during the day with limitations comparable to those noted for 5.0 MHz. These frequencies also provide daytime reception at fairly close range, 200 miles or so, and can be used when 5 MHz reception is poor. The half-wavelength horizontal antenna should be selected for short distances. A quarter-wavelength vertical antenna is suitable at greater distances (see figure 5.11).

ANTENNAS FOR THE 14 TO 15 MHZ RANGE.

These frequencies are most favorable for long range, daytime reception. They are not usable for short-range reception except during periods of sunspot maximum. However, for long range reception, they are the most favored frequencies during both sunspot cycle conditions. Under average conditions, the maximum wave angle is limited to 30 degrees or less depending on the density of the ionized layer.

During maximum sunspot cycle, reception may even be possible during the night in some locations. During minimum sunspot cycle, they are useful only during the daylight hours and dawn and dusk periods. Both horizontal and vertical antennas are suitable.

ANTENNAS FOR 20 MHz.

The 20 MHz frequency is normally the best to use for daytime reception and will be optimum at either noon or a few hours past noon. Signals at this frequency arrive at very low wave angles and are useful only for long distance reception. During minimum sunspot cycles, reception is poor but improves during the winter. During maximum sunspot cycles, the reception is excellent at night and during the day. The vertical dipole that favors low wave angle radiation has been used at this frequency with favorable results. Construction details for a 20 MHz antenna are shown in figure 5.12.

ANTENNAS FOR 25 MHz.

The 25 MHz frequency is best during daylight hours except during the summer. Reception is especially good during maximum sunspot cycle and very poor during minimum sunspot cycle. It is used only for long distance reception because a low radiation angle is required for the signal to be returned back to the earth from the ionosphere. Design details of a different vertical antenna for use at 25 MHz are also shown in figure 5.12.

VERTICAL ANTENNA CONSTRUCTION.

The vertical antenna which is favorable to low wave-angle reception is preferable to the horizontal half-wavelength antenna and can readily be constructed as shown in figure 5.12. The dimension of the ground radials and orientation as shown should be used to yield approximately a 50 ohm antenna impedance. For a 70 - 90 ohm antenna, the half-wavelength dipole mounted vertically will yield approximately the correct impedance. In order to prevent interaction between the feed line and the lower half of the dipole, which disturbs the radiation pattern, extend the feed line horizontally outward several feet from the antenna before dropping it vertically to the ground.

5.4 USE OF HF BROADCASTS FOR

TIME CALIBRATIONS

Now that we have a receiver and an antenna, we can proceed to make time calibrations. If your accuracy requirements are low, you can use the voice time-of-day announcements. However, if you want higher accuracy, you will have to measure the seconds ticks or decode the WWV/WWVH time code.

5.4.1 TIME-OF-DAY ANNOUNCEMENTS

Time of day is available from many sources in the United States and Canada. Radio and television stations mention the time frequently and, in fact, use the time of day to schedule their own operations. The telephone companies offer time-of-day services in most locations. But where does the time come from?

Most time-of-day services in the U. S. start at the National Bureau of Standards. An NBS telephone service is available by calling (303) 499-7111. In addition, WWV and WWVH broadcast voice time-of-day announcements once each minute. This is also the case for CHU where the time announcements are given alternately in French and English. Using the WWV voice announcement and the tone following the words, "At the tone, XXXX hours XXXX minutes Coordinated Universal Time," a person can check a wall clock or wristwatch to within a fraction of a second. The UTC time that is announced can be converted to local time by using the map in Chapter 2.

5.4.2 USING THE SECONDS TICKS

For accurate time recovery, we cannot just listen. We must rely on electronic equipment. By using an electronic oscilloscope to

where the maximum signal is received. The frequency of the HF signal generator is then adjusted for peak receiver output.

The audio signal generator is set to a 1 kHz output frequency. A high accuracy 1 kHz signal is not required. The HF generator is externally modulated by the 1 kHz signal. The Oscilloscope sweep rate is set to 100 microseconds/division with positive external triggering from the 1 kHz signal. The vertical amplifier gain is set high for a large vertical deflection. The vertical position control is adjusted for zero baseline with no input signal.

Initially, the 1 kHz signal generator is connected to the vertical input of the oscilloscope. The trigger level control is adjusted so that the trace crosses over or touches the horizontal center line at the left. The horizontal position control can be adjusted so that the signal crosses over the first division on the left as shown in figure 5.14. The crossover point of the undelayed signal will serve as the zero delay reference point.

A local clock pulse at 1 pps second rate is used to trigger the oscilloscope sweep. At some time interval later during the sweep, the seconds pulse appears on the display as shown. The time interval from the start of the sweep to the point where the tick appears is the total time difference between the local clock and the transmitting station. By subtracting the propagation and receiver time delays from the measured value, the local clock time error can be determined. The equation to determine time error at a receiving location is:

The receiver is tuned to the station and the oscilloscope sweep rate set at 0.1 s/ division. Listening to the broadcast will help judge the quality of reception and fading. The tick will typically appear as shown in figure 5.16. CHU signals can be used as well (see fig. 5.17). If the tick is one division or more from the left side of the scope display, the time of the local clock is corrected until the tick falls within the first division from the left side. If the local time tick is late, the received tick will be heard before the sweep starts. If this is the case, the local clock should be adjusted until the tick appears. After the local seconds pulse has been properly adjusted and appears within the first division (0.1 second in time), the sweep rate is increased to say, 5 ms/division. Using this greater resolution, the local clock is adjusted until the leading edge of the received pulse starts at a time equal to the propagation delay time plus the receiver delay time after the trigger as shown in figure 5.18.

The sweep rate should be increased to the highest rate possible without allowing the total sweep time to become faster than the combined propagation and receiver delay time