Making Sense of Antenna Analyzer Readings
Almost every ham today owns an antenna analyzer. These handy tools display the complex impedance of your antenna system. But for many, the numbers — Z, R, X, +j, –j — are still mysterious. Let’s break them down in practical terms, so you can read your analyzer like a pro.
What Does Z Really Mean?
The analyzer shows the impedance Z, which is a combination of resistance (R) and reactance (X). Ideally, you want your antenna feedpoint to be as close as possible to 50 Ω purely resistive (R = 50, X = 0). That’s the sweet spot for most radios and coax.
Antennas rarely land exactly on 50 Ω. That’s where matching systems enter:
- Core transformer (multiband): e.g., 49:1 or 4:1 to bring extreme impedances into a manageable range.
- Transmission-line transformer (monoband): a chosen section of coax/ladderline (¼-wave, ½-wave, 1⁄12-wave tricks) to nudge Z with very low loss.
Important: these are not neutral adapters. They become part of the antenna system. They introduce R and can alter X by their very nature (conductor loss, core loss, parasitics).
Some commercial designs add parts (e.g., a capacitor in parallel with the transformer) to make the feedpoint look “prettier” to the radio. That doesn’t improve how the antenna radiates — it just hides real behavior. At RF.Guru we avoid such cosmetics. We prefer pure antennas, pure transformers. If you need extra range, use a proper transmatch (tuner) rather than masking physics.
Understanding X: The Reactance Part
X tells you if your system appears inductive or capacitive at that frequency:
- +j (positive X): inductive appearance — often below the resonant point, but sign can flip around standing-wave dips due to reflections.
- –j (negative X): capacitive appearance — often above resonance, with the same caveat about sign flips near dips.
±j is not a strict “too long / too short” detector. Reflections along the line (where the tuner sits, line length, chokes) can show +j or –j even when radiator length isn’t the culprit.
What Values Are Reasonable?
Reactance is normal. The real question is whether your system (transformer/line/tuner) can work efficiently with it:
- |X| < 50 Ω: practically invisible; often inside an internal tuner’s range.
- |X| = 50–150 Ω: routine for modern tuners; SWR rises but remains easy.
- |X| ≥ 200 Ω: still workable. With good coax, even ~5:1 SWR typically adds < 1 dB extra line loss across 20–30 m runs when matched properly.
- Key point: With a decent low-loss HF coax (10–13 mm foam PE), matched loss around 14 MHz is typically about ~0.9 dB per 30 m (indicative).
- Tuner at the shack: the line carries standing waves. Expect a modest increase over the matched loss. In practice for ~30 m, total line loss ≈ 1.0–1.2 dB at 5:1 SWR — i.e., < 1 dB extra compared to 1:1.
- Tuner at the feedpoint: the line is re-matched; you’re back near the matched-loss figure (≈ 0.9 dB / 30 m in this example).
- When does it get ugly? Very long runs or lossy/undersized coax. Keep lines reasonable and use quality cable; then even 3–5:1 is not catastrophic.
These are practical, conservative rules of thumb — the exact number depends on cable spec, length, and where the tuner sits.
Practical Takeaways
- Read R and X together; SWR alone hides the cause.
- Transformers and stubs become part of the antenna; they shape R and X — they are not neutral dongles.
- ±j near dips is normal; don’t chase “zero X” dogmatically.
- There are no perfect antennas — only in textbooks.
Mini-FAQ
- Why does my analyzer show –j? — Capacitive appearance. Often means resonance is above your frequency — but reflections can also produce –j around dips.
- Is a little reactance bad? — No. Tuners comfortably handle ±100 Ω. With good coax and sensible lengths, even 3–5:1 keeps total line loss low.
- Are transformers just adapters? — No. They add R and can alter X. We avoid “masking” parts (e.g., parallel capacitors) that only make Z look pretty to the rig.
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