Nuts and Volts - January 2016

than 1, and is written as a ratio such as 1:1 or 1.5:1 or 3:1, and so forth.

SWR can also be calculated as the ratio of the feed line Z0 and the load impedance — whichever is greater than 1. Since you already know the feed line’s Z0 and can measure the load’s impedance, this is a lot more convenient than trying to measure maximum and minimum voltage inside the line: Another convenient way to measure SWR is by using forward power (Pf) traveling from the generator to the load and reflected power (Pr) traveling in the opposite direction:

Figure 3 shows a chart that converts any combination of forward and reflected power into an SWR value.

Effects of SWR (Radio and Non-Radio)

Why do we care so much about SWR? A clue was provided earlier in explaining that if the characteristic impedance of a line does not match the impedance of whatever load it is attached to, some of the power applied to the line is reflected at the load. Since we would like all of our expensive transmitter output power to be put to work as a radiated signal, it would be good to minimize reflected power at the load. SWR is just a convenient way to measure the quality of the impedance match between our line and the load. The lower the SWR, the better the match!

Non-radio folks also care about SWR — especially with regard to high speed digital data. Data signals may “just” be voltage levels corresponding to 0 and 1, but it takes very high frequency components to make the sharp edges and narrow pulses our designs require.

A 100 Mbit/s data stream contains signal components in excess of 1 GHz! At those frequencies, every wire and PCB (printed circuit board) signal trace has to be treated as a type of parallel-conductor transmission line because it is.

If the Z0 of a PCB trace does not match the output impedance of whatever circuit is generating the signal or the input impedance of whatever is receiving the signal, severe ringing, overshoot, undershoot, or multiple false transitions and glitches can occur. Search for application notes on “signal integrity” for detailed information about what PCB designers must do to control the data paths at today’s signaling rates.

Video signals — particularly analog video — can suffer from impedance mismatches, resulting in ghost images and distorted pictures.

The solution is to understand when transmission line considerations apply and terminate the signal traces in appropriately.

Measuring Impedance and SWR

To make accurate measurements at RF, instruments must be designed for that purpose. Low frequency multimeters simply can’t be used. Nevertheless, inexpensive versions are available that don’t cost a lot and still provide useful information. You just have to know where to shop!

SWR Meters

The most common transmission line instrument in a ham or CB station is the basic SWR meter shown in Figure 4A. The meter is used by setting the CAL control for a full-scale reading for forward power (FWD), then switching to the reflected (REF) position to read SWR. The meter sensitivity varies with frequency, requiring readjustment when using different bands; accuracy is low compared to a lab instrument. Available online and from CB shops for under $30, these meters give a good idea whether SWR is high or not, and are useful in monitoring output power while operating or when adjusting an antenna.

More advanced meters like the Daiwa CN-101 in Figure 4B provide simultaneous power and SWR measurements with a crossed-needle meter. The unit displays both forward and reflected power with

January 2016 17 FIGURE 4. Inexpensive SWR meters (A) are useful up to about 30 MHz at power levels up to 100 watts. The more expensive meter (B) displays forward and reflected power simultaneously along with SWR on a crossed-needle display.

Skin Effect Above about 1 kHz, AC currents flow in an increasingly thin layer along the surface of conductors. This is the skin effect ( https://en.wikipedia.org/wiki/Skin_effect). It occurs because eddy currents inside the conductor create magnetic fields that push current to the outer surface of the conductor. At 1 MHz in copper, most current is restricted to the conductor’s outer 0.1 mm, and by 1 GHz, current is squeezed into a layer just a few μm thick.