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 


      
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