160 Meter Antenna Arcing

A few weeks ago my "small" 160 meter antenna malfunctioned. At first it was only with a kilowatt, then it deteriorated until I was only able to run QRP without the problem occurring. It pretty well took me off top band other than playing around as a QRP entry in the Stew Perry contest in October.

The antenna is the 160 meter mode of my 80 meter vertical yagi. It's base matched by an L-network that is switched in when 160 meters is selected. Although only half height the antenna exploits the extensive radial system of the 80 meter array. In comparative testing it appears to be -6 db relative to my usual full size wire vertical. While not a wonderful antenna it allows me to operate year-round on top band. The full size wire vertical must be removed in spring to keep radial wires are out of the way of farm equipment used for haying. 

The first symptom of trouble was erratic SWR changes while transmitting with high power. At lower power, although the SWR was more steady, the rig detected a problem and reduced power. A low volume sizzle was heard in the headphones. This was my first hint of arcing.

Troubleshooting the antenna was . Since it is so far from the shack I could not do a transmit test while also being present at the antenna. Visual inspections uncovered nothing amiss.

My suspicion first fell on the L-network capacitors. There is a 2000 pf vintage transmitting mica in parallel with several 100 pf 1 kV low loss capacitors. Old style mica high current capacitors are known to occasionally suffer internal connection failures after several decades. Although the small ceramic capacitors are calculated (current, voltage, Q) as more than adequate to the application and have not failed in other networks I've built, well, you never know.

I first substituted an identical 2000 pf mica capacitor. The problem persisted. Cutting the 100 pf ceramic capacitors out of the circuit again had no effect. I turned elsewhere to find the problem.

The DPDT relay that switches the tower connection between the 80 meter L-networks and the 160 meter L-network has its 8 A contacts wired in parallel for greater current capacity. I disconnected the 80 meter connection and shorted the relay contacts with a wire. Again, no change. I didn't really expect the problem to be the relay since it continued to work properly on receive and low power which would not be the case had the contacts been ablated by arcing.

My attention next shifted to the L-network coil. It is large, to maximize Q, and is exposed to the weather. It is wound with weathered THHN wire I scavenged from an old antenna. The high voltage across its terminals could possibly arc through the weathered wire insulation to an adjacent turn or to a nearby conductor. A close inspection showed no evidence of arcing.

Having failed to diagnose the problem visually or by component substitution and bypass I asked a friend to help. We communicated by handheld. He transmitted at power just high enough to elicit the arcing while I poked around the antenna. Despite the audible sizzle of arcing it took a few minutes to pinpoint the location. It can be difficult to localize a quiet sizzle outdoors with components of the antenna bound together and hidden by weather covers.

The problem was not at all what I expected. The arc was from the tower to a few of the 12 VDC control wires. There is a bundle of Cat5 cables that connect the central control box to the 80 meter parasitic elements and to the shack. The cables are buried and are only exposed at the tower base.

 
When I completed the 80 meter array, long before adding the 160 meter L-network, I made a temporary weather cover for the control cables. There's a lot of them! I bundled and wrapped them in a black polyethylene sheet that was handy. I taped the wrap closed and taped that to the lowest tower brace.

On 80 meters this isn't a problem because the tower base is at a low RF potential, where current is maximum and voltage is minimum. The base of a vertical is like the centre of a dipole. This is not the case on 160 meters where the relatively large transformation ratio of the 160 meter L-network is a pussycat at the 50 Ω port but a monster with bloody fangs where it connects to the antenna.

TLW shows 1200 volts across the coil, and therefore on the antenna port, with 1000 watts applied. The control cables were an easily accessible path to RF ground through the inadequate layers of thin plastic. It punched a hole through the polyethylene sheet (left side, above) and then burned through the thin plastic covers on the two crimp connectors closest to the tower brace.

Therein lies the problem with temporary solutions: you fail to follow up to replace it with a permanent solution. This is a lesson I never seem to learn. For those following the blog you'll know I have far too many projects on the go. I forget or I choose to focus on other things. Small maintenance items like this tend to be put off and finally forgotten. Sometimes these oversights come back to bite me.

A more robust weather cover for the control cables is now in place. It's simply a short length of PVC pipe with the cable bundles stuffed inside and taped top and bottom to protect against rain and insects, respectively. I cleaned and painted over the scorched area of the tower brace (visible in the picture). This time it ought to last.

RF is weird. You might expect that high RF power leaking into the control wires would damage the control system components. It didn't. All the 80 and 160 meter modes of the array continue to perform as before. It probably helped that all the control lines entering and exiting the central control box have RF chokes. However the parasitic element boxes do not have chokes on their end of the control lines since they did not appear to be necessary. Exactly where the RF decided to travel is unclear. 

Soon I'll have my winter 160 meter antenna ready and this lesser antenna will lie idle until next summer. Among the alternatives available to me, I'm trying something different this year for 160 meters. Its RF arc potential is even greater than for the antenna just discussed. More on the new antenna in a future article.