[Lowfer] I need some help to build a Norwegian Hifer beacon

[Lyle Koehler]

There is a simple HiFER transmitter design on my web site at
http://www.computerpro.com/~lyle/proto/LFproto.htm#HiFER  This design uses
an Epson programmable oscillator which is available in the US from Digi-Key;
I'm sure there are European suppliers as well. The transmitter will provide
an output power of up to 10 milliwatts and has a simple output adjustment
potentiometer.

In the US, we are permitted to radiate 10,000 uV/m at 30 meters, which
corresponds to approximately 1.8 milliwatts into a half-wave dipole in free
space. How does this relate to your regulations, which allow 42 dBuA/m at 10
meters? There is probably a much faster way to do the calculation, but here
is the long way. First, 42dB is a field strength ratio of 125.89, so
42dBuA/m is 0.0001.259 A/m. To convert to volts/meter, multiply by 377
(actually 120 times Pi); the impedance of free space. This gives a field
strength of 0.0474 V/m at 10 meters. At 30 meters, it would be 1/3 as much,
or 0.0158 V/m (15,820 uV/m). In other words, your rules allow a field
strength that is 1.582 times greater than the US part 15 rules. The power
ratio is the square of the field strength ratio, so if we can run 1.8
milliwatts to a dipole, you can run about 4.5 milliwatts.

This calculation would be accurate only in the far field, and will be
slightly in error for distances as close as 10 meters. If you need a really
accurate number, you would have to do a more complex calculation.
Fortunately there is a free antenna analysis program called NF (near field)
that will do the job, and I think it can still be found as nf.zip on the
ARRL BBS at http://www.arrl.org/files/bbs/programs/
(When I tried that URL just now, the site would not come up, but I think my
ISP is having problems.) Anyway, I modeled a half-wave free-space 13.55 MHz
dipole in NF and selected the "near field" calculation, which provides the
results in either electric or magnetic units. At 1 watt transmitter power
(apparently the lowest power that NF will recognize) and a distance of 10
meters, the field strength is 0.0019 A/m. Taking (0.000125/0.0019) squared,
and multiplying it by 1 watt, I come up with an allowable power of 4.4
milliwatts, which is very close to the result of my first calculation. You
should be OK with 4 milliwatts to a dipole.

Lyle, K0LR