Frequency Management

12.5.1.1 Definitions. In the context of this chapter, frequency management is the art/technology of selecting an optimum frequency for HF communication at a certain time of day between any two points on the earth. One important factor is omitted in the process, that is, the interference that may be present in the path, affecting one or both ends of the path.

We will be using the terms MUF, LUF, and FOT (OWF). The MUF and LUF are the upper and lower limiting frequencies for skywave communication between points X and Y. The MUF is the maximum usable frequency on an oblique incidence path; the LUF is the lowest usable frequency.

The concept of MUF is most important for the HF link design engineer for skywave links. We will find that the MUF is related to the critical frequency by the secant law. The optimum working frequency (OWF; original derivation from a French term), sometimes called FOT (from the French, frequence optimum de travail), is often taken as 0.85 X MUF. Little is mentioned in the literature about the LUF, since it is so system sensitive.

The MUF is a function of the sunspot number, time of day, latitude, day of the year, and so on, which are things completely out of our control. The LUF, on the other hand, is somewhat under our control. If we hold a transmitting station EIRP constant, as the operating frequency decreases, the available power at the distant receiver normally decreases owing to increased ionospheric absorption. Furthermore, the noise power increases so that the signal-to-noise ratio deteriorates and the circuit reliability decreases. The minimum frequency below which the reliability is unacceptable is called the lowest usable frequency (LUF). The LUF depends on transmitter power; antenna gain at the desired takeoff angle (TOA); factors that determine transmission loss over the path, such as frequency, season, sunspot number, and geographical location; and the external noise level. One of the primary factors is ionospheric absorption and hence, since this varies with the solar zenith angle, the LUF peaks at about noon. Consequently, in selecting a frequency, it is necessary to ascertain whether the LUF exceeds this frequency.

When applying computer prediction programs such as IONCAP, under certain situations we will find the LUF exceeds the MUF. This tells us, given the input parameters to the program, that the link is unworkable. We may be able to shift the LUF downward in frequency by relaxing link reliability (time availability), reducing signal-to-noise ratio requirements, increasing EIRP, and increasing receive antenna directional gain.

One factor in the transmission loss formula for HF is D-layer absorption. It varies as 1/f2. For this reason we want to operate at the highest possible frequency to minimize D-layer absorption. Suppose we operated at the MUF. It is a boundary limit and would be unstable with heavy fading. We choose a frequency as close as possible to the MUF yet stay out of boundary conditions. The operating frequency goes under two names: FOT or OWF, both defined earlier. The OWF is usually 0.85 the value of the MUF for F2 operation. Our objective is to keep our transmitter frequency as close to the OWF as possible. This is done to minimize atmospheric absorption but yet not too close to the MUF to reduce ordinary-extraordinary ray fading. As the hours of the day pass, the OWF will move upward or downward and through the cycle of our 24-h day. It could move one or even two octaves from maximum to minimum frequency. How do we know when and where to move to in frequency?

12.5.1.2 Methods. There are six general methods in use today to select the best frequency or the OWF. We might call these methods the frequency management function. The six methods are:

1. By experience.

2. Use of CRPL (Central Radio Propagation Laboratory) predictions.

3. Carrying out one's own predictions by one of several computer programs available.

4. Use of ionospheric sounders.

5. Use of distant broadcast facilities.

6. Self- and embedded sounding.

experience. Many old-time operators still rely on experience. First they listen to their receivers, then they judge if an operating frequency change is required. It is the receive side of a link that commands a distant transmitter. An operator may well feel that "yesterday I had to QSY* at 7 p.m. to 13.7 MHz, so today I will do the same.'' Listening to his/her receiver will confirm or deny this belief. When he/she hears his/her present operating frequency (from the distant end) start to take deep fades and/or the signal level starts to drop, it is time to start searching for a new frequency. The operator checks other assigned frequencies on a spare receiver and listens for other identifi

*A "Q" signal is an internationally recognized three-letter operational signal used among operators. A typical Q signal is QSY, which means "change your frequency to_.''Q signals were used almost exclusively on CW (Morse) circuits and their use has extended to teleprinter and even voice circuits.

able signals near these frequencies to determine conditions to select a better frequency. If conditions are found to be better on a new frequency, the transmitter operator is ordered to change frequency (QSY at the distant end). This, in essence, is the experience method. We will address this issue further on from a somewhat different perspective.

crpl predictions. Here, of course, we are dealing with the use of predictions issued by the U.S. Institute of Telecommunication Sciences, located in Boulder, Colorado. The predictions are published monthly, three months in advance of their effective dates (Ref. 13).

Consider a HF circuit designed for 95% time availability * (propagation reliability) and assume a minimum signal-to-noise ratio of 12 dB Hz. The median receive signal level (RSL) must be increased on the order of 14 dB to overcome slow variations of skywave field intensity and atmospheric noise, and 11 dB to overcome fast variations of skywave field intensity. Therefore a rough order of magnitude value of signal-to-median-noise ratio with sufficient margin for 90% of the days is 37 dB for M-ary frequency shift keying (MFSK) data/telegraph transmission for a bit error rate (BER) of 1 X 10 4. See CCIR Rec. 339-6 (Ref. 14).

predictions by pc. Quite accurate propagation predictions for a HF path can be carried out on a personal computer. A very widely accepted program is IONCAP, which stands for Ionospheric Communication Analysis and Prediction Program. It is written in Fortran (ANSI) and is divided into seven largely independent subsections (Refs. 10 and 11):

1. Input subroutines.

2. path geometry subroutines.

3. Antenna pattern subroutines.

4. Ionospheric parameter subroutines.

5. Maximum usable frequency (MUF) subroutines.

6. System performance subroutines.

7. output subroutines.

Table 12.1 is a listing of 30 available output methods. The IONCAP computer program performs four basic analysis tasks. These tasks are summarized below. Note that Es indicates highest observed frequency of the ordinary component of sporadic E and HPF means highest probable frequency.

* We define "time availability" as the percentage of time a certain performance objective (e.g., BER) is met.

TABLE 12.1 IONCAP—Available Output Methods

Method

Description of Method

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