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Vintage Receivers


  • Solves antenna matching problem with two phased antennasProvides convenient and flexible antenna switchingVacuum relays provide reliable high power operation
  • Easily tested on the ground before tower installation

Tower Mounted Stack Match

Stacked rotary beams are popular. However, connecting two 50 Ohm antennas together presents some problems. The resulting impedance of the total antenna is now 25 Ohms and the resulting SWR presented to the transmitter is 2:1.

A method of easily switching between individual antennas and the combined pair is also needed. The two antenna selectable stack match described here gives solutions to these problems.

This station accessory consists of two separate sections. One is the switch selectable control box and power supply that sits on the operating bench, and the other is the remotely mounted relay switching box that mounts on the tower. Four Jennings  RJ1A-26  SPDT vacuum relays with 26 Volt coils are used 1. These provide reliable legal limit antenna switching.

Power to drive these relays is supplied using the antenna coax center conductor. RF chokes and bypass capacitors keep the RF isolated from the DC supply voltages. A home made 50 to 25-ohm balun transformer is used to match the 25 Ohm antenna impedance when both antennas are selected at the same time 2. This balun consists of 14-trifilar turns of #14 wire wound on a T-200-6 core 3. I assembled this transformer inside a round tin enclosure with a lid, and potted the transformer after assembly and testing. This potting procedure or toroid enclosure is not necessary.

Two one-kV mica capacitors with unmarked values are shown on the schematic of the toroid transformer. These are selected and their values determined during testing. Choose values that provide the lowest SWR using a test frequency of 30 MHZ from an SWR analyzer. These capacitors compensate for stray inductance values of the toroid transformer. Once these capacitor values are experimentally determined, just solder them in place permanently. Expect values required to be about 10 to 30 pF or so.

Clamping diodes are used across three relays to suppress switching transients caused by the relay coils. A Neon gas bulb is used for the same purpose for the relay that has bipolar DC voltages applied to it.

The station control box switch selector has a regulated 26 Volt DC power supply. Plus 26 Volts applied to the coax connector selects the lowest antenna. Minus 26 Volts selects the highest antenna, while no voltage supplied selects both antennas.

A single supply polarity is used. The isolated common of the power supply is reversed with the supply voltage to provide the necessary apparent minus voltage. This switching logic is done by the control box front panel mounted antenna selector switch. This is a two pole three position on-center off-on toggle selector switch.

The relay switching components are mounted in a die-cast aluminum box that also resides inside a weather resistant enclosure. The outside enclosure I used has a clear removable swing away front door that provides access to the internal relay components.

You may use nearly any type of outside weather enclosure provided it gives protection to the inner components. I got my outside enclosure from a surplus source, and no more are available there. These specific types of enclosures are expensive if purchased new, so search through your junk box, flea markets or on the Internet for an alternative that will work for you.

This selectable antenna stack match is easy to test without installing it on the tower. Connect a SWR analyzer to the "transmitter" coax connector. Connect 50 ohm PL259 terminators on each of the "upper" and "lower" antenna connectors. Test for a low SWR reading while selecting all antenna combinations. Once you are satisfied with this test, you are ready to install the remotely mounted relay box on your tower.

Use the same length of coax from each antenna is required. This keeps the
antennas in proper phase when both are active at once. Use the identical
type of coax preferably from the same reel.

The only precaution during use is to remember to never hot switch the relays while you are transmitting! Only change antennas when you are listening. Following this simple precaution, I have obtained good operation from this two antenna stack match for six years while running legal limit power. No problems have developed since installation.


Download complete one page schematic PDF format


1. This circuit design will allow for most vacuum relay coil voltages to work. The power supply voltage regulator is an adjustable type. Simply adjust the variable resistor to set the control box voltage to match your specific coil voltages. I built another version of this stack match using vacuum relays with six-Volt coils. To avoid increasing the power dissipation of the voltage regulator IC, reduce the input voltage to the regulator by choosing a power transformer with a smaller secondary voltage.

2. Sevick, Jerry "Transmission Line Transformers" , Published by ARRL, 1987, Chapter Six "Transformer Designs with Impedance Ratios Less than 4:1", pg 6-11.

3. An interesting alternative to this balun appeared in February, 1994, QST article by Dean Straw, N6BV and Fred Hopengarten, K1VR. Their condensed material appears in the ARRL Antenna Book, 1997, 18th Edition. Page 11-29 describes the 25 Ohm to 50 Ohm balun wound with coax on a T-200A toroid core. The stack match described in this section of the Antenna Handbook does not mount remotely, and requires two identical coax runs to the ham shack. Otherwise, their system is very similar to what I describe here.