Aerial Benchmark Scheme
Preparation and Implementation
of Benchmarking
Many of the tests carried out on aerials at Blake UK have been influenced by
the introduction of a new Aerial Benchmark Scheme in 2003. Four new standards
were created for UHF TV aerials. These new standards were created to help make
aerial choice easier, by ensuring that aerials used for Digital reception meet
the minimum requirements for a given standard, this is with respect to mechanical
construction and electrical performance. Aerials that achieve a Benchmark pass
will have packaging labels that display the level of the pass, be it Standard
1, Standard 2 etc. Typical tests carried out on aerials by Blake UK include:
Forward Gain
Forward gain is measured in dBd, which is gain relative to that of a simple dipole.
It has been the practice by some to quote gain as dB or dBi, which is gain relative
to that of a mathematical device called an isotropic source; this produces an
apparent gain that is 2.2dB higher than it would be if measured in dBd.
Return Loss
Return loss is a measure of the ability of the aerial to accept and radiate any
RF signal sent to it. The loss of returned signal to the source is measured and
presented as return loss in decibels. The return loss measure specifies the degree
of convergence of aerial terminal impedance to 75ohms, which is the impedance
of the cable to which it will be connected.
Polar Plots
Polar plots are a measure of an aerials ability to receive signal in a given direction.
Generally most aerials require directivity and are measured over 360deg. The best
response for a terrestrial digital receive aerial is a narrow acceptance angle/main
lobe, and any other rear/side lobes not exceeding -16dB.
Cross Polar Rejection
Cross-polar plots show an aerials ability to reject opposite polarity signals,
it defines the ability of an aerial to ignore signals likely to cause co-channel
interference from distant transmitters using the same channels but radiating signals
rotated 90deg to the wanted signals.
Feeder Pickup
Feeder pickup measure specifies the degree to which an aerial can reject interfering
signals conveyed to the aerial along the outer of the coaxial cable. Many spurious
signals created around the home such as those from light switches and thermostats
can find its way on to the coax cable and have an effect on the reception.
All tests carried out are largely based on the measurement methods specified
in BS 5640:Part 2: 1978, IEC 60597-2: 1977 "Aerials for the reception of sound
and television broadcasting in the frequency range 30 MHz to 1 GHz, methods of
measurement of electrical performance parameters" and also CCIR REC 419.
The following is an example of the different levels of gain that a wide band
aerial must achieve for any of the 4 standards.
