It is recommended that a diode is inserted in the feed line to avoid
reverse voltage and that a fuse is also included to prevent burn-out of the
choke should the coax be inadvertently shorted either through mishandling or a
mechanical fault.Powering
Remote Units
Feeding the Mast Head Units.
Most
popular ex-TV 2.4GHz down converters are powered via the signal coax. This
requires the injection of the supply power into the same coax that feeds the
converted signal down to the receiver. Similarly, some of the popular low noise
preamplifiers can be powered via their output terminal.
Some users modify their units for a separate power feed usually involving drilling extra holes and modifying the internal circuitry. Such modifications can lead to problems of weather sealing, signal leakage and often require great care in drilling the case. There are easy fixes which do not entail intricate surgery and leave the units intact.
First
requirement is a power feed to bring the volts up the feed coax. This circuit is
simple using readily available parts. Power inserters may be purchased from
several sources or home-constructed. An easy answer is to purchase a two-way TV
splitter and modify the internal circuitry. Cheap splitters use resistors to
achieve impedance matching, these are removed and replaced by the circuit below.
This makes a neat yet economical and easy power inserter.
It is recommended that a diode is inserted in the feed line to avoid
reverse voltage and that a fuse is also included to prevent burn-out of the
choke should the coax be inadvertently shorted either through mishandling or a
mechanical fault.
Wiring is non-critical provided normal VHF techniques using short leads is followed.
C1 - .01uF C2 - .001uF L1 – 5.6uH
For powering masthead low noise preamplifiers, a similar circuit, with C2 omitted, mounted in the feed cable (on the output side of the down converter) can be used to extract power to feed around the unit to the pre-amplifier. In this case care must be taken to ensure lead lengths are very short (use SMDs if possible) around C1. Of course the DC Input now becomes DC Output. Use good shielding to avoid leakage or stray pick-up especially from any adjacent transmit antenna. Use of a converted cheap TV splitter may not be practical here unless it is in a completely enclosed metal container
or
units such as the DEM 13ULNA which can be fed via the output terminal, the
addition of a suitable choke to units like the Drake converter is a successful
alternative. For the popular California Amplifier a minor modification around
the Test Output terminal permits volts to be extracted to externally feed the
ULNA.
Note that the suggested Drake modification will not work for units such as the California Amplifier and the TranSystem AIDC 3733 unless a blocking capacitor (about 6pF) is inserted between the input terminal and the input circuitry.
California Amplifier.
The input terminal is directly connected to the input of the first stage.
Adding a blocking capacitor would require some microsurgery so the easy route
was taken using the Test terminal to extract the volts for powering the DEM
13ULNA.
The
ULNA has a coaxial bypass capacitor mounted on the side of the enclosure to
permit alternative external power use.
Modification involves adding a 560nH choke between the 10V regulator output and the Test terminal. The terminal is isolated by removing components linking the Test output to the normal IF output.
In
the unit depicted the capacitor and 560ohm resistor have been removed but in
second unit the resistor was de-soldered and swung away on one lead so the
circuit can easily be restored should this ever be required.
This Drake has been modified to the <2dB standard as promulgated. In
this modification the 10 volt line is brought up to the pad seen lower left of
centre in the accompanying picture.
A 220 nH choke has been inserted to couple this 10 volts to the input terminal. Apart from ensur-ing the choke is mounted as close as possible to the input terminal no additional bypassing or lead routing has been necessary.
Ensure the choke lies almost in line with the input lead to clear the internal shield.
This system has been tested both in use, and by bench measurements with no discernable per-formance deterioration.
Take
care! Once this modification is incorporated the input terminal is live and
shorting it to ground could damage the Drake or the power feed system.
NORSAT 2500 - Option 1.
The NorSat 2500 lends itself to either power feed
modification. The same cautionary message must be given as for the Drake above.
If the unit is modified to feed the voltage out the 2.4GHz input terminal,
shorting this input by a matching unit, antenna using an earthed line or a test
signal from a DC earthed unit could cause damage to the downconverter or power
source.
In the first option power is fed out the input terminal.
The input is connected directly to an earthed shorting line for input
protection. This line is terminated at a square earth pad.
Carefully cut the line at the pad and bridge the cut with
a 1000pF SMD capacitor. A 220nH choke has one lead cut very short, bent and
soldered to the input terminal side of the 1000pF.
Extend the other lead to reach the 10 volt terminal of
the voltage regulator (78N10) and insulate with 'spaghetti' or heat shrink. Take
care as to what might be connected to the input.
NORSAT 2500 - Option 2.
The second option copies the recommendation for the
California Amplifier.
The third terminal on the Norsat is labeled Test Out.
This function is not required in amateur use and the terminal is a convenient
option for extracting volts.
An attenuator pad samples the normal output (less the DC
in). Close to the terminal is a SMD capacitor. This can be carefully unsoldered
and moved out of the way (as shown in the photo above and accompanying).
A 560nH choke has one lead cut short and soldered to the
terminal. The other lead is insulated and taken to the 10V terminal of the 78N10
voltage regulator.
A refinement can be made by soldering a 1000pF bypass
capacitor from the terminal; side of the 560nH choke to the earth pad adjacent
to the moved SMD capacitor. Ensure leads are kept very short.
TranSystem AIDC 3733
No attempt has been made to modify the Transystem unit.
This presents its own problems since the only option, apart from modifying the
case, is to bring the power out the input terminal.
This modification calls for some very careful work at the
input terminal. The input is connected directly to a DC earthed filter.
Unmodified there is insufficient space to add a blocking capacitor and the
required decoupling choke.
Elsewhere this modification has been done. It requires
removing some of the insulation around the connector input lead to make room for
the required components. To do this the input filter must be temporarily
removed. An added complication is that on refitting this filter it must be
retuned thus making replacement a challenge.
Some parts are on order and when they arrive a trial
modification will be done and the results reported. While the modification is
not impossible, some extra skill may be needed to achieve a satisfactory result.
Comments, criticism, improvements welcome to ZL3QL at tcarrell@nzart.org.nz