Beverage Maintenance and Repair

For the past few weeks I found myself rationalizing the puzzling behaviour of my 175 meter long northeast Beverage (my only receive antenna at the moment). European stations were often no better copy on the Beverage than on the transmit vertical. My rationalization was the possible presence of skew path, a frequent phenomenon on 160 meters.

When I found myself making the same rationalization almost every night I decided that something must be up. I pulled out my antenna analyzer and swept the SWR from 1 to 5 MHz.

This is very bad. Nominally the SWR should be about 1.5 with a small to moderate undulation. The 1.5 SWR comes from measuring the nominal 75 Ω impedance of the RG6 transmission line and antenna matching transformer with a 50 Ω analyzer. The undulation is mostly due to the termination resistor not being exactly the surge impedance of the antenna.

This SWR is far worse than that and is quite different to what it used to be. Clearly something was amiss. Since it was time for annual maintenance of the antenna I grabbed a ladder, my snowshoes and tree pruning implements and headed into the bush. After uncovering the feed point from the tangle of hawthorn trees that had sprouted around it I checked the connections and all looked good.

The bush clearing took 2 hours of hard work, slogging through the snow and bush and climbing the ladder in snowshoes to cut the limbs that threatened the wire. On the termination box containing the resistor the nuts were not snug but not tight. Everything else looked good.

Returning indoors the SWR showed no improvement. The next day I headed straight to the termination with my tools and opened the box. The adjacent picture is what I discovered. (Note: I spread the halves slightly apart to make the damage more visible.)

Other than the long trudge back and forth through the bush troubleshooting is pretty easy. The only parts are resistor, transformer, long wire, ground rods and coax. It is no surprise I found the trouble so easily. I pocketed the box and trudged back to the shack. In a minute it was disassembled and the halves of the 470 Ω resistor inspected.

There are no char marks to indicate a lightning surge. This is any case highly unlikely during our winter. As W8JI has written that is a common failure mode. He recommends carbon composition resistors for their relatively good tolerance to millisecond surges from near strikes. If not for that any suitably valued resistor other than wire wound works well as 1.8 MHz.

I suspect a pre-existing stress fracture or a couple of years of thermal cycling between -35° C and 35° C did it in. That these resistors are many decades old may have played a role. I have a bunch of them so it is easy to replace.

Rather than a direct replacement I opted for a lower value resistor installed without the box. I used the same technique on my temporary west Beverage last winter. The 390 Ω resistor is in the ground rod clamp with a thin stranded wire and small wire nuts to connect the resistor and antenna wire.

Thin flexible wire is necessary to avoid fatigue failure of the resistor. Alternatively use a box or, as some do, a fuse holder or other fixture to take the mechanical stress.

This is all very temporary. I am considering "twinning" the northeast Beverage this winter to make it reversible between northeast and southwest. I'll use the box to house the termination transformer. There will be no termination resistor, just a ground connection from the transformer secondary winding.

Satisfied with my temporary repair I hauled everything back out of the bush and headed for the shack. I couldn't try it out immediately since it was still daylight.

Sometime between then and sunset I remembered that I ought to recheck the SWR. I pulled out the analyzer and had a look. The improvement should be obvious.

The undulations can be reduced with more fine tuning of the termination resistor. Some imperfection is allowable in the termination resistor without noticably affecting the antenna pattern. I am not going to bother.

After the sun set I got on 160 meters and quickly found a few European stations. This time there was no need to rationalize about skew path. Signal-to-noise (SNR) comparisons between the Beverage and vertical were no contest: the Beverage was by far the superior, just as it used to be.

The break of the resistor made the Beverage unterminated and therefore bidirectional. I did notice stations to the southwest came in pretty well on the Beverage during the CQ 160 contest yet I did not wonder too much about it at the time. I was simply happy that I could work Europe and the US on the Beverage and didn't worry about the fictional propagation to explain what I heard.

I am now clearing bush to run a reversible Beverage for north and south directions as part of my quest for full compass coverage. This will be my first receive antenna of this type so it should prove interesting. More on this when the antenna is complete.

It isn't a rush job so it may not be ready for use until late February or March. Even then it'll need a remote switching system for which I am in the process of gathering parts.