Lightning Supression for Rhombic Antennas: Harper's "Damage from electrical storms may be minimized by the usual expedients employed on exposed communication and transmission lines. At receiving locations, where a coaxial line connects the antenna to the apparatus, spark gaps or carbon protector blocks are installed between each side of the antenna output and ground as shown on Figure 38. On all except direct hits, this protection has been found sufficient to prevent appreciable damage to the coupling unit or station apparatus. If an open-wire line is to be used instead of the coaxial, an aditional set of protectors will be required where the line enters the station building.
Trasmitting stations are protected by horn gaps to ground across the termination and across the open-wire line at the point where it enters the building as shown on Fig. 43. Additional protection is furnished by grounding the short-circuited far end of the dissipative terminating line.
Pole damage has been reduced by use of sectionalized ground conductors on the poles. The top of these conductors may be terminated in an elevated rod, or it may be looped over the pole top. Available data indicate no difference in the protection afforded by these methods. The grounded conductor is divided into ten foot sections by two inch gaps as shown on Figure 39.
Due to the necessity of preventing the lightning protective apparatus from interfering with the transmission properties of the system, special precautions must be taken to avoid the introduction of lumped capacity irregularities, such as massive horn gaps or high capacity receiving antenna protectors. By minimizing the physical size of these elements, installing them in the most desireable circuit location, and giving attention to the length and arrangement of the connecting leads, it has been found possible to reduce any tendancy to cause impedance irregularities.
On account of the impossibility of securing complete protection with any practical means, it has been found expedient to maintain a stock of especially vulnerable parts such as insulators, wire, coupling transformers and protectors."
My Experience with Rhombics and Lightning
Having read the Harper's section related to lightning, especially the last sentance, I thought it would be of interest to relate my experience in the Summer of 1996.
My single "compromise" rhombic (210 feet per leg, tilt angle 64 degrees) was hit by a bolt of lightning during an intense storm. The bolt struck a 1/4 inch aircraft cable I had used to support the antenna in a large (50 foot) tree. This cable did not have direct contact with the antenna curtain per se and is analagous to the support strike referred to in Harper. The cable was melted outright in the center with obvious evidence of vaporization of the metal and "splatter." The strike either branched into my feedline or induced a charge. I had constructed a plastic enclosure for 4 RF relays that served to switch the termination and feedlines end to end. This box ended up looking like it had been shot with a shotgun (fragments were scattered 50 yards away). The relays were melted as well. Inside the radio room no direct damage but I was there when the hit occurred and I saw a 6 foot blue arc to ground. All very impressive. Since this hit I have bonded all grounds to a common earth (a ground loop took out our well control box and some consumer electronics in this strike) and I have installed several big knife switches to isolate the rhombic during bad weather and to ground the curtain. I also have purchased from the WireMan several spark plug air gaps that should do the trick. I am mulling over using the center tap ground method used by W6AM on his terminations (Haper refers to grounding the end of nircrome wires commonly used in commercial applications).