The hardware of the PI6ANH repeater
Mainly the antenna and the repeater-box itself belongs to
the hardware of the system. As any other usual repeater the ATV repeater
consists of a power pack, a receiver, a transmitter and the steering system.
Compared to a phone-repeater there are a couple of main differences in
the modules itself. For example identification is necessary by law in form
of a testpicture and not only by CW-tones. Also, handling video compared
with audio only (as with phone repeater) is quite different. Especially
the bandwith of video and HF plays a much more important role.-
The repeater main-unit
The various modules are build into a waterproof box with two doors, which
makes maintenance very easy. It took about two years until we have the
hardware as it is.
Now lets take a closer look on the different modules in the opened repeater-box:
The PI6ANH repeater block
Left door from top to bottom:
The blower (not too good to be seen) on the top pushes the warm air out,
that gets pressed in by the lower blower. The speed of the two blowers
is regulated by a temperature sensor. So the temperature keeps in narrow
range during the seasons, which is very important for stable operation.
The first row of boards (under the blower) is meant for 10 GHz. For the
10.4 GHz input of the repeater, we use an modified TV-Sat LNC (Low Noise
Converter), which is assembled directly at a 10 GHz slot-antenna.
On the board itself a simple satellite tuner block (by Sharp / 27 MHz
bandwith) with FM-Demodulation is mounted. The outcoming signal (baseband)
is led to an FM-ATV
Baseband-Processor, which finally puts out clear Audio and Video. The
usually free-running satellite tuner gets a PLL
(Phase Locked Loop), which can easily tuned to any frequency by dipswitches.
A synch pulse
detector (on the very left) watches the band permanently for video
and switches the transmitter (with priority related to 23cm) on, if a 15.625
kHz line frequency signal is recognized.
The middle row of boards consists of the blower-speed temperature-regulation
and the color testpicture card. A picture of 128x64 dots is burnt into
an EPROM,- meanwhile a new test-picture board is installed with 8 different
easy programmable pictures, which get slowly scanned all the time. See
some of these pictures in "General
technical information". The hard- and software will also soon be introduced...
The task of the board-row below to the bottom is for the sysop's (system-operator)
remote-control and for general time steering. There is a 8-number DTMF
decoder to shut the repeater down (signals are delivered by a crystal 2m
handheld). With a 2450 Hz sinus carrier, the repeater can be set into a
10 minute test-picture mode. Additionally every full hour the repeater
switches on for 10min automaticlly to give people without a HAM-licence
a chance to adjust their receivers.
Right door from top to bottom:
Easy to recognize is the power block module, which is designed for heavy
duty. It is certainly secured against short-circuits and switches
off when voltage rizes over 14V immediately.
The next row shows the 1.2 GHz receiving unit with a high quality TV-satellite
(tuner by Sharp with 16 MHz bandwith) with optimized threshold. (A 40dB
Pre-Amplifier with more than 40dB gain and noise less than 1dB is mounted
directly under the 1.2 GHz antenna). Left of the tuner is again a PLL-board
to keep the receiver right on frequency. Annother ATV-Squelch
(synch line detector) is installed,- now for 23cm.
The line in the middle represents the FM-ATV
Baseband-Processor unit for the TV-satellite tuner and several switching-
and timer prints (for switching video between testpicture generator and
1.2 GHz input video etc...). So far we work with a 6 MHz audio subcarrier
(instead of 5.5 MHz) to reduce audio-effects in the video. We plan to add
annother audiocarrier, which transmits the audio from the 2m ATV-meeting
frequency on 144.750 MHz.
The most important block in the repeater is taking nearly 50% of the space
in that door. It's the whole 2.4 GHz transmitting unit with a baseband
processing unit for transmitting, the 2.4 GHz-oscillator locked with a
PLL (SP 5070), the driver (2x MSA1104), the final two-staged 5W amplifier
(MGF904 and MGF905), the coax relais, some protection circuits and a supervision
for the locked PLL. A closer look on this is in preperation.
The antenna system
The antenna system is about to change at this very moment. The for the
time being used antenna for reception on 1.2 GHz is the well-known
Alford-Slot antenna. The horizontal polarized antenna has a gain of about
5 dBd,- the bandwith and the gain over the entire band from 1240 MHz to
1300 MHz (this is the allowed frequency range for 23cm in Germany and the
Netherlands) is quite good.
For some detailed information on that antenna, visit the Brookdale
ATV Repeater Home Page (WB3QCD Alford-Slot Antenna for 430-440 MHz).
2.4 GHz FM-ATV signals on long distance makes it necessary to fight for
any dB, which might be available anywhere in the system.
The easiest way is still a good high-gain antenna (semiconductors with
more than 5W HF on 2.4 GHz are still too expensive for the average HAM).
The "11-Element-Slot" - Antenna (very similar to the DC0BV slot antenna)
had been recalculated to the repeaters frequency and to a 180 degree (halfsided)
radiation pattern. So far, we only could measure (with commercial help)
the gain in 0 degree direction and the SWR over the whole band.-
The results were very satisfying for us: 13 dBd gain with a (-3 dBd)
bandwith of 40 MHz and a return-loss of more than 20 dB (!). The raw aluminium
block has the sizes of 80x40x3 cm and is standard and should therefore
be available for only a little money. A sketch with all lenghts and impedance
transformations is in work. (Has anyone else experience with such antenna
Compared to the dipol groups (ca. 7dBd gain) we used before, it is
an additional gain of 6dB (= 4x times HF-Power).
On the picture: DL4EBJ (Johannes) with 11-Element-Slot Antenna.
One easy way to receive the repeaters 13cm-signals is described in "QRV
on 13cm FM-ATV" (still in German only).
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J. Köring, Bahnstraße 3B, 47551 Bedburg-Hau,
Copyright 1996 by Johannes
Köring (DL4EBJ). All Rights Reserved.