Wave properties, precisely
Three facts the Technician track stated loosely and Extra tests exactly.
How are the component fields of an electromagnetic wave oriented?
They are at right angles.
In what direction does an electromagnetic wave travel?
It travels at a right angle to the electric and magnetic fields.
All three — electric field, magnetic field, direction of travel — are mutually perpendicular.
What determines the speed of electromagnetic waves through a medium?
The index of refraction.
Which is where velocity factor comes from: a coax’s dielectric has an index of refraction, and the wave slows accordingly.
What are circularly polarized electromagnetic waves?
Waves with rotating electric and magnetic fields.
The rotation produced by two perpendicular linear antennas fed in quadrature, from the antennas module.
Earth-Moon-Earth
What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by EME?
12,000 miles, if the moon is “visible” by both stations
The only requirement is that both stations can see the Moon at once — which, given the Earth’s curvature, means up to about half the planet apart.
What characterizes libration fading of an EME signal?
A fluttery, irregular fading.
The Moon wobbles slightly in its rotation (libration), so the many reflecting features on its surface shift relative to each other and their reflections combine and cancel irregularly. It is multipath, on a 480,000-mile path, and it gives EME signals their characteristic flutter.
When scheduling EME contacts, which of these conditions will generally result in the least path loss?
When the Moon is at perigee.
Perigee is the Moon’s closest approach — around 356,000 km against 406,000 at apogee. Path loss goes as the fourth power of distance for a two-way reflection, so the difference between perigee and apogee is a couple of decibels. On a path with almost no margin, a couple of decibels is worth scheduling around.
Meteor scatter
When a meteor strikes the Earth’s atmosphere, a linear ionized region is formed at what region of the ionosphere?
The E region
Around 100 km — the E region altitude, which is why meteor-scatter distances resemble sporadic-E distances.
Which of the following frequency ranges is most suited for meteor-scatter communications?
28 MHz - 148 MHz
Ten metres through 2 metres, with 6 metres the sweet spot from the Technician track. Below that the ionosphere carries the signal anyway; above it the trails do not reflect well.
Auroral propagation
What is most likely to result in auroral propagation?
Severe geomagnetic storms.
Which of these emission modes is best for auroral propagation?
CW.
Auroral reflection Doppler-smears a signal badly — the “raspy” quality from the Technician track. SSB voice becomes unintelligible; CW survives because a smeared tone is still a tone with recognisable timing. This is the general rule for degraded paths: the less structure the signal carries per second, the more abuse it survives.
Tropospheric ducting
Atmospheric ducts capable of propagating microwave signals often form over what geographic feature?
Large bodies of water.
Water surfaces produce the stable temperature inversions ducting needs. This is why long microwave paths over sea are routine and the same paths over land are not.
What is a typical range for tropospheric duct propagation of microwave signals?
100 miles to 300 miles.
When the MUF drops
What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?
Switch to a lower frequency HF band
Directly from the General MUF lesson: above the MUF the signal leaves. Move down until you are below it again — which is why an evening on HF is a walk down the bands as the ionosphere weakens.
Check yourself
- Why does a perigee EME schedule matter when the difference is only about 2 dB?
- An aurora is running and you want to work it. Which mode, and why not SSB?
- The band goes quiet mid-contact. Up a band or down?
Answers
- Because an EME path has almost no margin — two decibels is the difference between copy and no copy.
- CW. Auroral Doppler smearing destroys voice intelligibility, and a smeared tone is still readable.
- Down. The MUF has fallen below your frequency; a lower band brings you back under it.