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Automatic Antenna Relay for Lightning Protection

No Station Should Be Without One

by Mark Persons W0MH
CQ Magazine article
October, 2021
Pages 51-53

There you are, driving along and minding your own business while chatting on 2 meters.  Then you spot lightning ahead.  Your thoughts immediately turn to your HF ham station (mine is in Photo A).  “Did I remember to disconnect the antenna from my rig?”  “Should I turn back or continue on?”

Well, if you had constructed and installed an automatic antenna relay, your rig  would be safe.  I built just such a device for that very reason for use on coaxial cables.  A DPDT relay connects my linear amplifier to a 5-position antenna switch when the rig is on.  When off, the amplifier’s RF center conductor is disconnected from the antenna and connected to ground, and the antenna center conductor goes to ground as well.  That is triple protection.

Photo B.  Connector end of the antenna relay box.

The project revolves around the capabilities of a Potter & Brumfield/Tyco Electronics PRD-11DY0-12, DPDT 12 VDC relay.  It has silver plated contacts with a rating of 20 amperes at 277 VAC (781 volts peak to peak).   For perspective, 1500 watts into 50 ohms is 5.48 amperes with 274 volts RMS.  Peak-to-peak voltage is 772 volts.  As a radio broadcast engineer, I successfully used this relay at 1-KW AM broadcast stations over the years.  It was perfect for selecting a main or auxiliary transmitter to the antenna.  Never had a problem.

The antenna relay needs 12 VDC power that is on only when the transceiver is on.  Not all rigs can do this.  In my case, power comes from the accessory connector of my Icom IC-7300.  It has a 13-pin DIN connector with power output only when the rig is on.  I found a used connector in my junk box and joined it to a two-conductor cable.  The current is fairly low, as only 169 mA is required to run the relay coil, plus 10 mA for the LED lamp. 

There is a diode across the relay coil to prevent the relay from producing a voltage spike the instant power is shut off.  This sort of EMF transient could damage the device that is powering the box.  I originally started with a fuse in series with the power, but later switched to a diode to “steer” power to the relay.  Lacking that, accidentally connecting the 13.8 volts backwards would have looked like a short circuit because of the diode across the relay coil.

The parts list is in the sidebar. Small items came from my junk box and hamfests. The box I chose is as small as practical to house the relay.  You might prefer something else.  Let me remind you that the box should be metal, as opposed to plastic or PVC.   The goal is to keep RF radiation inside the box to prevent harming someone.  I drilled four more holes for screws to increase mechanical integrity and to help keep RF inside. 

Parts List with prices for the larger items:
1 – P&B/Tyco Electronics PRD-11DY0-12, DPDT, 12 VDC relay, $57.55 at Digi-Key
1 - Hammond 1411L, 5” x 4” x3” grey aluminum utility box, $12.38 at Digi-Key
2 - SO-239 chassis mount connectors
1 - Green LED indicator with mount
1 - 1 K ohm/half-watt resistor
2 - 1N4004 diode
1 - Accessory connector for the rig

Building the Relay

Please refer to the photos as we discuss the construction of the relay, starting with Photo B. The schematic is in Figure 1. The antenna relay described here is not high-tech.  It is an easy project that almost any ham can tackle.  My advice is to take time and pride in the final result.

Photo C gives us a look at the inside of the box. Use good judgement, along with a ruler, to center the relay and connectors in the chassis for a clean appearance.  A drill press was used, but you could get by with a hand-held drill if you are careful.  I sometimes drill a small pilot hole for position accuracy, then enlarge with a larger bit.

Photo C.  Inside the relay box.

Taking a look at Photo D, the relay's RF connections are 1-7/8 inches apart, so it was logical to use the same spacing for the SO-239 connectors.  Starting with a 1/4-inch hole, I used a 5/8-inch chassis punch for the SO-239 connectors.  From there, #12 solid copper wire runs to the RF relay.   It is easy to do and is good for 20 amperes of current … at least at DC, but it is a little less at RF frequencies.  I used crimp-terminals on the ends of the two wires that go to ground.  Instead of crimping, I soldered them on.  The idea behind the terminal is to avoid a situation where dissimilar metals (copper and aluminum) are bolted together.  Corrosion could compromise the connection, especially in a humid environment. 

Keeping RF lead lengths short is very important for low VSWR.  I started by using a wire between the two normally-open relay contacts but changed to a brass 6-32 bolt with three nuts and lock washers.  You can see that in Photo D.  Finally, I soldered some 1/4-inch braid in parallel with the wires on the relay armature and out to the SO-239 connectors.  This further reduced inductance.  Testing confirmed the braid did not hamper mechanical operation of the relay. 

Photo D.  Top view of the box showing a brass bold joining normally-open (N.O.) contacts on the relay.


In Photo E, a green LED indicates the relay is allowing RF to pass through.  As an added touch, I added four stick-on feet, not shown in the photo.   

You’ll notice I used a 10-32 bolt as a ground point near the RF connectors.  This should be connected to station ground, along with your rig and other station equipment.  That’s just good practice in any ham shack.  Retain a copy of this article, or your schematic version of this project, in station files for future reference. 

Photo E.  Side of box showing the LED and control cable.

Pass-through power loss is very low with this design.  Return loss measurements showed the VSWR was 1.01:1 on 160 meters, 1.02:1 on 80, 1.03:1 on 40, 1.04:1 on 30, 1.05:1 on 20, 1.06:1 on 17, 1.07:1 on 15, 1.08:1 on 12, 1.09:1 on 10, increasing to 1.17:1 on 6 meters.  I don’t recommend this configuration for use above 54 MHz.  In my opinion, it is a small price to pay for the security the relay affords.   It’s your choice.

There is one thing … this relay protects ham equipment from high voltage between the coaxial cable shield and inner conductor.  It is effective when the antenna coax shield is tied to the building electrical service entrance/electrical panel ground.  That arrangement is known as a single point ground where lightning will want to go to ground at that point, rather than going to the radio equipment.  Think of the rig as a “stub” from the service entrance.  It gets power and RF from the same ground point.  There is no reason for lightning to go there.  An additional ground at the shack is a bad choice because then the radios can be in the lightning path to ground. 

I now have much more confidence, knowing that my rig is as safe from lightning as I can make it, without the hassle of disconnecting the antenna after each ham session.   

Note:  Mark Persons W0MH is a retired radio broadcast engineer.  He was first licensed as WN0AXD at the age 15 in 1962.  Mark received the Engineer of the Year award from the Society of Broadcast Engineers in 2018 and their Lifetime Achievement Award in 2020.  He currently mentors four radio broadcast engineers and is a member of the National Radio Systems Committee, which developes improved technical standards for radio broadcast with ultimate adoption by the FCC.

01/03/2022 Email: Hi Mark. I read your article in the October 2021 CQ Magazine about the antenna grounding relay and my first thought was “Well, Duh!” That thought was prompted by the fact I had everything required to build the project in my junk box and that I had, several years ago, built a similar project to automatically ground my ladder line fed 80m loop, though that relay was mounted in a non-metallic box. Thank you for a well thought out and well written article.  And thanks for the impetus to get into the shop and actually build the project. 73, Ron NY0A, Dassel, Minnesota.

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