CB/Ham Field-Strength SWR Meter
(210-0533) Operation Faxback Doc. # 47578
INTRODUCTION
Your RadioShack CB/Ham Field-Strength SWR Meter helps you tune your CB or
amateur radio system for the best performance.
CB and amateur radio systems work best when the antenna system's impedance
matches the transmitter's output impedance as closely as possible.
The meter's standing wave ratio (SWR) function helps you trim your antenna
to the precise length you need for the maximum transmitted power.
The meter's field strength function helps you position your antenna for
the best coverage and determine the effectiveness of changes you make to
your antenna, antenna cable, and transmitter.
Carefully read all of these instructions to get the best use from this
meter.
WARNING: You can use this meter to make measurements on equipment that
uses high voltages. Carefully observe all safety precautions
provided with the equipment you are testing.
Note: You need a separate 50-ohm coaxial cable (such as RG-58U) with a
PL-259 connector connected to both ends (such as Cat. No. 278-0968,
not supplied), long enough to reach from your radio's antenna jack
to the back of the meter.
MEASURING SWR
Measuring SWR shows you how much of your radio's transmit power is
reflected from the antenna back into the antenna cable, generating heat
and wasting power. With this information, you can adjust the length of
your antenna or antenna cable so your antenna and radio combination can
produce the maximum power they are capable of producing.
For more information about measuring SWR, see the ARRL Handbook, available
at your local library.
CONNECTING THE METER
To connect this meter to your CB or amateur radio and your antenna, you
need RG-58U coaxial cable (not supplied) with a PL-259 connector on both
ends (such as RadioShack Cat. No. 278-968). The cable must reach from
the back of the meter to your radio's antenna jack.
Follow these steps to connect the meter to your radio and antenna.
1. Turn off your radio. Then disconnect the antenna from the radio and
plug it into ANTENNA on the back of the meter.
2. Plug the coaxial cable into the radio's antenna jack and into
TRANSMITTER on the back of the meter.
CALIBRATING THE METER/MEASURING SWR
1. Set CALIBRATION/FS/SWR on the front of the meter to CALIBRATION/FS.
2. Turn on your radio. Then select a channel or frequency on your
transmitter and hold down its transmit key. Do not speak into the
microphone.
Notes: An SWR reading will be different for different frequencies
(channels). If you transmit on one channel more often then
any other, select that channel. If you transmit on several
channels, choose a frequency in the middle of the range of
channels you use. (For example, if you transmit on all 40 CB
channels, choose Channel 20, because it is midway between
Channel 1 and Channel 40.)
If you use a CB that has sideboard modes (SSB), do not select
any of these modes. (Use only AM for SWR measurements).
If you have an amateur radio, select the continuous wave (CW)
or tuning mode to check the SWR.
3. While you hold down the transmit key, rotate the meter's knob until
the needle points to CAL.
4. Release the transmit key, then set CALIBRATION/FS/SWR to SWR.
5. Press the transmit key again, then read the SWR by noting the needle's
position on the upper SWR scale. A meter reading of 1 on the upper
scale is ideal.
The figure shown between the upper and lower scales on the meter
(% REF POWER) indicates the percentage of power that is wasted
through reflection of the signal from the antenna back into the
antenna cable. For example, an SWR reading of 1.5 also means that 4%
of your signal power is lost. However, 96% of the radio power is more
than enough for almost all applications.
6. Repeat Steps 1-5 for each channel or frequency you want to get an SWR
reading for.
Interpreting SWR Readings
An ideal SWR reading is 1.0, but this reading is usually possible only
under laboratory conditions or with a dummy load. Actual antenna
installations have higher readings. The information below will help you
interpret the readings you get.
SWR Efficiency Interpretation
1.0 to 1.5 Excellent The antenna cable and the antenna length
match the transmitter's output
requirements almost perfectly.
1.5 to 2.0 Very good The antenna, the cable, and the
transmitter operate very efficiently.
2.0 to 3.0 Acceptable The antenna, the cable, and the
transmitter operate with some loss. If
possible, adjust your antenna or antenna
mounting system to improve.
Above 3.0 Inefficient Adjust your antenna or antenna mounting
system to improve efficiency.
IMPROVING SWR
There are several ways to improve the SWR of your radio/antenna
combination. Try these first.
Be sure you are using the type of cable recommended for your
equipment. If the manufacturer recommends a 50-ohm cable, do not
substitute another type that has a different impedance.
Confirm that you mounted your antenna according to the manufacturer's
instructions. The angle and the base arrangement can affect the SWR
reading.
Adjust the length of your antenna according to the instructions
provided by the manufacturer. A change of as little as 1/8 inch can
make a measurable difference.
See your radio's and antenna's owner's manuals.
MEASURING FIELD STRENGTH
Your meter can measure the low levels of radio frequency (RF)
generated by a radio transmitter. You can use these measurements to
map the transmitting direction and coverage pattern of your antenna, or
to compare your radio and antenna's efficiency before and after an
adjustment. You do not need to connect the meter to the antenna or the
radio to measure field strength.
It takes two people to measure field strength. For an accurate
measurement, you will need to map the site around your antenna.
Then, while someone presses the radio's transmit key, you will need to
walk around the site around your antenna with the meter, marking the
readings you see.
Hints: In addition to the meter, we recommend you use the following
items to help you measure field strength:
Several photocopies of the template
36 objects such as rocks or weighted pieces of paper
25 feet of rope or strong cord
Calculator
Pencil
Compass
Make sure there is at least 25 feet of open space in all
directions around your antenna.
PREPARING THE ANTENNA SITE
Before you use the meter to measure field strength, you should mark the
site around the antenna in a pattern that will help you make the
measurement.
An easy way to do this is to map the area around your antenna.
The template shows an imaginary circle around an antenna, with the circle
divided by 18 spokes representing imaginary straight lines measured from
the center of the circle to 36 points spaced equally around the circle.
Each straight line is 25 feet in length. Four equidistant concentric
circles are drawn between the center and the outside circle. The circles
are numbered from 1 (the circle closest to the center) to number 5 (the
outside circle). The numbers on the template (1-5) are a guide to show
your where to mark the field strength level the meter displays as you
measure.
Hint: To help measure to a circular path around the antenna, tie a slip
knot in a 25-foot piece of rope or strong cord, slide the knot
loosely around your antenna then use the rope as a guide as you
walk around the antenna.
Follow these steps to prepare the antenna site.
1. Starting at the antenna, use a compass to find due north. Then walk
due north and stop about 15 to 25 feet away from the antenna.
2. Use an object such as a rock or a weighted piece of paper to mark
where you are standing.
3. Walk around the antenna in a circle, using more objects to mark 35
more evenly spaced marks on the path.
Hint: You can quickly find the space to leave between each mark by
using a calculator. Measure the diameter of the marked circle
(from one side of the circle to the other). Then, divide the
diameter by 11.46.
For example, for a 50-foot diameter circle:
50 feet / 11.46 = 4.363 feet
So, you would place a mark every 4.363 feet (4 feet, 3 inches).
CALIBRATING THE METER/MEASURING FIELD STRENGTH
IMPORTANT: Be sure to follow any radio service regulations that limit the
amount of time you can transmit without a break or station
identification.
1. Remove the supplied short wire antenna attached to the bottom of the
meter, then insert it into FS ANT on top of the meter.
2. Set CALIBRATION/FS/SWR to CALIBRATION/FS.
3. To calibrate the meter, have your helper select a channel or frequency
on your transmitter and hold down its transmit key. Tell the helper
not to speak into the microphone.
Then, while the helper holds down the transmit key, walk around inside
the marks you made using the steps in "Preparing the Antenna Site"
above, covering as much area inside the circle as you can. As you
walk, watch the meter's needle. If it moves to the right of 5 on the
FIELD STRENGTH scale, rotate the knob on the front of the meter until
the needle lines up with 5 on the scale.
Notes: If you use a CB that has sideband modes (SSB), do not select
any of these modes. (Use only AM for field strength
measurements).
If you have an amateur radio, select the continuous wave (CW)
or tuning mode to check the field strength.
4. To measure field strength, as the helper continues to hold down the
transmit key, move to the marked spot on the circle due north of the
antenna, then watch the meter's needle.
Note the reading on the FIELD STRENGTH scale and place a dot on the
template corresponding to the reading at that point.
5. Move to each marked spot along the circular path then repeat Step 4
for each one, taking another reading and recording that reading on the
template.
USING FIELD STRENGTH READINGS TO TUNE YOUR RADIO AND ANTENNA
You can use field strength measurements to monitor the effectiveness of
changes you make to your radio and antenna while you make them. Changes
you might make include:
Installing a new antenna or antenna cable
Adjusting the length of an antenna
Changing the antenna mount on your vehicle
Adjusting your radio's transmitter
You do not need to connect the meter to the antenna or the radio to
measure field strength, but you will need two people. While someone
presses the radio's transmit key, you will need to note the readings you
see on the meter.
IMPORTANT: Be sure to follow any radio service regulations that limit the
amount of time you can transmit without a break or station
identification.
1. Remove the supplied shorted wire antenna attached to the bottom of the
meter, then insert it into FS ANT on top of the meter.
2. Set CALIBRATION/FS/SWR to CALIBRATION/FS.
3. To calibrate the meter, have your helper select a channel or frequency
on your transmitter and hold down its transmit key. Tell the helper
not to speak into the microphone.
Then, while the helper holds down the transmit key, walk around and
adjust the meter's knob until the meter's needle moves to 3 on the
FIELD STRENGTH scale.
Notes: If you use a CB that has sideband modes (SSB), do not select
any of these modes. (Use only AM for field strength
measurements).
If you have the amateur ratio, select the continuous wave (CW)
or tuning mode to check the field strength.
4. Have the helper release the transmit key, then make a change to your
radio or antenna (such as adjusting the antenna or transmitter or
adding a new antenna).
5. When the change is completed, have the helper hold down the transmit
key again while you watch the meter's needle.
If the field strength reading is higher, you have probably improved
the power output and efficiency of your radio and antenna. If the
field strength reading is lower, repeat Steps 4 and 5.
Note: If you move the physical location, or change the transmission
direction, of your antenna, you might have to recalibrate the
meter and measure the field strength again.
See "Calibrating the Meter/Measuring Field Strength" above.
(BR/tc 5/26/98)