Learning Objectives

1. I can calculate input impedance, feed considerations, and the role of baluns in an antenna feed system.

Part I — measurement

Part I — measurement. Calibrate the NanoVNA over 50 kHz to 5 MHz with an open, short, and load, then measure a series of known loads: an open, a short, a capacitor across a frequency sweep, a matched 50 Ω resistor, and a 1 kΩ mismatch. You finish by characterizing an unknown filter as a black box.

Part II — design and build

Part II — design and build. You are handed a stand-in for an electrically small antenna: a 15 Ω resistor in series with a 1 nF capacitor, which at 2 MHz presents about \(15 - j79.6\ \Omega\) — a small radiation resistance behind a large capacitive reactance, exactly the situation Lesson 4 described. Unmatched, that is a 12:1 VSWR. You design an L-network for it, source the parts yourself, build it, and measure the improvement — then sweep to find the band over which the match survives.

Equipment

Beyond the VNA and its cables, each team builds a mock antenna from two components:

You will not get what you asked for

Those two values are the whole input to the design. The matching network’s inductor and capacitor are what you calculate — and then go and find. They are not on your bench: they are across the hallway in the electronics lab, in whatever bins happen to be stocked. Take the closest thing to each of your two values, measure the capacitor you came back with, and record all four numbers.

You will not get what you asked for. That is the normal case, and the lab asks you to price it: how much did the substitution cost, and is the result good enough for what the radio needs? One of those two parts matters far more than the other, and finding out which is most of the lesson.

Before lab

Read the whole packet, skim the VNA user’s guide, and work the Part II design by hand so you arrive with component values already in front of you. The Lesson 4 page walks the same design one step at a time, and the feed-match widget there lets you check your answer on a Smith chart before you touch a component.

Lab packet

The lab sheet is the turn-in document: Lab sheet (PDF). It already contains a blank Smith chart to plot on. If you want spares, or a combined impedance-admittance chart for the Part II design:

• Blank Smith chart (PDF, print) — the same chart bound into the packet.

• Impedance-admittance chart (PDF, color) — red impedance grid over blue admittance grid. This is the one to use when you cross from \(Z\) to \(Y\) in Step 5 of the design.

• Blank Smith chart (SVG) — vector, for zooming on screen.

Where this is going

Matching is the last piece of the antenna-as-a-circuit picture. From here Lesson 5 leaves the terminals behind and asks where the radiated field actually begins — the field regions — which is the question you have to answer before you can measure a pattern at all. The VNA skills you build here return in Module 2, where S-parameters become the standard language for describing every antenna and network you meet.