Many new hams, CB operators, and even boaters with a marine radio get their first standing wave ratio (SWR) meter from a convenient RadioShack. An antenna with an SWR greater than 2:1 may be in need of work! If you use an antenna tuner to match a “long-wire” antenna, then this device is indispensable.
Their catalog # 2100534 fills the bill, but unfortunately the panel layout & markings may easily lead to confusion. Exactly what does one do with the three switches and the knob? Friends whom I’ve tried to help were completely lost because the front panel intersperses switches for the two different capabilities of this meter.
Figure 1 shows improved panel artwork with thick borders that more clearly separate the functions and better explains the controls. Using a photocopier’s enlarge/shrink function, make the top border about 3-3/8″. Cut out the switch rectangles and knob circle. Then tape it over the commercial front panel…
Now to explain the meter’s operation: The instrument has two completely separate functions: measuring output POWER and measuring Standing Wave Ratio (SWR). The middle switch chooses which function.
To read POWER, the user moves the middle switch to its highest position, then selects from three different power ranges with the left-most switch. Both “average” and “peak envelope power” (PEP) can be measured, depending on the setting of the right-most switch.
In voice single side band, the PEP can be quite a bit larger than the average, and thus this meter can help the user avoid turning the mic gain up so high that he splatters his signal, taking up much more bandwidth than proper. (If turning up the mike gain no longer increases the PEP output, for sure you are beyond the limit! Back off a bit!) The rotary knob isn’t used at all.
To read the SWR, the user first positions the middle switch into the “FORW CAL” (forward relative power) position, sends out a steady signal (careful not to do this longer than safe for your transmitter’s amplifier!) and quickly adjusts the rotary knob on the right until the needle is at “full scale”.
There are multiple ways to create the steady carrier needed to allow the adjustment, depending on your transmitter: there may be a “TUNE” position; or you may be able to hold your Morse Code key down, or with an AM CB rig, simply push the mike button. Then flick the middle switch into REV (“reverse relative power”) and read the SWR from the top scales marked SWR on the meter. Try to keep your SWR 2:1 or lower!
Figure 1: Artwork for new panel markings.
Figure 2 — New artwork on SWR / Power meter