Tuning for several bands

A fan dipole, a trap vertical, a multiband EFHW: one feed line, several bands, and every knob you turn can move all of them. Tuning such an antenna means watching more than the band you are on. The workbench gives you two ways to do it, and both start from the same list of bands:

• By hand. Turn the knobs yourself and watch every band’s impedance move on the Smith chart at once.
• With the optimizer. Mark the knobs it may move and let it balance the bands for you, worst band first.

Which one to reach for depends on how much the knobs interact, and the antenna below shows how to tell.

The example: a fan dipole for 12 and 10 m

multiband.twoband_fan_dipole is two dipoles bonded at one feed, one cut for 12 m and one for 10 m. Its per-band knobs are grouped under band 0 (the 12 m element) and band 1 (the 10 m element); the one that matters here is each band’s length. The numbers on this page are momwire over average ground, the workbench’s default.

Set up the bands

Both ways start from the band list, in the Optimize gear menu (the gear beside the Optimize toggle, next to Live):

1. Open the gear menu and tick Several bands at once. On a design whose bands have their own frequency knobs, as this one’s do, the list starts with those: here 24.97 and 28.47, the frequencies its 12 m and 10 m bands are set for. On other designs it starts with the measurement frequency.
2. Add any other band you care about: type it in MHz and press add. Up to eight; × removes one.
3. Close the menu.

The list belongs to the design you set it on, and it stays while you work on that design.

Tuning by hand

With a band list set, the Smith chart lists its bands in its bottom-left corner, each in the colour its marker will be drawn in. Click a band to show it. From then on, every knob you drag shows that band’s impedance moving on the chart with a short trail behind it, alongside the live dot for the measurement frequency. Click it again to hide it.

Each band shown costs one more solve per knob change. That is why none is shown to start with, and why the toggles sit on the chart: show the bands the knob in your hand affects, hide the rest. The extra solves are cheap (the design is built once and read at every band, about twice as fast as a full solve per band), but on a large design they add up.

Now drag band 0 length. Measured on this antenna, 5 cm either way:

Change	12 m (24.97 MHz)	10 m (28.47 MHz)
as designed	52.4 − j5.1 Ω, SWR 1.12	50.8 − j5.7 Ω, SWR 1.12
band 0 (12 m) length −5 cm	50.3 − j15.6 Ω, SWR 1.36	50.7 − j6.4 Ω, SWR 1.14
band 0 (12 m) length +5 cm	54.7 + j6.3 Ω, SWR 1.16	50.8 − j5.2 Ω, SWR 1.11
band 1 (10 m) length −5 cm	52.4 − j5.3 Ω, SWR 1.12	44.2 − j39.5 Ω, SWR 2.29
band 1 (10 m) length +5 cm	52.4 − j4.9 Ω, SWR 1.11	56.4 + j20.8 Ω, SWR 1.50

Each element moves its own band and barely touches the other: 5 cm on the 12 m element swings 12 m’s reactance by about 11 Ω and 10 m’s by half an ohm. On the chart, the 12 m ring travels while the 10 m ring sits still. An antenna like this tunes by hand, one knob per band, in turn: set band 0’s length until the 12 m ring crosses the real axis near the centre, then band 1’s for the 10 m ring, then a final touch on the first.

A few things help along the way:

• The live dot is the measurement frequency, and the readout, the SWR and the pattern follow it. It is drawn over the band rings, so it stays visible when the dial sits on one of your bands.
• Touching a band’s knob moves the dial, on this design, while the dial’s lock is closed: the dial jumps to that band’s own freq knob, the same frequency the band list started from. So the live dot lands on the ring of the band you are tuning, with its numbers in the readout. Open the lock to keep the dial where you put it.
• To see the whole span, a frequency sweep across both bands (the SWR view is the plainest) shows both dips. With auto re-run on, it re-sweeps half a second after you let go of a knob; with it off, the curve fades as stale until you press run, and the ring that marks the measurement frequency on it disappears, since it would mark the antenna as it was.

When the knobs fight

Hand tuning works because each knob here mostly drives one band. Not every multiband antenna is that kind. On UR0GT’s three-band vertical (160/80/40 m), the height, the top wire and the coupling capacitor each move every band: fix 40 m and 80 m drifts, fix 80 m and 160 m drifts. You can still watch all three rings while you try, and the chart shows you quickly that you are chasing your tail. That is the job for the optimizer.

Letting the optimizer do it

The band optimizer uses the same band list. It tunes the knobs you mark for the best compromise across all the bands at once.

1. Mark the knobs it may move. Right-click a knob, tick Optimize this knob, and set its Optimize range, the bounds it searches. (With a knob focused, o toggles the mark.) Knobs inside a band group are marked like any other. Here: band 0 length and band 1 length, each left on the range the design gives it.
2. Turn on Optimize. With the band list set, the run tunes for every band in it rather than for the measurement frequency.
3. Watch it run. The readout shows each band’s SWR and the worst one, and each band’s impedance moves on the Smith chart in its own colour, the worst band’s ring drawn brightest.

It minimizes a blend of the worst band and the average:

(1 − w) · worst band’s SWR + w · mean of the bands’ SWR

Balance in the gear menu sets w (default 0.5). At 0 it improves the worst band only, which lets it drag the easy bands down to the hardest one’s SWR; the average term stops that, trading a little on the worst band for a lot on the rest. On the UR0GT vertical, w = 0.2 settles on SWR 1.86 on all three bands, and w = 0.5 on 1.93 / 1.93 / 1.34: 0.07 worse on the worst band, half an SWR unit better on 40 m.

When it finishes, a table gives each band’s SWR before and after, plus the worst and the mean. On the fan dipole, from the design’s own lengths:

Band	Before	After
12 m, 24.97 MHz	52.4 − j5.1 Ω, SWR 1.117	53.3 − j0.7 Ω, SWR 1.067
10 m, 28.47 MHz	50.8 − j5.7 Ω, SWR 1.122	51.9 − j0.4 Ω, SWR 1.038

It got there by lengthening the 12 m element 2.0 cm (5.877 → 5.897 m) and the 10 m element 9 mm (5.190 → 5.199 m), in 59 evaluations. Small moves, because the design was close; from an antenna you have dragged far off, it travels further and the before column is worse. Read the notes under the table: one says when a knob ended at the edge of its range (the best value may lie past it: widen the range and run again), another when no band is near a match.

Opening the gear menu pauses Optimize, and closing it resumes with one run for all the edits, so changing the bands or the balance never starts several runs.

Exact resonance on every band

The workbench’s band run minimizes SWR. The command line can also solve for an exact condition on every band, which is worth trying when you have as many knobs as conditions. Resonance (X = 0) is one condition per band, so two bands and two lengths:

python -m antennaknobs optimize --builder multiband.twoband_fan_dipole \
    --bands 24.97:res,28.47:res --mode root \
    --params bands.0.length bands.1.length

On the fan dipole this finds both reactances below 1 mΩ in 9 evaluations, SWR 1.068 / 1.039. When there is no exact answer within the knobs’ ranges, the root form says so (no root in the box, or which knob moves no band) rather than returning a near miss; the minimax form is the one to reach for first. Optimizing across bands covers the forms, the per-band objective, feed and Z₀, and the band-by-band sequential mode.

Keeping a run

A run you want to come back to can be kept. Under a finished band run’s table, Keep… saves it as a study: where it started, the knobs it moved with their ranges, the bands, the balance, and the answer. It is listed under Studies in the design’s analysis picker afterwards, where picking it jumps the knobs to its answer and Run again from its start repeats it. That is the easy way back to the same setup after you change something else about the antenna. The command line’s optimize --bands … --keep NAME does the same; see Keeping a run.

On the command line

The same run as the workbench’s, as one command:

python -m antennaknobs optimize --builder multiband.twoband_fan_dipole \
    --bands 24.97,28.47 --params bands.0.length bands.1.length

It prints the knobs’ ranges, the per-band table before and after, and a params block to paste over the design’s. --mean-weight is the balance, --bound NAME=LO:HI a knob’s range, and --ground the ground (average ground when not given).