Microsoft Word - PM-2458 FAQ FAQ
t-meter family of Broadband Wireless Power Meters

What is the t-meter?
The t-meter is a low-cost, hand-held power meter for the 2.4, 5.3, and 5.8GHz
frequency bands, with model-specific coverage in 900MHz, 3.5GHz and 4.9GHz. The
PM-900, PM-3500 and PM-4900 also provide improved coverage across the 5.4 to 5.7
GHz band. The t-meter is capable of measuring forward and reflected power in
addition to VSWR.

What is the t-meter used for?
The t-meter is used to verify the installation of broadband wireless systems operating
in the bands which your model covers. It can be used to measure the RF power out of a
radio or amplifier, test coaxial connector terminations for low reflected power, and verify
the VSWR of an antenna. Additionally, the meter can be used to calculate transmission
line loss.

What type of equipment can the t-meter be used with?
The t-meter can be used with most equipment that occupies the frequency bands which
your model is specified to cover. The meter works particularly well with spread
spectrum radios (802.11b DSSS and FHSS). Additionally, the meter works with
proprietary modulation techniques.

Does the t-meter work with 802.11a devices?
Yes, the t-meter will work with most 802.11a devices*. The 802.11a specification uses
OFDM modulation but also defines many other aspects of the signal. When an 802.11a
device is not transmitting data, it will transmit short bursts (48uSec) of energy. These
OFDM bursts contain four distinct regions resulting in a power envelope that is not
constant with time. The bursts output power peak-to-average can vary as much as 8dB
across the burst. The t-meter is designed to measure peak power, so what a user sees
on the display of the t-meter is a power measurement that is quite a bit higher than the
radio manufacturers power output specification (average power), as the meter samples
these varying peaks. The meter will not display an average or constant power reading.

*Older t-meter model PM-2458 units (serial numbers 4134 and smaller) do not work
well with an 802.11a device. The leveling circuit is not fast enough to measure the
48uSec bursts.

The PM-2458 model does not specify coverage from 5.47 5.725GHz. Can it be
used to measure HiperLAN devices?
Yes. Set the PM-2458 meter to the 5.3GHz band when testing both HiperLAN/1 and
HiperLAN/2. HiperLAN/1 is similar to the IEEE 802.11b specification, but resides at
5.15GHz to 5.35GHz. The 5.3GHz setting on the PM-2458 will work well when
measuring HiperLAN/1 signals. HiperLAN/2 speeds along at up to 54Mbps similar
to 802.11a, but its spectrum is allotted in the 5.47GHz to 5.725GHz. Due to the high
crest factor of these signals, the 5.3GHz setting on the PM-2458 gives more accurate
results than the 5.8GHz setting when testing HiperLAN/2 as well.
t-meter models PM-4900, PM-3500 and PM-900 all feature specified coverage from
5.47GHz to 5.725GHz.

Why does the t-meter

indicate a low VSWR when I have a complete short or open
at the other end of my transmission line?
The t-meter is capable of detecting 0 to 20dBm of forward power, and 20 to 17dBm
of reflected power (when no in-line attenuators are used). It is necessary to consider the
round trip attenuation of the signal from the source. For instance, if the system in
question has a source with an output of 20dBm, and 100 of transmission line is used that
has 7dB of attenuation for the appropriate frequency, the round trip attenuation for the
forward and reflected signal is 14dB. Assuming the meter is connected in-line between
the source and transmission line, it will see 20dBm forward and 6dBm reflected (20dBm-
14dB) with a complete short or open at the end of the line. This would result in a return
loss of 14dB, or a VSWR of 1.5:1. A bad VSWR will be measured to be better than it
actually is since the reflected power is attenuated by the cables loss.

How do I know if a connector is bad?
The best way to test your transmission line terminations is to place the t-meter between
a signal source and the termination in question while using the included 50
load on the
opposite end of the line. A good termination should return a VSWR of 1.2:1. It is
possible that the termination could be good and that a problem somewhere else in the
transmission line could cause a higher VSWR. It will be important to keep this in mind
while testing. Depending on the system configuration it might be possible to use the
included 50
load to isolate the problem to a section of the transmission line.

How do I know if an antenna is bad?
Most antenna manufacturers indicate a minimum VSWR of 1.5:1. The meter can be
placed between a signal source and the antenna to get a VSWR reading. If the VSWR is
worse than 1.5:1, the antenna should be considered suspicious.

Does the t-meter require Maintenance?
Normal wear to the RF input and output connectors can lead to damaged center
conductors and coupler failure. It is important that all connectors are kept clean. Clean
outer surfaces with isopropyl alcohol and a clean cloth. Clean the inside of the connector
with compressed air to remove loose dirt. Carefully remove any additional dirt with a
nonmetallic stick and clean cloth soaked in alcohol. To minimize wear, do not rotate the
center conductor as you tighten the coupling nut. Rotation of the center conductor leads
to wear on both the male and female pins causing increased forces on the coupler.
Additional tips on the proper care of RF connectors can be found in the source listed
below:

Extending the Life of Type N Connectors, Application Note, Wide Band Engineering,
Phoenix, AZ, 1998.
http://www.wbecoinc.com/AN107.pdf

Does the t-meter require calibration?
The t-meter is a verification and trouble shooting tool used in the field to verify the
operation of wireless system components. As such, a periodic calibration cycle is not
specified. Should a calibrated measurement be required, contact Praxsym to discuss
whether a calibration process is available for the particular measurement you will be
making.


How much does the t-meter cost, and where do I purchase one?
The t-meter is available from several wireless product distributors. Visit
www.vswrmeter.com
, or contact Praxsym for a complete list of distributors.

How long is the warranty on the t-meter?
Praxsym covers the t-meter for a period of one year. The warranty covers
manufacturer defects.

How do I get customer support on the t-meter?
The distributor you purchased the meter from will be the first point of contact for
questions concerning the operation of the meter. If the issue warrants additional support
it will be escalated appropriately to Praxsym.

Where do I find the specifications for the t-meter?
Specs can be found at
www.vswrmeter.com
.

Where do I find a manual for the t-meter?
An electronic copy of the manual can be found at
www.vswrmeter.com
.

How do I become a registered distributor for the t-meter?
For more information about becoming a registered distributor, contact Praxsym.