Comm/Nav Web Training

HF - High Frequency

ARC-190
Frequency Range · 3-30 MHz
Power Out · 300 watts

Some words about antenna lengths...

Every comm/nav troop should be able to figure out the perfect antenna length for any given wavelength. It's pretty simple as long as you can remember that the velocity of light is approximately 300,000,000 meters per second. Now take the speed of light and divide by your frequency:

300,000,000 meters/sec

=100 meters
3,000,000 (cycles)/sec

If you can't remember the speed of light, just remember that the perfect antenna for 3 MHz (the lowest freq of the HF range) would be a bit longer than a C-5 (which is 75.3 meters long!). You can figure it out backwards by simply taking 3,000,000 Hz and multiplying it by 100 meters to find out the velocity of light. (Actually, quarter wavelength antennas are typically used, but hopefully you get the point)

Anyway, the point of all this is that I'm leading to the fact that the perfect antenna for 3 MHz is quite long, which is why we need the antenna coupler, without which, we would incur system debilitating standing waves.

Standing Waves taken verbatim from TO 31Z-10-4

(1) When radio waves are fed to a wire conductor of infinite length, the RF waves of electromagnetic energy move along the wire. Because of the electrical resistance of the wire, the amplitude of the waves gradually diminishes, but the waves continue to travel as long as the wire does not come to an end. These waves of energy are called "traveling waves."

(2) In practice, however, a wire conductor (such as an antenna) has a finite length. Therefore, the traveling waves stop abruptly when they reach the end of the wire. At this point, since the current has stopped flowing, the magnetic field surrounding the wire collapses, and in doing so, the collapsing lines of force cut across the wire and induce a voltage in the wire, according to Lenz's law. This voltage causes a current to flow back toward the initial source. If a continuous succession of waves is fed to the wire, they will be continually reflected back toward the source. The waves moving from the transmitter toward the end of the wire are called "incident waves", while those which are reflected back are called "reflected waves."

(3) With a continuous flow of incident waves away from the transmitter and a continuous flow of reflected waves returning toward the transmitter, it is obvious that these waves, traveling in the same conductor, must pass each other. At certain point along the conductor both the incident and reflected waves will reach their maximum positive (or negative) values, at the same instant. At these points they reinforce each other. At certain other points along the conductor both of the waves reach a zero value at the same time; here the resultant wave is zero. In a conductor, such as an antenna, which has a finite length, the points at which the resultant wave reaches its maximum (positive or negative) and its zero values are stationary. This is so, even though both the incident and reflected waves are moving. Since the resultant wave, in effect, stands still on the line, with only its amplitude changing from maximum positive, through zero, to maximum negative values, the wave is referred to as a "standing wave."

To reduce standing waves in most transmission lines, you need to match impedances (the impedances being the ratio of voltage to current).

The key here is represented in the formula for impedence, mismatches of which will cause standing waves:

$Z = \sqrt{R^2 + (X_L - X_C)^2}$

Given that capacative reactance (

$X_L = 2 \pi fL$

)

and inductive reactance (

$X_c = \frac {1}{2 \pi fC}$

) are both frequency sensitive (f), different frequencies are going

to result in different impedances at the antenna, resulting in an impedance mismatch if not compensated for. Hence, the use of an antenna coupler in the ARC-190 system.

ARC-190 Basics

RT-1341 - The receiver/transmitter modulates audio into RF and vice versa. It is notable that there are two IF frequencies, 97.8 and 1.8 MHz. This leads to one of the key features of the system: dual conversion; simply meaning that the R/T uses two stages of heterodyning.

Remember now, that when you mix a signal, you get 4 signals: 2 original, 1 sum and 1 difference. The highest quality signal lies in the difference signal, so the HF system uses it. However, using the difference signal can increase the occurence of unwanted and interfering image signals. To reduce this interference, the R/T simply employs a second stage of coversion or hetrodyning.

Another key feature of the R/T is its use of frequency synthesizers. Many radios use a VCO (Voltage-Controlled Oscillator) because it is a simple and effective way of producing a large number of frequencies, but the drawback of these oscillators is that output frequencies are not as highly accurate as those in crystal oscillators. And while the crystal oscillator is highly stable, it can only produce one freqency which would seem to make it unsuitable for a system like the ARC-190 which has 30,000 channels. So to maintain a high degree of accuracy and a wide selection of frequencies, the system uses a frequency synthesizer which is basically a bank of crystal oscillators, the outputs of which are mixed to form the desired frequency.

CU-2275 coupler - The HF coupler provides the impedance matching necessary for transmission on such a wide range of low frequencies. When keying on a new frequency, you can hear the 'tune tone' on your headsets. During this duration, the R/T generates a 100 watt signal which is used by the coupler to electrically tune the antenna to match the input impedance.

Note that the coupler is pressurized to 7±1 PSI with dry nitrogen (air). This serves three functions which a scary comm/nav troop like myself often memorizes verbatim:
(1) Prevent high voltage arcing
(2) Prevent corrosion
(3) Provide a uniform cooling medium

Of course, the primary function is to prevent high voltage arcing which would otherwise occur at high altitudes, while the other 2 provide fine trivia.

C-10828 Control - The control provides access to the 30 preset channels, 8 modes, 30,000 normal channels, test functions, squelch and power on/off. You should know the 8 different modes of operation: UV (Upper Voice), LV (Lower Voice), UD (Upper Data), LD (Lower Data), CW (Continuous Wave), AME (Amplitude Modulated Equivalent), P (Preset) and A (not used).

Continuous Wave is seldom used... it's basically for morse code transmissions. There are the 4 single sideband modes, which ought to be fairly self-explanatory. A is not used and will generate a control fault if selected. Preset accesses one of the 30 preset channels. And AME is simply AM transmission with one of the sidebands removed. Note that it is not the same as AM.

Something every comm/nav troop needs to know about HF are the 3 user faults. The first, already mentioned, is a control fault generated by selecting A. You can also generate a control fault by selecting a frequency below the usable range (below 2 MHz). And finally you can generate an R/T fault by selecting an unused preset, since the presets are stored in the R/T. By default, a brand new R/T doesn't have any presets in it, so if you select a channel that has not had a frequency programmed into it, the control is simply trying to tell you that you aren't going to be transmitting on anything until you put something in there.

Basic Troubleshooting

The ARC-190 is a relatively decent system, however the R/T and transmission line connectors go bad, and nearly as frequently, the coupler fails.

The most useful tool in troubleshooting the system is the built in receive and transmission test cricuits. By performing this test on a variety of frequencies, troubleshooting the system can become easy. If the problem is frequency sensitive, meaning that it generally faults only on certain frequencies, then you more than likely have a coupler or transmission line problem, regardless of the fault indicated. The ever trusty TDR can help you to determine if it's the latter. And as for R/T problems, your friend the wattmeter can often narrow the problem down to this LRU, but don't forget: in SSB transmission, there won't be any power output unless you modulate the signal. So I recommend checking power out in AM, which should give you a healthy 300 watts or so. And if you are getting 0 watts, make sure the secure side of the system is in bypass, as applicable. On rare occasions, when all else seems to have failed and the wiring looks good, remember that you have the R/T mount, lightning arrestor and on some aircraft, a coupler mount and feedline and on other aircraft, the leadthru mast and longwire. The leadthru mast or feedline is very easy to test... simply ohm the leads and you should have a short.

Take the HF test
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