the FCC recently began allowing amateur operators in the US to use the 2200m and 630m (136kHz and 475kHz) bands. one of the hurdles for using these bands is that wire antennas are extremely inefficient. for 2200m, a typical wire antenna (a fan antenna or top loaded inverted L) takes about 250 or more watts to achieve the 5 watt ERP limit. magnetic loops can be much more efficient, because ground conductivity and other environmental factors don't reduce the efficiency. the problem with mag loops, however is their small RF "aperture". but i think i saw the solution to the aperture problem when i was in the army. this was in the form of an array of loop antennas strung out about 5ft apart across a field. the whole array was about 30 or 40 ft. it was a nice "stealth" antenna as well because the loops were about 18" diameter. these loops were being used in the HF band, and i actually heard that station on a communications receiver several times on frequencies from 2Mhz to 15Mhz. a mag loop for 2200M is going to be a bit bigger, but the real challenge is making an inexpensive high voltage tuning cap. most transmitting mag loops use vacuum variables, which can get pricey. using a standard air variable can be a problem because the air gap voltage is usually only several hundred volts. so i'm looking at some "out of the box" ideas, like making a variable cap from pieces of single-sided PCB, or concentric pieces of PVC pipe with copper foil wrapped around the PVC pipes. i've got a lot of books from the 1910-1920 era, like "Practical Wireless Telegraphy" and there are some very interesting cap designs (such as a cap that operates like opening and closing a book). a lot of this type of technology is used in constructing Tesla coils ( FCC rules say spark-gap transmitters, damped wave, are type B emissions, and are strictly verboten).
so the main hurdle is the antenna array. another "out of the box" idea i had is to use a Howland Current Source to drive the antennas, rather than a typical single ended RF amp.
i would like to see if anybody has any interesting ideas for the capacitor. the tuning cap needs to be in the neighborhood of 2nF, and i'm not sure yet what the voltage will be, maybe 10kV. the cap needs to be able to handle relatively high antenna current ~10A or so. it will also need to be enclosed in a weatherproof enclosure.
so the main hurdle is the antenna array. another "out of the box" idea i had is to use a Howland Current Source to drive the antennas, rather than a typical single ended RF amp.
i would like to see if anybody has any interesting ideas for the capacitor. the tuning cap needs to be in the neighborhood of 2nF, and i'm not sure yet what the voltage will be, maybe 10kV. the cap needs to be able to handle relatively high antenna current ~10A or so. it will also need to be enclosed in a weatherproof enclosure.
