A typical receive / transmitter system for ATV working.
|The block diagram
above shows a typical receive /transmitter system for ATV working.
|The above system Is
run at my qth, with home made antenna systems, each antenna has a gain
of about 16dB.The antennas are separated by only 600mm centres, with
feeder run of 4 meters of LDF4-50 to each
The pre-amp has approximately 35dB gain; the
receiver is an old analogue satellite receiver. The transmitter runs 12
watts output to the antenna. With the above system I am able to receive
my own signal from the local repeater GB3TM when I am in transmit mode.
To operate on GB3TM my receive frequency is 1316 MHz, my transmitter
operating on 1249 MHz. Without the band-pass filter in circuit the
transmitter completely blocked out the received signal from GB3TM, as
the pre-amp and front end of the receiver were overloaded
|The path from my qth to GB3TM is about
20miles, and is not a line of sight path. As can be seen from the
band-pass characteristics of the filter, when inserted into the receive
system it give an additional rejection of at least 50dB of the
transmitter signal (diagram)
||Users of GB3GW ATV repeater are also using the filter.
At the time of writing these notes, GB3GW is the only ATV repeater in
the UK with a transmit / receive split frequency of only 30 MHz.
Again the filter
allows users to see there own signal from the repeater when in transmit
mode, although the attenuation at 30 MHz is much reduced, it adds valuable
extra attenuation about 30dB.
If the filter is
inserted into the transmitter antenna feeder system. The filter is tuned to the
transmitter frequency; this will give attenuation to rf outside its pass-band
frequency. The result of this is that the rf passed to the antenna from the
transmitter will have any spurii attenuated; also the side-band noise generated
by the transmitter will be much reduced. The attenuation of the side-band noise
was essential on GB3GW repeater, due to the receive frequency being only 30MHz
low of the transmitter.
GB3GW and GB3TM are two ATV repeaters in
North West Wales,
maintained and administered by The Arfon Repeater Group. (www.arfon.info
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