>The instruction sheet is rather basic, and if any radio experts would
>care to contribute any extra guidance on VSWR meter use, it would be
>most welcome, and will be added to the manufacturer's instructions.
I don't know about expert, but after 27 years of ham radio, I've got a
fair amount of experience of using SWR meters and will have a go. My
apologies if I get the level wrong.
An SWR meter measures the "goodness" of an antenna system including
the coaxial cable but excluding the radiating effectiveness of the
antenna itself. The equivalent circuit of an antenna is a combination
of inductance and capacitance (and a small amount of resistance) that
form a tuned circuit with an optimum operating frequency.
Firstly, a bit about electrical and antenna theory. By Ohms Law,
Power = Voltage times Current. The RF power output from a transmitter
(typically 5 - 10 watts for VHF Comm. comprises an alternating voltage
and an alternating current. VSWR stands for Voltage Standing Wave
Ratio and for our purposes here, the current aspect can be ignored.
When an RF signal is applied to an antenna, it travels along the
antenna, producing instantaneous voltages (or standing waves) that
vary with the phase of the signal. When the signal hits the end of
the antenna, it is reflected producing another voltage standing wave
that varies with phase. If the antenna length is exactly one or more
quarter-wavelengths, the forward and reflected voltages will be in
phase and the instantaneous voltages will combine and be radiated.
The ratio of forward to reflected standing waves is 1:1.
However, if the antenna length is slightly more or less than one or
more quarter-wavelengths, the forward and reflected voltages will be
out of phase which has two effects. Firstly the combined,
instantaneous voltage is less and, therefore, less power is radiated.
Secondly, that non-radiated power has to go somewhere, and that's
usually back along the coaxial cable to the transmitter where it is
hopefully dissipated through the transmitter chassis back to the
battery, but could get into any other wiring causing interference.
The VSWR meter contains directional couplers which enable it to
separate and measure the forward and reflected power levels and
display them as a ratio.... 1:1 being the ideal, up to 2:1 would be
acceptable, anything higher needs investigation and rectification.
Most meters are also able to measure the forward power level but few
are calibrated accurately and this should be used as an indication
only.
At the crudest level, the meter will have one meter with a switch to
indicate forward or reflected power. Others have two separate meters,
and some have a single meter with cross-over pointers. All will have
a pot to adjust the forward power level. However, the operation is
the same.
Remove the coaxial cable from the back of the transmitter and
connect it to the ant socket on the meter.
Connect a separate length of coaxial cable between the transmitter
and the xmit socket on the meter.
If possible set the transmitter to low power output.
Adjust the frequency to somewhere near the middle of the available
frequencies BUT remember that you have to actually transmit to use
the meter, so pick a frequency that is NOT used in your area AND
always listen to make sure it is clear.
If the meter has a power measuring option, it will have a switch to
allow setting the meter for reading VSWR (probably marked SET).
Switch it to this position.
Press the PTT button on the transmitter.
Adjust the meter to show full scale deflection.
Read the VSWR level (if the meter has a single meter, you may have
to switch it to SWR).
As mentioned above, a VSWR of 1:1 is the ideal, up to 2:1 is
acceptable, 10:1 indicates significant problem (e.g. broken or
short-circuited coaxial cable).
If you are using the SWR meter to adjust the length of an antenna, cut
it at least 2cm longer than the theoretical length then take three
readings - bottom of band, mid-band and top of band (remembering not
to cause interference). As the antenna is too long it should have a
better SWR at the lowest frequency. Cut about 1cm from the antenna
and repeat the measurements (they should be better than before but
still not perfect). Continue cutting (maybe less than 1cm as you
approach 1:1) until you achieve an optimum reading mid-band. If you
find that you get a better SWR above mid-band, STOP because you've cut
off too much. As long as the SWR is below 2:1 all will be well.
And that's all there is to it!
Peter Barwick
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