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.

DC Input

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.

Drake 2880

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