Beverage Lightning Protection

Beverage receive antennas are very susceptible to lightning. They are long wires that at close to the ground and grounded at both ends. While direct strikes are not rare, the greater threat is typically secondary strikes (low current lightning branch) and inducted current from a nearby strike. I can personally attest to the lightning risk, when my Beverage system has been struck not once but twice.

This summer I removed the head end electronics of my three reversible Beverage antennas, disconnected the feed lines and directly grounded the antenna wires. I rarely operate 160 meters during the warm months since noise is high, activity in the northern hemisphere is low and the radials of the primary 160 meter vertical are removed during the haying season.

One of my summer projects was to add lightning protection to the Beverage antennas. It isn't difficult and there is ample information available on how to do it. Nevertheless I moved slowly. I wanted to better understand how the protection systems work before ordering parts and making the modifications. I am now better informed though far from being an expert. 

When I was satisfied with what I learned I ordered the parts and made the modifications to the Beverage head ends. After completion I tested them to confirm they still worked as they should and then reinstalled them in the field. The Beverage system is back in operation and ready for the fall and winter season.

The design I settled on is a melding of the methods I gleaned from W0BTU and VE6WZ. Those weren't my sole sources but they were well documented with good explanations. I also read what W8JI and ON4UN (Low Band DXing) had to say on the topic and I delved deeper into the circuits and explanations from commercial lightning protection device vendors. Any mistakes or misunderstandings are my own!

I believe it will be helpful to first review a well-known circuit for coaxial lightning protection. The circuit above is used in a variety of products made by Array Solutions. I chose it in particular because they are open with their design and their products are used by many hams.

Lightning and atmospheric static discharge have both RF and DC components. A blocking capacitor on the ungrounded centre conductor is not effective on its own since the surge has nowhere to go and the potential will build until it exceed the capacitor's breakdown voltage. The capacitor holds the charge at bay, briefly, while other components ground the surge. 

There is no DC component in the RF signals hams work with so an RFC (choke) grounds the DC component of the surge or static buildup while blocking RF. If the DC charge is large enough or increases faster than the charge can be grounded via the RFC, or the RF potential is large, the GDT (gas discharge tube fires (conducts) and it has a short-term ability to conduct kiloamps of charge. The diagram text explains the other components.

It should be obvious that the choice of capacitor and RFC affects normal operation. The capacitor in particular should have a low reactance and high Q over the operating frequency range suitable to the power rating and maximum SWR. A higher power rating is recommended even with less than legal limit power unless a low SWR is certain.

A DC surge with a slow rise time and moderate current might be handled entirely by the RFC if it does not overheat and fail from grounding the surge energy. This is desirable since the GDT will fail from repeated firing and conducting high currents, so we want to reduce how often it fires. For maximum protection they should be replaced after several secondary strikes or one primary strike. Since the GDT typically fails open you cannot easily determine that it has failed.

It should be obvious that the GDT firing voltage should be higher than the maximum voltage for the transmitter power into a 50 Ω load or the higher voltage due to a mismatch. For example, 1000 watts into a 50 Ω load has an RMS voltage of about 225 and a peak voltage of 320. Increase the power and the voltage rises. SWR multiplies the maximum voltage that can be present. Array solutions selection of a 1200 volt GDT is sensible.

There is more to it than that. The capacitor should be rated to hold off the surge being grounded by the RFC and GDT. How high the voltage grows depends on how quickly and effectively the surge can be grounded. Both the surge and working voltage ratings are relevant. The longer it takes the charge to flow to ground, the longer a high voltage is applied to the capacitor.

It is not only the current capacity of the RFC and GDT, but also the ESR (equivalent series resistance) of the ground rod's connection to the ground and how quickly the earth charge within the ground rod's "reach" is depleted. An excellent ground connection is no guarantee of the protector's protection during a direct lightning strike. But that's a subject well beyond the scope of this article. 

With measurements, I estimate that the ESR of the 4' copper clad ground rods that I use for my Beverages, in my local soil, is between 100 and 150 Ω, more or less. That doesn't appear to affect Beverage performance but it is not low enough for the best lightning protection. The sooner the GDT fires the faster the charge can be grounded. That motivated my choice of a 75 rather than a 90 volt GDT.

A receiving antenna has less extreme requirements than transmit antennas because the signal level is very low. A modest amount of signal loss is acceptable and the GDT can be chosen that fires at a much lower voltage. Consider the following open-wire Beverage protection system by W0BTU.

The GDT are 90 volts, the RFC is replaced by a 33 kΩ resistor and the capacitors are ordinary ceramic bypass devices. Resistors are cheaper than RFC and can be effective, for RF and not just DC static and surges. Clearly it is far less expensive to protect receive antennas than transmit antennas. We must protect both conductors of the open-wire line, doubling the component count.

The capacitor can have a lower voltage rating commensurate with the GDT firing voltage. Since the frequencies are low -- typically down to 1.8 MHz -- the capacitor value must be high for a low reactance. At 1.8 MHz a 0.1 μF capacitor has a reactance of about 1 Ω, which is negligible in series with a 600 Ω antenna. The reactance is lower at higher frequencies so we size the capacitor for the lowest operating frequency. A smaller high voltage capacitor would also suit, except on the feed line side of the unit where the impedance is typically 50 or 75 Ω.

My implementation is modelled on W0BTU's unit. Differences include: 

• 75 volt 5 kA Bourns GDT were specified by VE6WZ in his Beverage system and I wanted to keep the voltage as low as possible for maximum protection from even minor induction events. I am confident that Steve made a well informed choice.
  
• The 33 kΩ resistors that drain charge to protect the GDT are ½ watt rather than 1 watt. It isn't a big change and I had them in stock. If a resistor fails the GDT will fire more often and fail sooner.
  
• The coupling capacitors are 0.1 μF and 1000 working volts. Notice the size in comparison to the 0.1 μF capacitors on the feed line side of the unit (blue, lower right). The 630 volt devices are much smaller. I could have used the 630 volt capacitors from my stock but opted for the larger capacitors for their higher power dissipation.
• I don't directly protect the relay as W0BTU does. The resistor and GDT on the coax centre conductor offers limited protection from lightning conduction between Beverage head ends via the remote switch; the coax shield is already grounded at the remote switch and has limited GDT protection in the head end via the secondary windings of the transformers. The current iteration of the Beverage remote switch uses SPDT reed relays to ground the coax centre conductors of all but the active Beverage. That in itself is good protection. I have being doing the same for control lines wherever feasible.
  
• The capacitors are mounted on the PCB but not the GDT and resistor. By direct wiring them to ground I keep the high voltage and high current surges away from sensitive components where the narrow separation of copper pads can offer an alternative and perhaps more enticing path to ground. Stranded interconnect wires allow easy removal of the PCB and connectors from the enclosure for service. 

The above design was used for both the northeast-southwest and east-west reversible open wire Beverages.

The 470 Ω resistor between the Beverage wires was fitted temporarily to test that the added components do not affect normal operation of the unit. Above is a test of the modified north-south RG6 reversible Beverage head end. The SWR curve is not perfect since I used a 51 Ω resistor on the antenna rather than 75 Ω, the analyzer is normalized to 50 Ω and the coax between the analyzer and unit is RG6. This was merely a sanity check that I had made no serious mistakes since all the head end units were in good working order.

Here's a closer look at the reversible RG6 Beverage head end. The antenna port is on the right and the feed line port is on the left. The GDT and resistor pair protect both conductors of the antenna coax. The outer conductor protection is obviously needed. Inner conductor protection is in case of strong coupling between conductors or from the far end via the reflection transformer. I want to avoid a protection path via the fragile transformer windings.

I did not protect the reflection transformers. In previous lightning strikes the reflection transformers were unaffected so I didn't feel the urgency. I will probably go ahead and add the protection after the flurry of fall antenna projects come to an end. It isn't a priority.

The Beverages head ends are back in service -- well, after locating a cold solder joint. I am now ready for the 160 meter season, except...I am in the process of modifying my big vertical. For the time being I have only 4 radials installed, which I rolled out earlier than usual to work E51D. I'll have more to say about improvements to the vertical once the work is complete. One of my objectives this year has been to improve my 160 meter signal.

I also plan to protect the multitude of control lines and rotator cables. Although many are grounded by relays when not energized, others cannot be grounded that way. Unlike RF transmission lines, DC lines cannot be protected by series blocking capacitors. I have not yet settled on a design. 

There are many months to work on it before the arrival of the 2024 lightning season. That is also when I will find out how well the Beverage protection works. My intent is to leave them connected year round.