 X C 2FC 1 X 2340 C 2 136x103 500pf VWS 4/2019 WB5OYP Vienna Wireless Society Capacitive Reactance of the antenna 4 Wire Top Load Hat C = 860 pf 4 wires X 60 ft X 1.5 pf = 360 pf Total Capacitance = 500 pf + 360 pf = 860 pf 1 X C 2FC 1 X 1361 C 2 136x103 860pf Adding capacitance, Lowering the capacitive reactance of the antenna is a good thing as we’ll see.. VWS 4/2019 WB5OYP Vienna Wireless Society Assume the Effective Height of our 100 ft antenna is 50 ft No top load wires 50 ft = 15.24 meters 2 2 H Eff RRADIATION 160 2 2 15.24 RRADIATION 160 0.077 2200 VWS 4/2019 WB5OYP Vienna Wireless Society Assume the Effective Height of our same 100 ft antenna is now 60 ft by adding some top load wires 60 ft = 18.28 meters 2 2 H Eff Now with top load we have RRADIATION 160 1.4 times the Radiation R versus no top load wires 2 0.077 ohms vs 0.108 ohms 2 18.28 RRADIATION 160 0.108 2200 VWS 4/2019 WB5OYP Vienna Wireless Society Antenna tower height 100 ft. No top load so Effective Height = 50 ft C = 500 pf Xc = 2340 ohms Estimate loading coil Q = 250 Estimate loss in loading coil: RLOSS= Xc/Q = 2340/250 = 9.36 ohms loss resistance in loading coil Assume no ground loss (ground discussion another time) Rtotal = Rrad + Rtotal = 0.077 + 9.36 = 9.437 ohms Efficiency = Rr/Rtotal = 0.077/ 9.437 = .008 = 0.8 % VWS 4/2019 WB5OYP Vienna Wireless Society Antenna tower height 100 ft. Top Load so Effective Height = 60 ft C = 860 pf Xc = 1361 ohms Estimate loading coil Q = 250 Estimate loss in loading coil: RLOSS = Xc/Q = 1361/250 = 5.44 ohms loss resistance in loading coil Assume no ground loss Rtotal = Rrad + Rtotal = 0.108 + 5.44 = 5.552 ohms Efficiency = Rr/Rtotal = 0.108/ 5.552 = .019 = 1.9 % Efficiency has increased from 0.8% to 1.9% by adding topload wires VWS 4/2019 WB5OYP Vienna Wireless Society P = I2 R Ft I2 = P/R = 1/0.018 Current Distribution I = 7.4 amps 50’ tall, 1/4” diameter wire using Mininec E = I x Xc No Topload E = 7.4 A x 9.14K Ω High Capacitance = High Xc = 9.14 K Ω E = 67.6 kV Rr = 0.018 Ω, Pr = 1 Watt @ 7.4 A, Voltage = 67.6 kV RMS Modeling by Peder Hansen W8EDI VWS 4/2019 WB5OYP Vienna Wireless Society 50 ft tall, ¼ in diameter wire, using Mininec Four wire Topload wires, 50 ft long. All wires ¼ in. Increased Capacitance (lowers Xc) = 2.449 K Ω Rr = 0.058 Ω, Pr = 1 Watt @ 4.15A, Voltage = 10.17 kV RMS Modeling by Peder Hansen W8EDI VWS 4/2019 WB5OYP Vienna Wireless Society Ft 50 ft tall, ¼ in.
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