Introduction
Recently I discovered the NanoVNA vector network analyzer. I had seen articles in QST magazine over the last several years and was intrigued (Ref.1/2). I remember years ago testing VHF/UHF/uW antennas with large signal generators, directional couplers and scopes all weighing hundreds of pounds. Now the same thing and more can be done with a small lightweight hand held instrument. I decided to apply this to my existing HF antennas to see what their characteristics are.
Theory Background
Figure 1 shows a simple example of matching a load to a source. The source has an impedance of 50ohms, and the load impedance is variable. To show this graphically, LTspice (Ref.3) is used with the antenna load resistor a function of time. When t=1second, the load resistor is 50ohms and the maximum power is transferred to the antenna. The green trace is the voltage across the antenna and the blue trace is the current through the antenna. The red trace is the power into the antenna. So for maximum power transfer, the load resistance must match the source resistance.
For ac RF waves, we must consider complex impedance. Generally the transmitter is designed to have a source impedance of either 50ohms or 75ohms. Coaxial transmission lines are also typically 50 or 75ohms. If the antenna is not matched or tuned correctly, then it’s impedance will be complex and not a pure resistive 50/75ohms. In this case standing waves will result on the transmission line. A typical amateur radio transmitter such as the Elecraft K2 will survive SWR <= 2:1. An antenna matching unit will increase performance.
SWR Testing Tx Line & Tx Line + Ant
Figure 2 shows the SWR of the RG-58U coaxial transmission line terminated at the antenna in 50ohm. The value is 1.25max/1.05 minimum. Figure 3 shows the combined SWR of the HF antenna with the transmission line. Without the countepoise wires, the resonant SWR is >3:1, but with the counterpoise the SWR is <2.5:1 at resonance. The mounting is not optimum as it’s on a metal beam with a concrete base, whereas it is designed to mount above real earth.
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References
#1. – “NanoVNA V2 Plus4 50KHz – 4GHz Two-port Vector Network Analyzer”, P.Salas AD5X, QST Magazine April 2022
#2. – “NanoVNA Vector Network Analyzer”, P.Salas AD5X, QST Magazine May 2020
#3. – “LTspice Simulation”, Analog Devices
https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html
