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January 2006

gizmo to measure LNB voltageIt's hard to know what is going on inside an LNB or downlead just by looking at it. Although Doris Stokes may be able to tell you that you're receiving Sky News just by holding the cable, the rest of us need some instrumentation.
To check the signal strength you can use a meter at the dish and a spectrum analyser will show you what's on, but if you want to know if the LNB is receiving power, what voltage is supplied for polarity selection, or whether the 22kHz tone is on or off, many instruments - particularly older ones - won't help.
SFM's Little Gizmo does all this in the size of a box of matches with two F-connectors (a push-on female and a screw-on male) and a miniature set of traffic lights on one end made up of three LEDs.
The unit is connected in-line with the LNB cable - it doesn't matter which way around the connectors are used (it depends whether you are testing the system at the dish end or the receiver end of the cable) - and the unit instantly displays the status of the downlead power signals with the LEDs.
The red LED lights when power is available. The yellow LED shows the polarity - on for horizontal and off for vertical. The green LED comes on when a 22kHz tone is present on the cable.
The SFM meter is ideal for verifying that all is well when replacing an LNB. Also it's an excellent tool for checking that your hard-to-program digital receiver or your motley collection of switches and tone inserters are behaving as expected.
The Little Gizmo is very simple and relatively cheap too. So don't go up a ladder without one!

Reviewed by Robert Wesley in "What Satellite TV" magazine April 1998.

Little Gizmo Price: £19
No longer available


Low cost method

If you simply want to measure the LNB voltage on one occasion you might not want to spend much money!

You can measure the voltage coming out of the tuner "F" connector quite simply by disconnecting the LNB cable and pushing a bare wire into the tuner connector. Now set your voltmeter to 20 volts dc, put the red probe on the bare wire and the black probe on the screw thread of the connector. With vertical polarisation selected you the meter should indicate from 13 to 14 volts. With horizontal selected, the meter should indicate 17 to 18 volts. However, this measurement is not necessarily correct because the LNB is not connected!

coaxial cable with inductor fittedTo measure the voltage "under load" you might be able to remove the side of the tuner module and touch the red probe directly onto the "F" connector centre pin. This is often difficult or dangerous (mains voltages nearby!) so a better method is to construct a simple adaptor like the one in this picture.

Solder two panel-mount "F" connectors together. The centre pins should be soldered together and the screw threads should be connected by fastening coax cable shielding braid between them. Add solder to make the braid rigid. You can now connect the red probe to the centre connection. If you want to see pictures while you are doing this, you need to solder a tiny "choke" or inductor to the centre pins and conect the meter to this instead. The inductor prevents the meter from affecting the picture.

However, there will still be picture degradation with this adaptor connected in line because it is causing an "impedance mismatch" which will affect some channels more than others.

A kit comprising two panel-mount "F" connectors, an inductor, two screw-on "F" plugs and a short length of cable is no longer available from SatCure. Don't order "LNB F-kit" price £2.00.

How to measure LNB voltage from a satellite receiver Return to index page