Not 'off topic' at all Johnny since it relates to Ham radio - There is no "magic" way to get high efficiency with an electrically-short antenna. All antennas are subject to same laws of physics. The underlying problem is high ohmic losses resulting from the unavoidably high RF currents flowing in any short antenna - doublet, Marconi or loop. These currents are inversely proportional to the size of the antenna in wavelengths, and increase to huge values as the antenna becomes very short (less than 1/8 wavelength).
That said, amazing results can be had from antennas that are astoundingly poor "on paper". Even commercial shipboard systems operating in the 425-500 kHz range had what we'd consider abysmal antennas. (Only the huge shore stations had anything like a "full size" antenna.) The typical end fed wire used with the 500 watt CW radios on ships were usually less than 200 feet long. At 425 kHz the antenna was only 1/10th of a wavelength long. That's equivalent to trying to work on 80 meters with a 20 foot end fed wire. Such an antenna might be only 4 or 5 percent efficient at best, but with 100 watts output that's still 4 or 5 watts radiated. As any QRP enthusiast will tell you, that's enough to "work the world". About ten years ago a group of us (the 600 meter research group) ran some experiments on that band with some very good results using marginal antennas and low power. Our objective was to explore exactly what your group is pursuing - the practicality of MF (300 kHz - 3 MHz) communications for emergency work even with the sort of antennas practicable on an urban setting. Unfortunately it was a very short-lived experiment caught up in a political struggle between the FCC and the US Coast Guard over control of the 600 meter band (that was before the current ARRL experimental group work on that band). We were using CW, since that's still the most spectrum-efficient mode with the greatest "bang for the watt", providing superb text messaging in the hands of trained operators. For emergency use, the famous "Gibson Girl" hand-cranked lifeboat transmitter only put out about 5 watts amplitude-modulated CW (MCW) on 500 kHz and had, under the most ideal conditions, an end fed wire 100 or 200 feet long (attached to a box kite or balloon). Frequently it was much, much shorter, depending upon weather conditions. Such a setup was considered to have a working range of up to 200 miles. I know of at least one well-documented case where such a transmitter was heard halfway around the world, causing ships in the Pacific to search for a lifeboat that was actually floating in the Mediterranean Sea, and they were hearing it on a small receiving loop antenna trying to home in on the signal's source! It seems that 600 meter communications using a very short antenna and moderate power should be entirely practical over tens of miles in the daytime and perhaps hundreds of miles at night. Whether 600 meters offers any advantage over a similar setup on 160 meters is an open question. Ron AC7AC -----Original Message----- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Johnny Siu Sent: Tuesday, December 14, 2010 7:42 AM To: Elecraft Subject: [Elecraft] OT: possible ham operation in the region of 415-526.5 Khz My questions are : 1. practically speaking, apart from huge traditional wire antenna, are there any other antennas of managable size (also reasonable efficiency) which can be easily set up under distress situation? 2. to make communciations of say within 200-300km, what ERP is required? 3. for a segment of 15Khz, voice communciation seems quite restrictive. How efficent will it be under digital mode (not CW) such as Pactor? I must admit that I am not a radio man by profession and have limited knowledge on MF radio operation. I am looking forward to your advice and apologise for the band width in this mail reflector. 73 Johnny VR2XMC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html