FDP in air – Feeding the antenna Given 288Ω input impedance of FDP in air, a common feed strategy...
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FDP in air – Feeding the antenna Given 288Ω input impedance of FDP in air, a common feed strategy is 300 ohm twin-lead or ladder line transmission line. Let’s see if this works even with two parallel flat traces Equations available for two parallel cylindrical wires Since using flat trace and not cylindrical wires, use approximation (Balanis, p. 514): a e = 0.25*a Where a e = effective radius if cylinder, a = trace width Given existing feed point of FDP has feed gap of 60 mils and trace width of 20 mils, calculate Zo of transmission line Simulate a quarter-wave differential line Add differential line to FDP in air to see if still
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Transcript of FDP in air – Feeding the antenna Given 288Ω input impedance of FDP in air, a common feed strategy...
FDP in air – Feeding the antenna
Given 288Ω input impedance of FDP in air, a common feed strategy is 300 ohm twin-lead or ladder line transmission line.
Let’s see if this works even with two parallel flat tracesEquations available for two parallel cylindrical wires
Since using flat trace and not cylindrical wires, use approximation (Balanis, p. 514):
ae = 0.25*a
Where ae = effective radius if cylinder, a = trace width
Given existing feed point of FDP has feed gap of 60 mils and trace width of 20 mils, calculate Zo of transmission line
Simulate a quarter-wave differential line
Add differential line to FDP in air to see if still matches
FDP in air – Feeding the antenna (cont.)
FDP in air – Feeding the antenna (cont)
To test if 333Ω is correct, let’s feed the antenna with a quarter wave (90°) line. Given a load of 288Ω, the impedance should be 385Ω at the end of the quarter wave transformer.
385*288333
* SLo ZZZ
FDP in air – Feeding the antenna (cont.)288Ω
50Ω
1205 mils90°
Simulate just the differential line to verify the Zo calculation and expected impedance at end of quarter-wave differential line• It works!
Momentum
FDP in air – Feeding the antenna (cont.)
50Ω
1205 mils90°
Now with the antenna and the same quarter-wave differential line• Not 385Ω at input anymore? Capacitance between lines and antenna?
Momentum
Test board 1
Test board 2Layer Thickness (mils) Characteristics
Top solder mask 1.00 εr = 3.8, tan δ = 0
Top 1.24 σ = 5.81*107 S/m
FR4 10.20 εr = 3.7940, tan δ = 0.0183
Ground 1.18 σ = 5.81*107 S/m
FR4 36.52 εr = 3.9920, tan δ = 0.0165
Vcc 1.18 σ = 5.81*107 S/m
FR4 10.20 εr = 3.7940, tan δ = 0.0183
Bottom 1.24 σ = 5.81*107 S/m
Bottom solder mask 1.00 εr = 3.8, tan δ = 0
TOTAL 63.8
On both boards, the FDP, feedlines, and layer stacking (~62 mil board) are identical. Test board 1 is for the balun measurement and board 2 is for the two-port measurement.
L = 1757 milsw = 83 milsg = 59 milsd = 237.5 milstrace width = 20 mils50Ω line
50Ω lines
L
w
gd
Test boards 1 & 2: Balun and Two-port Measurements and Simulations
Measurements SimulationsBalun Two-port One internal port Two internal ports
BW = 45MHz 62MHz 70MHz 63MHz
80-j*74Ω source
