9
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L A B E L I T A L Y S R L
- V i a S . A l l e n d e , 5 9 – 4 1 1 2 2 M O D E N A
T E L : 0 5 9 / 3 6 2 9 9 3 F A X : 0 5 9 / 3 7 6 0 5 6
P . I V A 0 2 5 7 8 7 5 0 3 6 2 C a p i t a l e S o c i a l e 3 2 . 5 0 0 E u r o
w e b - w w w . l a b e l i t a l y . c o m e - m a i l - i n f o @ l a b e l i t a l y . c o m
Some criteria for evaluating antennas
Typical cases of doubt:
• high reflected power,
• a high VSWR,
• a bad adaptation,
• It seems that the antenna absorbs abnormally power from the transmitter
To properly diagnose these apparent failures we have to find out if the cause is the same product
just purchased or poor installation of the same. The following F.A.Q. can help determine if the
product qualifies as defective by the manufacturer's warranty. Assuming that all connections are
clean, dry and tight; also that all the test equipments are in good condition and calibrated:
What VSWR you have measured?
Most of FM antennas have a match of 1.2:1 or 1.4:1 across a specified bandwidth. Performance at
a Voltage Standing Wave Ratio greater than 1.5:1 may be unsatisfactory. Performance at a Return
Loss of less than -15 dB may be unsatisfactory.
What test equipment did you use?
Check to see that it has been properly calibrated and that any connector adaptors are of good
quality. Poorly matched adaptors can invalidate the results.
Wattmeter/Power Meter?
These devices are inexpensive and therefore more common but can be inaccurate, particularly if
more than one RF carrier is present. Technicians who use them tell you how many Watts of power
is reflected back to the transmitter but often do not know the actual mismatch. The forward power
measurement is required to calculate the VSWR or Return Loss number. This can be necessary
because some transmitters have an output stage protection circuit which reduces power under
highly VSWR conditions.
Network Analyzer/Spectrum Analyzer with Tracking Generator?
These devices do not rely upon the site’s transmitter as a signal source. They can produce more
accurate and meaningful results but do not control the antenna to full power where arcing or
flashover would occur.
Did you perform the measurement directly at the antenna’s connector?
The technician may have chosen not to perform this test because it requires climbing the tower.
This procedure should be done to eliminate jumper cable effects. These cables could be defective
and cause the problem or absorbing the reflection which masks the problem.
What is your operational frequency?
Check to see if the antenna was ordered for the correct frequency. Several methods can be used
to determine an antenna’s frequency. If the technician has swept the response of the antenna he
will know the frequency of best match. That should be its designed frequency. The technician may
also measure the physical length so that we may compare it to a cut chart. This is a crude method.
If the antenna is of relatively new and the model number is known, the factory may still have the
production test data sheet which will identify its frequency by Serial Number.
Did you measure the antenna free and clear of metal objects?
A mounting too close to the tower can detune an antenna. The required spacing between the
antenna and any other metal object decreases as the operational frequency increases.