Step 1 — Identify the symptom category
A) “SWR is high everywhere”
Likely causes:
- Wrong ratio for the antenna type (common: using 9:1 as EFHW or using 49:1 on random wire)
- Antenna is not the assumed electrical length
- Bad connection or wrong tap
Action:
- Confirm antenna type: balanced dipole/loop vs random wire vs EFHW
- Verify taps and turn counts against the schematics above
B) “SWR is reasonable, but RF in the shack / hot mic / noisy receive”
Likely causes:
- Common-mode current on coax (choke missing or insufficient)
- End-fed system using coax as counterpoise
Action:
- Add/upgrade a 1:1 choke at the feedpoint
- Add a second choke 6–15 ft down the coax
- Provide a controlled counterpoise (especially on EFHW and random wire)
C) “It tunes, but performance is weak, and transformer gets warm”
Likely causes:
- Core saturation/heating due to high duty cycle or high mismatch
- Too small a core stack for the power
- Too many turns (excess loss) or too few turns (poor transformation/bandwidth)
Action:
- Move to a larger core or multiple cores
- Re-check turn count and core mix
- Reduce mismatch presented to transformer (shorten/lengthen radiator, change feed arrangement)
Step 2 — Validate with three simple measurements
1) Continuity sanity checks (power off)
- Coax center to shield should not be shorted (except through intended DC paths; most autotransformers should not look like a dead short).
- Confirm correct tap connection to coax center.
2) Common-mode check (operational)
Symptoms of common-mode current:
- Touching coax changes SWR or noise floor
- RF burns, audio distortion, keying issues
- RFI into USB devices
Fix:
- Stronger choke: more turns on Mix 31, or a larger multi-core choke.
3) “Known load” test (best quick proof)
Use resistors (non-inductive, power resistors if possible) to emulate expected impedances:
- For 4:1: attach ~200 Ω across balanced output; expect ~50 Ω at input.
- For 9:1: attach ~450 Ω from antenna terminal to ground; expect ~50 Ω at input.
- For 49:1: attach ~2450 Ω from antenna terminal to ground; expect ~50 Ω at input.
If it fails this test, the problem is in:
- Tap placement
- Turn count
- Wiring polarity / series-parallel misconnection (especially for 4:1 Guanella)
Fast “Which one did I accidentally build?” clues
- If you built an autotransformer with a 3-turn tap and 9-turn total, that is almost certainly a 9:1.
- If you built an autotransformer with 2-turn tap and 14 total, that is almost certainly a 49:1.
- If you built two identical bifilar 1:1 chokes and wired them series/parallel, you likely built a 4:1 current balun.