A Comprehensive Guide to Tuning an EFHW End-Fed Half-Wave Antenna

Whether you own an antenna analyser or just the SWR meter built into your rig

Well done — you've purchased your EFHW from www.radio-stuff.com. This is a comprehensive guide to tuning it.

An End-Fed Half-Wave (EFHW) is one of the most rewarding antennas an amateur can put up. One wire, one feedpoint, one coax run, and five bands. But it only delivers on that promise if the wire is the right electrical length, and "the right length" is never quite what the calculator told you. Your height above ground, the tree it's hanging in, the type of wire you used, and even how wet the garden is will all shift things.

We'll cover both camps — the analyser owners and the SWR-meter-only crowd — and we'll deal properly with the two questions everyone actually asks: what do I do if it's too long, and what on earth do I do if I've cut it too short?

Our worked example throughout is the classic 80 m EFHW, roughly 40 metres of wire on a 49:1 transformer, covering 80, 40, 20, 15 and 10 metres. If yours is a 40 m version, every principle here applies — just halve the lengths and double the frequencies.

1. What you'll need

Essential

• A tape measure — a long one, ideally 10 m or more

• Cable ties, or a few short lengths of strong cord

• Side cutters (to be kept firmly in your pocket until Section 7)

• A notebook and pen. Genuinely. This is not optional.

• 
  

One of these, for measuring

• An antenna analyser or NanoVNA (the easy route), or

• Your transceiver's built-in SWR meter, or an external SWR bridge (the patient route)

Nice to have

• A short coax jumper — 0.5 m or so — so you can plug the analyser straight into the transformer

• A second pair of hands

2. Safety first — this one matters

An EFHW is fed at a voltage maximum. That's the whole point of the design — the impedance at the end of a half-wave wire is enormous, and the voltage along with it. Under a legal-limit transmission, the far end of an EFHW can be sitting at several thousand volts of RF.

• Never touch the wire while transmitting. RF burns are deep, slow to heal, and unpleasant.

• Terminate the far end in an insulator, and keep it well out of reach of people, pets and livestock. Two and a half metres is a bare minimum; higher is better.

• Watch for overhead power lines. If the antenna can physically reach a power line at any point during hoisting, lowering, or in a gale, it is in the wrong place. This kills people every year.

• Lower the antenna before working on it. Every time. It is tempting to lean off a ladder and fiddle with the end. Don't.

• Disconnect before thunderstorms, and give the antenna a DC path to ground at the feedpoint for static bleed.

3. Understanding what you're actually tuning

Two minutes of theory here will save you two hours of trimming later.

The half wave and its awkward end

At the exact centre of a half-wave wire, the current is at a maximum and the impedance is a friendly 50-ish ohms. That's a dipole, and that's why we feed dipoles in the middle.

At the end of that same wire, the opposite is true: the current falls to zero and the voltage peaks. The impedance there is somewhere in the region of 2,000–3,000 ohms. Feed it with 50-ohm coax directly and you'd get an SWR around 50:1.

That's what the 49:1 transformer is for. It takes roughly 2,450 Ω and turns it into roughly 50 Ω. (49 × 50 = 2450 — that's where the ratio comes from.)

The transformer is a sealed, finished item and nothing in this guide requires you to open it. All the tuning happens at the wire.

The harmonic trick — why one wire does five bands

Here's the elegant bit. That high-impedance condition at the fed end doesn't only happen at the fundamental frequency. It happens every time the wire is a whole number of half-waves long.

So an 80 m EFHW cut for 3.55 MHz gives you:

Band	Wire is...	Harmonic	Frequency
80 m	½ λ	×1	3.55 MHz
40 m	1 λ	×2	7.10 MHz
20 m	2 λ	×4	14.20 MHz
15 m	3 λ	×6	21.30 MHz
10 m	4 λ	×8	28.40 MHz

Five bands, one wire, no traps, no tuner. Note that the ×3, ×5 and ×7 harmonics land at 10.65, 17.75 and 24.85 MHz — which are not the 30 m, 17 m and 12 m bands. Those WARC bands will need an ATU. That's normal and expected; it isn't a fault with your antenna.

The one rule you must remember

Long wire → low frequency. Short wire → high frequency.

Adding wire drags the resonant frequency down. Removing wire pushes it up. Everything in this guide flows from that single relationship.

And the trap that catches everyone

Because all five bands come from the same wire, when you change the length you change all five bands at once — but not by the same amount.

Trim 11 cm off an 80 m EFHW and you'll nudge 80 m up by a harmless 10 kHz. That same 11 cm shifts 10 m up by 80 kHz. This is exactly how people carefully trim 80 m into perfect shape and then discover they've walked 10 m out the top of the band.

Tune for your lowest band, but keep one eye on your highest.

4. Before you measure anything

More bad tuning sessions are caused by bad measurements than by bad antennas. Get these right first.

Measure at final height, in final position

This is the big one. An antenna's resonant frequency shifts substantially with height above ground and with what's near it. Tuning an antenna at 2 m and then hoisting it to 12 m guarantees you'll have to start again.

If you must lower it to trim — and you should, for safety — then hoist it fully back up before every measurement. Yes, it's tedious. It's also the only way this works.

Turn the ATU off

Your rig's automatic tuner will happily hide the truth from you. Switch it off, and switch off any external tuner too. You are trying to measure what the antenna is doing, not what the tuner is doing about it.

Measure at the transformer, not in the shack

This one is for analyser and NanoVNA owners. If you possibly can, take your measurements at the transformer itself, with the instrument connected directly to its coax socket. (If you're tuning with the SWR meter in your rig you have no choice but to read from the shack — see Section 6 for what that means for your readings).

Measure from the shack instead and you are measuring the antenna plus your entire feedline. Coax loss makes SWR look better than it really is — the reflected wave has to travel back down the cable, losing energy on the way, so the meter at your end sees less of it. A 30 m run of RG-58 might turn a genuine 3:1 at the antenna into a comfortable-looking 2:1 at the rig.

That's not a lie exactly — it is what your transmitter sees, and it's why your PA doesn't complain. But it isn't what the antenna is doing, and it will mislead you while tuning. The flattering effect gets worse the higher the band and the longer and thinner the cable, so it's precisely on 10 m, where you most need honest readings, that a shack measurement is least trustworthy.

Measuring at the transformer also removes any question of the feedline contributing to the readings.

Practical compromise: if the transformer is 12 m up a tree, do your coarse trimming from the shack and take one careful set of readings at the transformer before you make the final cut. The difference between the two sets is your feedline loss — worth knowing anyway.

Sort out the choke and the counterpoise

Two things you must get right before any reading means anything.

The choke. Without one, the outside of your coax braid becomes part of the antenna. Your readings will then change depending on how much coax you've used, where it runs, and whether you happen to be touching the rig — which makes tuning impossible. Use a proper ferrite-cored 1:1 current balun rated for your power level.

The counterpoise. The transformer needs something to push against — a return path for the current. Give it nothing and the coax braid becomes the counterpoise by default, at whatever random length your feedline happens to be, and your readings will wander.

There are two accepted ways to arrange this, and both work.

Option 1 — a counterpoise wire, choke at the transformer

A short counterpoise wire, around 0.05 λ at your lowest band — roughly 4 m for an 80 m EFHW — connected to the transformer's ground side. The choke sits right at the transformer and isolates the whole feedline.

Option 2 — two chokes, no counterpoise wire

Here you deliberately use a measured length of coax braid as the counterpoise instead of a separate wire. Place the first choke 0.05 λ back from the transformer — again about 4 m on 80 m — and that defined section of braid does the job. The choke stops everything beyond it from joining in. A second choke at the rig keeps any residual RF out of the shack.

This is how most commercial EFHW kits are supplied, and it's tidier on a portable antenna where a dangling counterpoise wire is one more thing to snag.

The thing that matters is that you use one of them. A counterpoise wire, or a defined braid length plus a choke — but not neither. An EFHW with no defined return path is the single most common cause of readings that change every time you look at them.

Check the weather

Measure on a dry day if you can, and know that a soaking wet antenna and wet ground will typically pull your resonance down by 20–50 kHz on 80 m. If you tune it perfectly in the rain, it'll be slightly high when it dries out.

Know your band edges

The band plan varies by region. This article uses IARU Region 1 (UK and Europe):

Band	Region 1 (UK/EU)	USA
80 m	3.500 – 3.800 MHz	3.500 – 4.000 MHz
40 m	7.000 – 7.200 MHz	7.000 – 7.300 MHz
20 m	14.000 – 14.350 MHz	14.000 – 14.350 MHz
15 m	21.000 – 21.450 MHz	21.000 – 21.450 MHz
10 m	28.000 – 29.700 MHz	28.000 – 29.700 MHz

5. Method A — tuning with an antenna analyser

If you have a NanoVNA, RigExpert, MFJ analyser or similar, this is straightforward. You get to see what's happening instead of inferring it.

Step 1 — Sweep wide

Connect the analyser where your rig would normally connect. Set a wide sweep — say 3.0 to 4.0 MHz — and find the SWR minimum. Sweeping wide matters, because if the dip is outside the band you need to know where it actually is, not just that it isn't where you wanted.

Record two things: the frequency of the dip, and the SWR value at the bottom of it.

Step 2 — Compare against target

For a five-band 80 m EFHW, aim to put the fundamental dip between 3.520 and 3.580 MHz, with 3.550 MHz as the bullseye.

That may look oddly low if you're an 80 m SSB operator, and it's worth understanding why it's the right answer. Look at the harmonic table again — the fundamental gets multiplied by 8 to reach 10 m. Put the fundamental at 3.65 MHz and your ×2 harmonic lands at 7.30 MHz, right off the top of the Region 1 40 m band. The 3.55 MHz target is the value that keeps all five bands in play. If you only ever work 80 m and don't care about the others, tune wherever you like.

Step 3 — Decide

What you see	What it means	Go to
Dip below ~3.52 MHz	Antenna is too long	Section 7
Dip between 3.52 and 3.58 MHz	On target	Step 4
Dip above ~3.60 MHz	Antenna is too short	Section 8
No dip at all, SWR high and flat	Something is broken	Section 11

Step 4 — Sweep every band

Once the fundamental is close, sweep the other four and write down where each dip lands and how low it goes.

Here is an actual 1–30 MHz sweep of a working 80 m EFHW, measured on an AA-230 ZOOM. This is what you're aiming for.

The red trace is the real antenna. Read off against it:

Band	Harmonic	Measured dip	Measured SWR
80 m	×1	3.57 MHz	2.1 : 1
40 m	×2	7.14 MHz	1.65 : 1
20 m	×4	14.28 MHz	1.15 : 1
15 m	×6	21.42 MHz	1.5 : 1
10 m	×8	28.56 MHz	1.45 : 1

Three things are worth noticing.

The dips sit almost exactly on whole multiples of the fundamental. 3.57, 7.14, 14.28, 21.42, 28.56 — each one is the fundamental times a whole number, and the harmonic bands land where the arithmetic says they should. That is the mark of a well-behaved multiband EFHW.

80 m is the worst band, not the best. At 2.1:1 the fundamental is the poorest match on the whole sweep, while 20 m comes in at a superb 1.15:1. That's normal, and it catches people out — they assume the band the antenna is "cut for" should be the flattest. It usually isn't.

You can see the ×3, ×5 and ×7 resonances doing nothing useful. They land at 10.71, 17.85 and 24.99 MHz — near, but mostly not in, the 30 m, 17 m and 12 m bands. Those need a tuner, and no amount of trimming will change that.

What the other two traces show

The amber and blue traces on that chart are the two ways this goes wrong, and they illustrate something important about where an error shows up.

Amber — the antenna is 3 % too long. Every dip moves down in frequency by the same 3 %. On 80 m that's about 100 kHz, which you could easily miss. By 10 m the same error has become nearly 900 kHz, and the dip has fallen clean out of the bottom of the band.

Blue — no compensating coil. Many multiband EFHWs include a small series inductor part-way along the wire. Its job is harmonic alignment: because an inductor's reactance rises with frequency, it adds far more electrical length on 10 m than it does on 80 m, which is exactly the correction needed. Without it the antenna looks progressively shorter — more capacitive — the higher you go, and each successive harmonic creeps further to the right. The fundamental is spot on; 20 m and 15 m have drifted out of the top of their bands.

This is the single most useful thing to take from the chart. At the fundamental all three antennas are within about 0.1 MHz of each other and look nearly identical. By 10 m they're 2.1 MHz apart. Errors that hide on 80 m are unmissable at the top of the sweep — which is why sweeping 10 m tells you far more about your antenna than sweeping 80 m does.

If your antenna came with a coil in the wire, leave it where it is. Its position is part of the design, and moving it will pull the higher bands around without doing much for 80 m.

Step 5 — Iterate

Make one change. Re-measure at full height. Repeat.

If you change two things at once, you will not know which one helped.

A reminder about where you measure from

This is worth repeating because it's where analyser owners most often fool themselves: measure at the transformer wherever you can (see Section 4). Readings taken at the shack end include your feedline loss, which flatters the SWR and hides how the antenna is really behaving — most of all on the higher bands.

If your readings look suspiciously good, walk the analyser out to the feedpoint and take a second set. The difference between the two is your coax loss.

6. Method B — tuning with only your rig's SWR meter

No analyser? No problem. This method is older than analysers and it works perfectly well. You just can't see the whole curve at once, so instead you sample it point by point and work out the shape.

The principle

You can't see where the dip is. But you can see which way the SWR is sloping, and that tells you which side of the dip you're standing on.

• SWR gets lower as you tune down the band → the dip is below you → wire is too long

• SWR gets lower as you tune up the band → the dip is above you → wire is too short

• SWR is roughly equal at both edges and lower in the middle → the dip is in the band → you're there

Step-by-step

1. Set up for minimum stress on everything

• ATU off

• Lowest power your rig will produce — 5 to 10 watts is plenty

• Use a mode that gives a steady carrier: CW, FM, AM, or your rig's TUNE function

• Keep each transmission to two or three seconds. You're taking a reading, not calling CQ.

2. Pick a listening spot first

Before you key up, listen. Make sure the frequency is clear. Then transmit briefly. Never transmit outside the band edges, and never outside your licence privileges — even for testing.

3. Take five readings across the band

For 80 m in Region 1, use 3.510, 3.575, 3.650, 3.725 and 3.790 MHz. Write down the SWR at each. Every single one — don't rely on memory.

4. Read the pattern

Your readings look like	Verdict
1.4 → 2.0 → 2.7 → 3.3 → 3.8 (rising)	Dip is below the band. Too long.
2.4 → 1.6 → 1.2 → 1.7 → 2.5 (V-shaped)	Dip is around 3.650. In the band.
3.8 → 3.1 → 2.6 → 2.0 → 1.5 (falling)	Dip is above the band. Too short.

The frequency with the lowest reading is, near enough, your resonant frequency. If the lowest reading is at one of the edges, then the true dip is outside the band and you can only say "it's somewhere beyond that edge" — you can't say how far.

The pro tip: use 10 m as a magnifying glass

This is the trick that makes SWR-meter tuning genuinely practical, and it's worth the price of admission on its own.

Remember the multiplier effect: a length error that shifts 80 m by 10 kHz shifts 10 m by 80 kHz. The higher bands amplify your error eightfold.

So if the 80 m dip is hiding somewhere outside the band where you can't measure it, go and sweep 10 m instead. Five readings across 28.0–29.0 MHz will show you a nice obvious V where 80 m showed you a barely perceptible slope. Work out where 10 m is resonant, divide by 8, and you have your fundamental — with eight times the resolution.

The same trick works in reverse for fine-tuning. Once you're close on 80 m, small final trims are far easier to judge on 15 m or 10 m.

Three habits worth having

Calibrate if you're using an external meter. Analogue cross-needle and single-needle meters usually need setting to full scale on the FORWARD position before you read

REFLECTED. Do it at every frequency, not just the first one.

Don't trust the last digit. 

The SWR meter in a typical transceiver is a rough instrument. A reading of 1.3 and a reading of 1.5 may well be the same thing. Look at trends across five readings, not at individual values.

Remember your coax is in the measurement. 

Tuning from the rig means you're stuck reading through the whole feedline, and coax loss always makes SWR look better than it is. That's mostly fine for this method, because you're looking for the shape of the curve rather than its absolute depth — and coax loss shifts every reading in the same direction, so the slope still points the right way.

Where it does bite is when you judge whether a band is "good enough". A shack reading of 1.8:1 on 10 m through 30 m of thin coax could easily be 3:1 at the antenna. If you can borrow an analyser for even one afternoon, take a single set of readings at the transformer to calibrate your expectations — then go back to tuning from the shack knowing what your numbers really mean.

7. If the antenna is TOO LONG

Good news: this is the problem you want. Wire is easy to remove and hard to put back.

Symptom: the SWR dip sits below your target frequency. On 80 m, that means below about 3.50 MHz.

The golden rule: fold, don't cut

Do not cut. Fold the excess back on itself at the far end and secure it with two cable ties. Folding back 30 cm removes 30 cm of electrical length just as effectively as cutting it off, and — crucially — you can undo it when you inevitably overshoot.

Cut only when you have confirmed the result on every band, at full height, and you're happy. Then re-terminate the end properly with a soldered or crimped loop and a fresh insulator.

How much to fold back

Work out how far you need to move, then use the table in Section 9. Or use the formula:

Wire to remove = Current length × (Frequency shift needed ÷ Current frequency)

Worked example. Your 80 m dip is at 3.480 MHz and you want 3.550 MHz. That's a 70 kHz climb. Your wire is 40.8 m.

Wire to remove = 40.8 m × (70 ÷ 3480) = 40.8 × 0.0201 = 0.82 m

Fold back 82 cm, re-hoist, re-measure. Expect to land close but not exactly on target — that's fine, that's what the second iteration is for.

Trimming discipline

1. Work in halves. If you think you need 80 cm, take 40 cm first. Measure. Then decide.

2. One end only. An EFHW is fed at one end and trimmed at the other. Never trim near the transformer.

3. Re-measure at full height every time. Every time. There are no shortcuts here.

4. Slow down as you close in. Big folds while you're more than 50 kHz out; 10–20 cm adjustments for the last stretch.

5. Check all five bands before you cut. The 80 m dip being perfect means nothing if you've pushed 10 m out of the band.

8. If the antenna is TOO SHORT

This is the one that makes people wince, but it's entirely fixable. You have several options, from the boringly practical to the genuinely clever.

Symptom: the SWR dip sits above your target. On 80 m, above about 3.60 MHz.

Option 1 — Splice on more wire (the right answer)

Just add wire to the far end. This is the correct fix and it restores full performance, because you're giving the antenna back the physical length it should have had.

Make the joint properly:

• Solder it, then cover with adhesive-lined heatshrink

• Or use a proper inline crimp connector, sealed with self-amalgamating tape

• Add a strain relief — twist the wires together mechanically before soldering, so the joint isn't carrying the load

• Add generously and fold back the excess. Adding 1.5 m and folding 60 cm of it back is much better than adding exactly 90 cm and discovering you needed 95 cm.

  
  

Option 2 — Add a tail at the end

If soldering up a tree isn't appealing, add a short length of wire at the insulator with a good mechanical connection, and let it hang down or run off at an angle. It doesn't need to be in line with the main wire — a drooping tail works. It's electrically slightly different from a straight extension, but for the last metre or so it's close enough.

Option 3 — Add a loading coil (when you can't add length)

If you're physically out of space — a small garden, a short run between two fixed points — a loading coil near the far end adds electrical length without adding physical length. An inductor placed in series with the wire slows the wave down, making the antenna behave as though it were longer.

• Place it about two-thirds to three-quarters of the way out from the feedpoint

• Leave a short "tail" of wire beyond the coil — this is what makes the coil efficient

• Expect to need on the order of 100–120 µH with a 6–8 m tail for a substantial 80 m shortening

• Be aware of the trade-off: loading coils cost you bandwidth and some efficiency, and — importantly for an EFHW — a coil sized for the fundamental will play havoc with the harmonic bands. This is a fix for a single-band problem, not a multiband one.

Option 4 — Change the geometry

Reconfiguring the antenna can buy you a surprising amount of electrical length for free:

• Lower it. Bringing the antenna closer to ground generally lowers its resonant frequency.

• Add a droop. Letting the far end sag, or running it as an inverted-L or sloper, effectively increases length.

• Increase the counterpoise. A longer counterpoise can pull the resonance down slightly.

These are coarse adjustments and they change your radiation pattern too — but if you're only 20 or 30 kHz out, they may be all you need.

Option 5 — Live with it

Be honest about whether you have a real problem. If your dip is at 3.62 MHz instead of 3.55, and the SWR across the part of the band you actually operate on is under 2:1, and the other four bands are fine — you have a working antenna. Your rig's ATU will mop up the rest with negligible loss. Not every imperfection needs correcting.

What won't fix it

Two things people reach for that don't help:

• Anything involving the transformer. The transformer sets the impedance match, not the resonant frequency. If your dip is in the wrong place, nothing you do at the transformer will move it — and there's nothing in there you need to adjust.

• Adding or moving the choke. A choke is essential for stable, trustworthy readings, and getting it wrong will make your measurements wander — but it will not move the dip to where you want it. Only wire length does that.

9. How much wire? The numbers

The relationship is beautifully simple:

The percentage change in length equals the percentage change in frequency.Shorten by 1 %, and every resonance climbs by 1 %.

The 80 m EFHW quick table

For a wire around 40.3 m long, resonant at 3.550 MHz:

Wire removed	80 m	40 m	20 m	15 m	10 m
5 cm (2 in)	+4 kHz	+9 kHz	+18 kHz	+26 kHz	+35 kHz
10 cm (4 in)	+9 kHz	+18 kHz	+35 kHz	+53 kHz	+70 kHz
20 cm (8 in)	+18 kHz	+35 kHz	+70 kHz	+106 kHz	+141 kHz
50 cm (20 in)	+44 kHz	+88 kHz	+176 kHz	+264 kHz	+352 kHz
1 m (3 ft 3 in)	+88 kHz	+176 kHz	+352 kHz	+529 kHz	+705 kHz

Adding wire moves everything the same distance in the opposite direction.

The numbers worth memorising

On an 80 m EFHW: about 1 cm of wire per 1 kHz on 80 m.Or in imperial: roughly half an inch per kHz.On a 40 m EFHW: about 0.3 cm per kHz (the wire is half as long, so it's half as sensitive per kHz — but each kHz on 40 m is proportionally smaller too).

Starting lengths, if you need to check your build

Target frequency	Half-wave length	Cut long (+4 %) for tuning
3.550 MHz (80 m)	40.3 m / 132 ft	41.9 m / 137 ft
7.100 MHz (40 m)	20.1 m / 66 ft	20.9 m / 69 ft
14.200 MHz (20 m)	10.1 m / 33 ft	10.5 m / 34 ft

The formulas: L (metres) = 143 ÷ f (MHz) or L (feet) = 468 ÷ f (MHz)

One thing that catches people out: insulated wire

If your antenna uses PVC-insulated wire rather than bare, it will resonate 2–5 % lower than a bare wire of the same length. The insulation's dielectric slows the wave down.

If you built with insulated wire and used a bare-wire calculator, this alone could explain a dip that's 70–170 kHz low on 80 m. Nothing is wrong — you just need to take a bit more off than you expected.

10. When the bands won't all play nicely

You've got 80 m spot on. 40 m and 20 m are lovely. And 10 m is sitting at 4:1 and sulking.

This is the normal end state of EFHW tuning, and it's worth saying plainly: a perfect five-band EFHW is a compromise, not a solved problem. Here's how to improve the odds.

Understand what you can and can't fix by trimming

Trimming moves all five dips together, in fixed proportion. You cannot move 10 m without moving 80 m. So if 80 m and 10 m disagree about where they want to be, no amount of trimming will satisfy both.

Choose your priority band and tune for that, then judge the others on whether they're merely acceptable.

Look at the installation, not the antenna

If a band's dip is in roughly the right place but the SWR at the bottom of it is stubbornly high — 2.5:1 or worse — the antenna itself is rarely the culprit. Work through these in order, because they're listed cheapest-and-most-likely first:

Check the choke and the counterpoise. A large fraction of "one band is bad" reports turn out to be common-mode current. Confirm your choke is fitted and working, and that you have either a counterpoise wire or the correct 0.05 λ of braid ahead of the first choke — not neither, and not a random length. Adding a second choke further down the feedline often clears it.

Raise it, or change its shape. Height above ground is the single biggest influence on feedpoint impedance. An EFHW at 6 m will not match as well as the same antenna at 12 m. If you can't go higher, try changing the configuration — sloper to inverted-L, or moving the far end away from a wall, gutter or fence.

Look at what's near the wire. Metal guttering, wire fencing, satellite dishes, foil-backed insulation and damp trees all detune an antenna, and they affect the higher bands most. A metre or two of clearance can be worth more than an hour of trimming.

Check the connectors and the coax. A corroded PL-259, water in a joint, or a tired old run of coax will all show up as a poor match that trimming cannot fix. Substituting a known-good short coax jumper at the transformer is a quick way to rule the feedline in or out.

And then just accept some of it

The WARC bands — 30 m, 17 m and 12 m — are not harmonics of your fundamental and will show a high SWR. That is by design, not by fault. Use an ATU on those bands, or don't use them on this antenna.

Likewise, 10 m on an 80 m EFHW is the eighth harmonic and the hardest to please. If it comes in at 2.5:1 across the bottom of the band, your rig's internal ATU will handle it and you'll never notice the difference on air.

11. Troubleshooting the weird stuff

Symptom	Likely cause	What to do
SWR high and flat everywhere, no dip on any band	Broken wire, or a bad connection somewhere in the feedline	Work through the checklist below before assuming the antenna is at fault.
SWR changes when you add or remove coax	Common-mode current on the braid	Fit a 1:1 choke at the feedpoint, and a second at the shack.
SWR changes when you touch the rig or the mic	Same problem — RF is coming back down the feedline	As above. Also check your station earth.
Dip is in the right place but minimum SWR is 2.5:1+	Installation — usually height, surroundings, or common-mode current	Work through Section 10. Failing that, accept it and use the ATU.
Fine when dry, terrible after rain	Water in a connector or a coax joint	Dry and re-seal every outdoor joint. Tape from the bottom up so water runs off, not in.
Everything shifts every time you take a reading	You're measuring at different heights	Full height, every single time.
SWR looks great at the rig but the antenna seems deaf	Coax loss masking a genuinely poor match	Measure at the transformer and compare. The gap is your feedline loss.
Resonance drifted over a season	Wire stretch, or the tree grew	Re-measure the physical length. Wire does creep under tension.
80 m is fine, 10 m is miles out	Normal — a small error on the fundamental is multiplied eightfold by the time you reach 10 m	Check Section 5 first. If 80 m is genuinely on target, accept it and use the ATU on 10 m.

No dip anywhere? Work through this before you cut anything

If the SWR is high and flat right across the spectrum with no dip on any band, the antenna is not mistuned — something is disconnected. Trimming will not help, and you'll only ruin a good wire. Check these in order:

1. The wire terminal at the transformer. Is the radiator still firmly attached, and is the connection clean? This is the joint that takes all the mechanical strain, and it's the one that fails.

2. The far end. Has the wire pulled out of the insulator, or broken where it was folded back?

3. Your coax. Substitute a short jumper straight into the transformer. If the fault disappears, the problem was the feedline, not the antenna.

4. Every connector. Look for corrosion, a loose centre pin, or a shield that isn't making contact. Give each one a gentle wiggle while watching the meter.

5. The counterpoise or choke connection. A detached counterpoise, or a choke that has come adrift, can produce badly behaved readings on every band.

6. Try a different band and a different rig if you can, to rule out your own equipment.

Don't open the transformer. There's nothing user-serviceable inside it, and opening it won't tell you anything you can act on — it will just let the weather in. If you've worked through the list above and the antenna still shows no resonance anywhere, get in touch with us at radio-stuff.com with your readings and we'll sort it out.

12. Write it all down

When you're finished, record all of this somewhere you'll find it again:

• Final wire length, measured with a tape from the transformer terminal to the insulator

• Height at each end, and the configuration (flat top / sloper / inverted-L)

• The resonant frequency and minimum SWR on every band

• Wire type — bare or insulated, and the gauge

• Counterpoise length — or, if you used the two-choke arrangement, the length of coax between the transformer and the first choke

• Where you took the readings — at the transformer or at the shack. Readings from the two places are not comparable.

• The date and the weather

Two years from now, when a gale brings the antenna down and you have to rebuild it, this note will save you an entire afternoon. It also gives you a baseline: if the SWR changes next winter, you'll know by how much, and from what.