Or “The horror of QRO and extreme antenna impedances”. Last year I started dabbling with an MFJ-998 auto tuner for a 80/160m antenna. Loyal readers might remember I spent numerous hours (days!) changing everything to get it right. As it turned out the MFJ automatic antenna tuner doesn’t like currents on the coax shield. That drives the device nuts. Which in turn drives the operator nuts.
I tried adding coax choke baluns everywhere but to no avail. I finally added a homebrew monster current choke. Refresh memory here. The link shows a picture of the inside.
Today I opened up this balun to replace the M4 bolts with a sturdier M6 size bolt. The horror! I used pieces of glue gun sticks to keep the stack of ferrite cores in place. Where the wires touch this glue, it was molten. The wire is kept tightly wound around the cores by means of white cable ties. Where it touches the wires, they’re turning slightly brown. Conclusion: things must have gotten pretty hot in there.
Now what? I really do not want the mayhem to start all over again. That bloody antenna with the MFJ needs to work and stay working like it did after I added this choking balun. There are several options from here.
See no evil, hear no evil. Just close the box, put it up and play at maximum volume. Will it work with the new radial arrangement? Will it stay in business? Will the wire insulation melt and short out the whole shebang? The question is whether the heating came from a “one time only” failure or do the wires become too hot when using the antenna for too long?
Replace the homebrew unit with a commercial one that is rated 2 kW or more. DX Engineering sells one (link) and Balundesigns has one too (link). The price is right when you consider the parts. My home made choke uses a stack of 5 cores costing 10 Euro (14 US $) each. The problem is that shipping costs 35-40 US$ with the chance of a zealous customs officer picking out my parcel and charge me import duties. On top of the the added cost, this procedure will then delay the delivery of my order. Hence this option is not really attractive.
Replace the 1.5mm² wire pair with thin high quality Teflon coax. I’ve seen some examples that use this coax rather than simple copper wire. The copper wire’s insulation is rated x Volt, with x being a number I don’t know by heart but it certainly isn’t rated for extreme voltages that might occur with the extreme impedances the antenna exhibits under a kW load.
Redo the whole thing myself using a combination of more ferrite cores (100?) over the coax and use the QRO rated Teflon coax. Then add the reengineered balun with the copper wire replaced by this same Teflon coax. Approximate cost with DX-Wire stuff is 78 Euro ex S/H. Better safe than sorry and knowing this antenna / MFJ thing combo needing A LOT of choking impedance, this overkill scenario seems the way to go. Especially since I had quite some fun on 160m last winter. Presumably the coming winter will be Low Band Time again, so the bloody thing needs to work and keep working.
Stuff to think about…