VHF Antenna Switch

When I was building a new VHF antenna, I needed to switch between the vertical and horizontal polarizations mounted on the same boom. To save on feedline, the switch has to sit close to the boom, which means a relay. The simplest option is an REV-14 for 800 UAH (price as of April 2018).

But where’s the saving in that, when the same money buys 20 meters of RG-213 on OLX? So I decided to reinvent the wheel.

I based it on a design proposed by A. F. Kardo-Sysoev, Doctor of Physical and Mathematical Sciences.

Sample pages from the report

Below is a quote from the article, which has been reposted many times online. Unfortunately, I don’t know the original source.

Building this relay gave the following results:

— SWR from 100 kHz to 500 MHz: 1 : 1.05;

— 500…1000 MHz: 1.05 : 1.1.

Building the relay:

— prepare the cable as shown in the figure and solder in the reed switch (any reed switch will do; it all depends on the “task” at hand);

— wrap the reed switch in PTFE film (not shown in the figure), slide the copper braid (shield) over it and solder as shown in the figure;

— make the L1 coil former from any dielectric material (PTFE, acrylic, ebonite, etc.). You can do without it altogether: glue a sleeve out of cardboard, or put insulating tape over the braid and wind the wire on top;

— wind the coil with PEL-2 or similar wire, 500 to 2000 turns, wire diameter 0.05…0.25 mm.

Coil resistance for different pull-in voltages:

• 6 V — 90 Ω
• 12 V — 170 Ω
• 27 V — 350 Ω

With the coil fixed over the reed switch, the whole assembly is wrapped in insulating tape and, if needed, coated with the sealant used in car repair.

Tests showed this relay to be highly reliable. With a 100 W transmitter there were no problems switching the antenna (switch only with the transmitter off).

What I needed was specifically an antenna switch, and I also wanted to be able to use it with other antennas. So I built the design you’ll see below.

First, I found some PET-155 wire, 0.25 mm in diameter. I’ll say right away that this is thick wire; 0.18–0.15 mm is better. I 3D-printed a coil former that slides over an MKS-52201 changeover reed switch. After winding however many turns it came to, I got the reed switch to change over at 4 volts and 100 mA.

Coil former

The easiest way to wind a coil

Coil fitted onto the reed switch

For the housing of the switch (commutator, relay, call it what you like) I used a 25x25x1.5 mm aluminum profile: the SO-239 connectors fit on it nicely and the coil with the reed switch fit inside.

Trying the SO-239 connectors on a 25x25x1.5 mm aluminum profile

I cut and drilled the profile, screwed on the connectors and soldered the contacts.

Inside

After I printed end caps and fitted a connector for the coil, this is the VHF antenna switch I ended up with.

Finished VHF antenna switch

With the relay inserted inline in the cable, SWR on 145.500 changed by 0.1 (from 1.2 to 1.3). That’s fine by me for now. The capacitance of the open contact relative to the common one turned out to be 2 pF. When working stations on the edge of audibility, switching the inline relay on and off had no effect on reception. Further use will show whether this design deserves to live.

Cost:

• reed switch — 20 UAH
• profile, 1 meter — 40 UAH (10 cm used)
• SO-239 connectors — 30 UAH (3 needed)
• wire, 100 grams — 200 UAH (20 grams used)

Total: 220 UAH, and the fun of building it is priceless :-)