User's Manual

Doc. No.
Rev.
Date
1.00
2011-12-08
Damm Cellular Systems A/S, Denmark
TETRAFLEX
®
V7.5 MANUAL - Antenna System for BS41x
TetraFlex® 7.5 Manual 4-7
Cables and connectors between antenna and BS41x shall be resistance to water ingress and
migration.
4.1.4 TX inter-modulation considerations
One of the most critical aspects about antenna systems for a multi-carrier base station is to
control the inter-modulation generated between the different transmitters in the BS.
Inter-modulation happens in many different places on different levels. One major place is the
TX output amplifier itself. When a signal from another TX reaches a TX output amplifier, all
typical transmitters will generate 3
rd
order IM products with a level typically 15dB below the
interfering signal. The BS41x Base Station is provided with double circulators in the TX
output to make sure, that IM3 products close in frequency will be at least 70dB below the
carrier.
At greater differences in frequency the inter-modulation products are further reduced by the
selectivity of the TX cavity combiner and the TX output filter. In the RX frequency band of the
BS spurious and 5
th
-order IM products measured at the TX antenna connector are verified to
be less that –106dBm. With the expected RX-TX antenna separation of 30 dB, any generated
product in the receiver input will be less than –136dBm, well below the RX sensitivity level.
This verification has been done with up to 16 simultaneous transmitters of 50W each.
For carrier output of 20W (+43dBm) the –106dBm gives a rejection of –149dB. To reach this
level, extreme care is needed to avoid inter-modulation at any point.
Some connector manufacturers have recently started to specify 2-tone 3
rd
-order IM for their
high quality connectors. These specifications typically is about –150dB. IM with more than 2
carriers are not specified. It is recommended only to use DIN 7/16mm connectors in the TX
antenna feeder section. N-connectors are an order of magnitude worse and should never be
used for more than 2 carriers.
Feeders shall be with solid screen and inner conductors to avoid any possibility for un-
linearity due to undefined connections between individual cores in the cable. High quality
RG214/U cable has been demonstrated to produce noise when bent mechanically in the lab,
simulating a windy condition on a mast.
The larger antenna vendors have in the last years established test set-ups to verify IM
properties of their cellular antennas for 900 and 1800MHz. For 400MHz this is almost non-
existent. It is recommended that if a tested antenna cannot be purchased, then ask the
vendor to confirm that all construction principles are identical to those used in verified
antennas in the 900 or 1800 MHz range.
One very critical thing to remember about inter-modulation is, that it is very difficult to predict
accurately. For antenna constructions two factors heavily degrade the performance. Over
years connectors etc. starts to corrode, degrading IM. Additionally, IM properties can also
degrade under windy conditions. Therefore a short verification of a new antenna set-up after
installation does not guarantee performance in the longer term.