Fun With Propagation Between the MUF and LUF

Get a hold of a buddy and see if you can make a QSO from 10 meters through 160 meters One autumn day in the late afternoon at the peak of Cycle 23, I ran into an old friend on 10 meters — Dave Henderson, NR1DX. We had worked together at Motorola in Fort Worth, Texas in the early 1980s — he was N0DH at the time. After leaving Motorola, he eventually ended up in the New England area — and thus his 1-land callsign. We had a nice chat on 10-meters, catching up with our personal lives and ham radio activities. Toward the end of the QSO we wondered if we could continue our QSO on 12-meters. So we QSY, and sure enough we easily worked each other there. This led to trying 15-meters, and then trying each lower band in succession. We ended up on 160 meters, but had to go to CW and turn on our amplifiers to make the QSO on Topband. That was a fun exercise —working Dave on all nine bands from 28 MHz down to 1.8 MHz in the span of several minutes. We had worked each other from the MUF (maximum useable frequency) to the LUF (lowest useable frequency). MUF: Nothing We Can Do What is the MUF? Its the highest frequency that is propagated over a path between any two given points at any given time. What determines the MUF? The MUF is determined solely by the state of the ionosphere —and most of the time its the state of the F2 region of the ionosphere. The MUF varies throughout the day, throughout the months, throughout a solar cycle, and is impacted by elevated geomagnetic field activity. Theres nothing you can do to your station to change the MUF. Of course you should try, if possible, to put most of your energy at the elevation angle dictated by the ionosphere. LUF: Lots We Can Do What is the LUF? Itthe lowest frequency on which you can hear the desired signal over a path between any two given points at any given time. The LUF is determined by the transmit power, the mode, the antenna gains, the losses, and the noise floor of your receiving system. The losses are primarily due to free space path loss, ionospheric absorption, and ground reflection loss —for multihop paths. As does the MUF, the LUF also varies throughout the day, throughout the months, throughout a solar cycle, and is impacted by elevated geomagnetic field activity. Since transmit power, the mode, antenna gains, and the noise floor of your receiving system affect the LUF, you can change the LUF because you can change those four items. You can: •Increase transmit power —as Dave and I did for our 160-meter QSO. •Go to a more effective mode —from SSB to CW as Dave and I did •Use a higher-gain antenna. For example, replace your in verted-V or vertical with a 4-Square. •Employ a low-noise receiving antenna —a Beverage, for example —to decrease your system noise floor. All of these will lower your LUF. Thus the LUF is very station-specific and depends on how much time and money you want to expend on improvements. The Impact of Absorption With respect to ionospheric absorption, it is good to understand where absorption occurs in the ionosphere. That’s easy, isn’t it? Absorption occurs in the D region, right? That correct for daytime conditions, but what about nighttime conditions when the D region, for all intents and purposes, goes away? Does absorption go away at night and thus worldwide 160-meter propagation at night is easy? If you ever done any DXing on 160 meters, you quickly realize that 1.8-MHz DXing is anything but easy. In truth, the absorbing region moves up to the lower E region at night. That easy to show with ray tracing from Proplab Pro, which includes the effects of the Earth magnetic field and electron-neutral collisions, both of which are important in determining ionospheric absorption. Figure 1 gives these results in terms of cumulative absorption versus altitude of the up-going wave —so we can see the altitude range over which absorption occurs. Figure 1 is for a one-hop path on 160-meters that is parallel to the terminator from nighttime (1000 UTC) to daytime (1400 UTC). During the day, absorption occurs in the D region (65-85 km, or 40-53 miles). But during the night, absorption occurs in the lower E region (85-95 km, or 53 to 59 miles). Figure 2 is the total absorption on this same path at the same times. The amount of absorption per hop is certainly lower at night than in the day, but there still absorption at night. The roughly 10-dB-per-hop at night adds up quickly in a multi-hop scenario, and suggests that multi-hop at night on 160 meters is limited to around 10,000 km —or 6,214 miles —after which the signal is below a typical noise level. This then leads to the assumption of another more efficient mode (ducting in the nighttime electron density valley above the E region peak) for our extremely long-distance QSOs on 160 meters. The Effects of Frequency Another interesting issue with ionospheric absorption is its dependence on frequency. Weall aware that absorption increases as we move from 10 meters to 160 meters. Does it keep increasing as we go below 1.8 MHz? No, it doesn’t . Figure 3 shows this data during the night for the same path as Figure 1 and Figure 2. Figure 3 is for the ordinary wave (the extraordinary wave incurs more absorption, and is usually ignored at low frequencies), and gives the expected results on 75 meters, on 160 meters, and at 1 MHz —increasing absorption as the frequency is lowered. But note what happens below 1 MHz —the ordinary wave absorption decreases down to 100 kHz. This says our amateur radio activities around 500 kHz and around 137 kHz will incur less absorption than on 160 meters. The reason for this is the interaction of the electron density and electron-neutral collision frequency versus altitude, electron gyro-frequency, and operating frequency. Of course this less absorption on the lower frequencies is offset by decreasing antenna efficiency and increasing man-made noise. Conduct Your Own Experiment So if youre looking for some fun with the MUF and LUF, get a hold of a buddy and see if you can make a QSO from 10 meters down to 160 meters. The best time would likely be the later afternoon or early evening when 10 meters is still open and darkness is approaching for 160 meters.

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