[600MRG] New Part 5 Grant

[D.J.J. Ring, Jr.]

Fascinating idea, Brian!
73
DR

Low Profile LF and MF antennas
Proposed Research and Experimentation
Brian Pease, W1IR, Amateur Extra
2/27/13
Most radio amateurs assume that verticals are the only practical
antennas for long distance
communication at low frequencies. Simulations of antenna performance
in the proposed 135.7 –
137.8kHz and 472 – 479kHz Amateur Radio Bands show that resonant
horizontal dipole antennas
located very close to the ground (but with no ground system at all)
have significant vertical E-field
radiation and can potentially perform nearly as well as a 15 meter (50
ft) top loaded vertical with a
ground system. This is especially true in locations with poor ground
conductivity where vertical
antennas tend to perform poorly. The dipoles are directional, with
maximum vertical radiation off the
ends. In one simulation, a resonant dipole at 475kHz positioned 2
meters (6.5 ft) above the ground
with ground conductivity of .002 S/m created a vertical Electric field
off it's ends only about 3dB less
than the 15 meter vertical mentioned above. Even a dipole 2 inches
above the ground radiates
significant energy in simulation. The Dipoles are also broadband. The
475kHz dipole 2 meters off the
ground has 30kHz bandwidth at the 2:1 SWR points, far wider than the
proposed band.
Resonant dipoles close to the ground are much shorter than in free
space, ~220 meters (715
ft)for the dipole 2” above ground vs ~315 meters (1033 ft) in free
space. They can be shortened further
by adding capacitive loading to the ends, loading coils, or with a
matching network at the center. For
very short antennas, the ends can be grounded, but this can add
considerable loss especially where
ground conductivity is poor.
I think that if these LF and MF bands become reality, these simple low
profile antennas could
allow many hams (who have some space) to experiment with LF or MF that
otherwise would not
attempt it because of the hassles and expense of a tower, ground
radial system, top-hat, and tuner.
These antennas can be run along the top of a fence or attached to
trees while standing on the ground.
Cheap Aluminum electric fence wire and insulators can be used. I have
the space to conduct some
experiments.
I propose to attempt the following:
•
Simulate several more antenna configurations, heights, and ground
conductivities for each band
using the NEC4 method of moments program.
•
Measure the actual conductivity of the ground where the antennas will
be erected to better
estimate expected performance.
•
Erect a limited number of antennas (one at a time in the same
location), tune them, and measure
and record lengths, bandwidths, etc. All antennas will be “in the
woods” well below the
existing tree canopy.
•
Transmit a carrier of known power from each antenna and measure the
vertical field strength off
the ends and sides in the far field several km away, using a shielded
Fairchild calibrated loop
antenna and a Rycom portable Selective Voltmeter with a preamp. Based
on the simulations,
the initial transmit power will be adjusted for an expected EIRP of ~1
Watt or less.
•
Compare measured to simulated performance and select the best designs.
•
If possible, use existing signal “grabbers” linked to the internet, or
direct contact with other
experimental stations to demonstrate long distance communication.
Possible modes are CW,
PSK31, and the narrow band weak signal FM digital modes created by
K1JT, especially WSPR
and his new JT9-1 mode designed especially for LF/MF communication.
I have the following equipment on hand now:
•
Materials to erect the antennas.
•
A homebuilt 25 Watt Class E amplifier for 472-479 kHz.
•
A Hafler P3000 300 Watt linear audio amplifier suitable for 135.7-137.8kHz.
•
An HP3586C Selective Level Meter as a receiver, with an RF output
suitable to drive the
amplifiers for the field strength testing.
•
A homebuild active loop antenna to receive these bands to select clear channels.
•
A Trimble GPS frequency standard to lock the 3586C frequency to a
fraction of a Hz accuracy.
•
A Fairchild calibrated loop antenna for the field strength measurements.
•
A battery operated Rycom 3121B Selective Level Meter with preamp for
field strength.
•
Equipment to assemble an RF impedance bridge for tuning/measuring the antennas.
•
An Advantest R3361A Spectrum Analyzer for measuring harmonic output of
the amplifiers, etc.
•
An IC-703 as a low level (SSB) signal source for the digital modes.
•
A laptop computer with software to send and receive the digital modes.
I will need to fabricate a ~100 Watt amplifier for 472-479 kHz, with
adjustable power level, and
a forward/reverse RF power meter that covers both bands. I will need
to assemble and calibrate the RF
impedance bridge.