Skip zone
| Skip zone | |
|---|---|
| Also called | Zone of silence, dead zone |
| What it is | The gap between the end of ground-wave range and the point where the first skywave hop returns |
| Skip distance | The shortest distance at which skywave signals return to earth |
| Grows with | Higher frequency and weaker ionization |
| Typical sizes | |
| 80 and 160 m at night | Often none |
| 40 m | Small or none by day; often several hundred miles at night |
| 20 m by day | Several hundred to about 1,000 miles |
| 10 and 15 m | Often more than 1,000 miles when open |
| Remedies | |
| Filling it in | NVIS on a lower band, VHF, or relay through another station |
The skip zone is the region around a transmitter where its signal cannot be heard, too far away for the ground wave to reach and too close for the skywave to come back down. Beyond it, the signal returns from the ionosphere and may be heard clearly over long distances. The skip zone explains a puzzle every new HF operator meets sooner or later: a station 3,000 miles away is loud, while another station only 300 miles away, working the same distant contact, cannot be heard at all.
How it forms
Close to the transmitter, signals travel by ground wave, which on HF fades out within a few miles to some tens of miles. Farther out, the only way to hear the station is by skywave, the signal refracted back to earth by the ionosphere.
Whether the ionosphere returns a signal depends on its frequency and on the angle at which it arrives. A signal sent nearly straight up meets the ionized layer head on, and it comes back only if its frequency is below the layer's critical frequency, the highest frequency returned from directly overhead. A signal sent at a lower angle meets the layer at a slant and is returned far more easily. So for any frequency above the critical frequency there is a steepest angle that still comes back, and that angle sets the skip distance, the shortest distance at which the skywave lands. Signals sent more steeply pass through into space. Everything between the end of the ground wave and the skip distance is the skip zone.
How frequency and time change it
The skip zone depends on how far the operating frequency is above the critical frequency.
- Higher frequency, longer skip. The higher the frequency, the shallower the angle needed and the farther out the first hop lands. On the 20-meter band by day the skip zone is typically several hundred to about a thousand miles, and on the 15 and 10-meter bands it often stretches beyond 1,000 miles.
- No skip zone below the critical frequency. When the operating frequency is below the critical frequency, even vertical signals return, and coverage is continuous from the transmitter outward. That is the principle of NVIS, used on 80 and 40 meters for regional nets.
- Night lengthens the skip. After dark the ionosphere weakens and the critical frequency falls. On the 40-meter band a regional daytime band often "goes long" in the evening: nearby stations disappear while distant ones remain or grow louder.
- The sun matters. Near the peak of the solar cycle the ionosphere is denser, critical frequencies are higher and skip zones are generally shorter.
When the skip distance reaches the longest possible single hop, roughly 2,500 miles for the F2 layer, the frequency has reached the maximum usable frequency for the longest paths, and the band is about to close.
Living with the skip zone
For an operator chasing distant stations, the skip zone is mostly a curiosity. It does explain why, in a DX pileup, stations a few hundred miles away calling the same rare station cannot hear each other and sometimes call over one another. For regional communication, though, it is a real problem, and there are several ways around it:
- Move down in frequency. A lower band, closer to or below the critical frequency, shrinks or closes the skip zone. Emergency nets often move from 40 to 80 meters in the evening for this reason.
- Use NVIS antennas. A low horizontal antenna aimed upward makes the most of the steep angles that fill the gap.
- Use VHF. For distances within the ground-wave and line-of-sight range, VHF and repeaters may simply be better.
- Relay. A third station outside the skip zone of both can pass messages between them.
Signals sometimes reach the skip zone after all. Some energy that lands far away is scattered back toward the transmitter by the ground or sea, and some is scattered sideways from regions of strong ionization. These backscatter and sidescatter signals are weak and have a hollow, echoing sound, but they let stations in each other's skip zone talk when the band is open to a distant area. Short-range openings by sporadic E on 10 and 6 meters can also fill in distances that are normally in the skip zone.