Hi All, I have a 70ft 80m self-supporting vertical, which I've baseloaded to bring it down to 160m. The antenna is about 800 ft. from my shack, fed with buried hardline. The vertical has sixty 120 ft radials fanning out over the desert floor.
When I built the baseloaded inductor I didn't have any suitable AirDux coil stock on hand, so I experimented using a toroid. I wrapped two 2.25 inch diameter T-43 cores together with fiberglass tape, and wound the wrapped core with thin RG316 coax. RG316 is thin, flexible, high-temperature coax, with a silver-plated shield, about 0.1 in in diameter. I shorted the inner conductor to the shield, so I was just using the coax as flexible, high temperature wire. I use a shunt coil of about 2 uH (23 ohms reactance at 1.8 MHz) made of plated copper tubing between the toroid and ground, reasoning that the RF current at this low Z end of the toroid will be pretty large. I didn't know how well the toroid would work in this application, but the assembly came to resonance very smoothly and seems to work well to 2000W. I use a DPDT vacuum relay to switch in the 160m toroid from my shack. As shown in the photos, (links below) the assembly is very compact. I mounted it in an inexpensive (about $22) NEMA waterproof enclosure.) To give a sense of the scale, one photo shows the toroid and shunt coil breadboarded on a piece of plastic and resting on a sheet of teflon at the base of the vertical (to keep it from arcing to the grounded radials.) http://www.w8zr.net/160amplifier/images/160m%20toroid%20assembly.jpg http://www.w8zr.net/160amplifier/images/160m breadboard test.jpg <http://www.w8zr.net/160amplifier/images/160m%20breadboard%20test.jpg> Here are some observations about this hookup. As expected, the toroid and wire get pretty hot at QRO power levels, although I didn't try to measure the temperature. (It takes two people to do this: one to key up the transmitter and a second to hold the IR thermometer.) However, intially the toroid rested on an acrylic (plexiglass) post, but the post got hot enough to soften and deform. I swapped the plexiglass for a ceramic post with no further problems. There's no evidence of core saturation, and the SWR is unchanged over the full power range. There's no visible effect of heating on the wires either, all of which use teflon insulation. I don't know how much power I'm wasting in heating the toroid, but I'd guess it be about 100-200W at full power, or roughly ten percent of total power. I don't know how that power loss would compare to using an air-wound coil. I can argue it both ways: an air wound coil wouldn't have any core losses, but the resistive loss in the wire would be several times greater than that in the toroid. In this sort of hookup, one has to be very careful about high voltage insulation. With my first toroid, I spaced the turns out around the core, and at about 500W the bottom of the winding flashed over to the top of the winding, even though the wire had teflon insulation. Now the winding uses only about 2/3 of the available core space, and I have an extra length (probably unneeded) of teflon sleeving over each end of the winding. The little coil wound of stranded 12 AWG white wire (with orange stripe) visible in the photo at the HV end of the toroid provides about 0.6 uH of inductance. Its purpose is to fine tune the inductance, since the toroid doesn't lend itself to fine adjustments. For this vertical, the shunt coil is about 2uH and the toroid is about 23uH. On 80m, the native 2:1 bandwidth on 80m is about 400 kHz (1.3 SWR minimum at 3550 kHz), while on 160m it is only about 35 kHz, with a minimum 1.15 SWR at 1815 kHz. There are only four connections to the NEMA enclosure: RF input, Output to the vertical, Ground, and +26V (approx) to power the vacuum relay. The arrangement is exceptionally convenient and compact, and can be connected to the vertical in five minutes. I use a 24V wall wart to power the relay, with a toggle switch and LED indicator back in the shack. I have a question for the antenna gurus on the list. I know base-loaded verticals have a bad rap compared to verticals with top capacity hats, the rationale being that most radiation occurs at the low Z, high current end of the antenna. However, with a toroid, almost all the electromagnetic energy circulates in the core of the toroid and does not radiate. If I'm right that the core is dissipating only about ten percent of the total power, then that would suggest that the toroidal baseloaded antenna is actually a very good radiator. I have no idea how its other parameters, e.g., angle of radiation, compares to a top-loaded antenna, or to a full 1/4 vertical. My previous iteration of this antenna used top-loading in a guyed vertical, and while the BW was a bit larger (and the maintenance was hugely larger!) I've not noticed any qualitative performance difference. Both versions seemed very competitive. 73, Jim W8ZR _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
