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RF / Switching In progress

HF to 2 m manual
three-antenna switch

One transceiver, three antennas, one knob. Completely passive: no power supply, no relays, no control wiring, nothing to fail in the dark. Everything below is what it takes to build one and, more to the point, how to prove it works before you key up.

Range
HF through 2 m
Power
~100 W HF and 6 m, 50 W on 2 m
Impedance
50 Ω
Switching
Physical rotary, break before make
Connectors
Four chassis N-female
External power
None
Do not

Never rotate the selector while transmitting. Release PTT and let the transmitter return to receive before changing antennas. The non-shorting switch briefly leaves the radio disconnected as it moves, which is desirable, but it does not make hot switching safe.

The one part that matters

The switch

An Electroswitch D4C0112N ceramic rotary switch, with the adjustable stops set so the shaft reaches only three positions.

Electroswitch does not specify the D4C0112N as a characterized 146 MHz coaxial component. Its low contact resistance, ceramic body, silver contacts and small size make it appropriate for a home-built switch when the RF connections are kept very short. That is a judgement, not a specification, which is why the VNA tests below are not optional.

D4 series datasheet DigiKey Newark

  • Ceramic insulation
  • Silver-plated brass contacts
  • One pole, up to 12 positions
  • Adjustable position stops
  • Non-shorting, break before make
  • About 3.5 mΩ initial contact resistance
  • 9 A contact-carrying capacity
  • 1,500 VAC dielectric strength between current-carrying components and ground
  • 1/4 inch shaft

What it costs

Bill of materials

About $70 to $110
Mostly the switch

The enclosure, the N connectors and the coax were already on the shelf, so they are listed as owner-supplied.

QtyComponentSpecificationPurpose
1Ceramic rotary switchElectroswitch D4C0112NSelects one of three antennas
1Pointer knob1/4 inch shaft, about 1.5 to 2 in diameterSelector control
4Chassis N-female connectorsOwner-suppliedOne radio input, three antenna outputs
A/R50 Ω coaxOwner-supplied, keep each internal run shortInternal RF connections
3Static-bleed resistors100 kΩ, 2 W, metal oxide or carbon compositionBleeds static from antenna conductors
1Copper or brass ground plate0.032 to 0.062 in thickCommon RF ground plane
1Tinned copper bonding braidShort and wideBonds lid and plate to chassis
1Chassis ground stud1/4-20 brass bolt, nuts, flat and internal-tooth washersExternal station ground
1 setMounting hardwareStainless or brass screws, nuts, internal-tooth washersAssembly and bonding
1Front panel labelRADIO, ANT 1, ANT 2, ANT 3, and DO NOT SWITCH WHILE TRANSMITTINGIdentification, and the reminder

How it goes together

Electrical arrangement

Only the coax center conductors are switched. Every connector body and every shield stays bonded to the enclosure and the ground plate, continuously.

HF TO 2 M THREE ANTENNA SWITCH SCHEMATIC DRAWN K4DIA SCALE NONE REV A SHEET 1 OF 2 RF GROUND PLATE RADIO J1 common S1 3 POSITION, NON-SHORTING ANT 1 J2 R1 100k 2W ANT 2 J3 R2 100k 2W ANT 3 J4 R3 100k 2W CONNECTOR BODIES AND SHIELDS NOTES: 1. Only the coaxial centre conductors are switched. 2. All connector bodies and coax shields bond to the RF ground plate. 3. R1 to R3 provide static bleed only. They are not lightning arrestors. 4. Keep exposed centre conductor under 1/4 in at every switch terminal. 5. Set the adjustable stops on S1 so only three contacts can be selected.
The three bleed resistors are static drains only. They are not lightning arrestors and they do not replace proper external grounding and surge protection at the building entry.

Where it sits

Physical layout

A compact star with the switch roughly centered between the four connectors, which is what keeps the leads short. If the enclosure will not allow it, the radio-to-common path gets priority for the shortest run.

HF TO 2 M THREE ANTENNA SWITCH PREFERRED LAYOUT DRAWN K4DIA SCALE NONE REV A SHEET 2 OF 2 1 2 3 4 5 6 ITEM DESCRIPTION 1 J2 N-type chassis, ANT 1 2 J3 N-type chassis, ANT 2 3 J4 N-type chassis, ANT 3 4 J1 N-type chassis, RADIO 5 S1 Rotary switch, ceramic, 3 position 6 RF ground plate, copper or brass NOTES: 1. Switch centred between the four connectors, so every internal lead stays short. 2. If the enclosure will not allow it, J1 to S1 takes priority for the shortest path. 3. Hole sizes to suit the parts bought. Confirm positions with a full size template before drilling.
Rust marks the radio-to-common path, the one that gets the shortest run when the layout has to compromise.

Getting it right

Construction

Enclosure and grounding

  1. Use a conductive metal enclosure wherever possible.
  2. If the box is plastic, install a continuous copper or brass ground plate behind all four N connectors and the switch.
  3. Remove paint, anodizing and oxidation under every connector flange and grounding point.
  4. Use internal-tooth star washers so they bite into bare metal.
  5. Bond a removable lid to the chassis with short, wide tinned braid.
  6. Bond the ground plate at several points, not through one small wire.

RF connections

  1. Keep every internal coax run as short as the box permits.
  2. Bring the shield within about 1/4 inch of each switch terminal.
  3. Keep exposed center conductor under about 1/4 inch.
  4. Bond each shield to the ground plate right beside its terminal.
  5. No long hookup wire between the coax and the switch.
  6. Do not bundle the three antenna conductors together.
  7. Separate the unused switch terminals as much as the layout allows.
  8. Secure the coax mechanically so a tug on an external cable cannot stress a joint.

Switch setup

  1. Find the common terminal with an ohmmeter.
  2. Identify three adjacent fixed contacts.
  3. Set the stops so only those three can be selected.
  4. Verify break-before-make operation with the ohmmeter.
  5. Confirm no position ever connects two antennas at once.
  6. Label the positions only after confirming the real contact sequence.

In order

Assembly sequence

  1. Lay out the four connectors and the switch before drilling anything.
  2. Check internal clearance for the switch body, the coax bend radius, and room to get a soldering iron in.
  3. Drill and deburr the connector, switch and grounding holes.
  4. Remove coating around every RF grounding surface.
  5. Install the N connectors and the rotary switch.
  6. Install and bond the RF ground plate.
  7. Cut and prepare the internal coax sections.
  8. Connect the radio port to the switch common terminal.
  9. Connect the three selected contacts to antenna ports 1, 2 and 3.
  10. Fit a bleed resistor from each antenna center conductor to the plate.
  11. Bond the lid and install the external ground stud.
  12. Check every path with an ohmmeter before a radio goes anywhere near it.
  13. Run the VNA tests.

Before any radio is connected

Continuity checks

PositionRadio to selected portRadio to other portsCenter to chassis
ANT 1ContinuityOpen but for the bleed resistorsAbout 100 kΩ through the selected port resistor
ANT 2ContinuityOpen but for the bleed resistorsAbout 100 kΩ through the selected port resistor
ANT 3ContinuityOpen but for the bleed resistorsAbout 100 kΩ through the selected port resistor

The exact readings shift with the three installed resistors and the measurement path. What matters is that only one low resistance RF path exists at a time, and that no center conductor is shorted to the chassis.

The part that decides it

VNA verification

Sweep 1 to 160 MHz
Watch 50 and 146 MHz

Return loss, selected path

  1. VNA Port 1 to RADIO.
  2. Known-good 50 Ω termination on the selected antenna port.
  3. Leave the other two antenna ports open.
  4. Sweep about 1 to 160 MHz.
  5. Record return loss or SWR at the amateur bands of interest, especially 50 MHz and 146 MHz.

Target Preferably 1.2:1 SWR or better at 146 MHz, and no sharp unexpected resonance anywhere from HF to 2 m.

Insertion loss

  1. VNA Port 1 to RADIO.
  2. VNA Port 2 to the selected antenna port.
  3. Terminate the two unused ports in 50 Ω.
  4. Sweep 1 to 160 MHz and record S21.

Target Preferably under 0.2 dB at 146 MHz. Expect better on HF.

Isolation

  1. Keep Port 1 on RADIO.
  2. Select one antenna.
  3. Move Port 2 to each unselected port in turn.
  4. Terminate the selected and remaining ports in 50 Ω.
  5. Sweep 1 to 160 MHz and record the worst case.

Target Preferably at least 40 dB at 146 MHz.

If 2 m SWR or isolation disappoints: shorten the exposed center conductors, improve the shield bonding around the switch, and put more physical distance between the antenna port conductors.

Living with it

Operating notes

  • Never change antennas while transmitting.
  • Reduce power for the first on-air tests.
  • Confirm acceptable SWR into each antenna before full power.
  • The bleed resistors are not lightning protection.
  • Disconnect antennas in severe weather, and use properly grounded surge protection at the building entry.

The finished unit is a passive one-radio, three-antenna selector needing no power supply, control wiring, relays or indicators. Performance through 2 m comes down to short RF paths, continuous shielding, and a low-inductance common ground.