A cheap 23cms signal strength meter / portable sniffer
These came round at Lidl last year for as I remember £7 and they were back again a year later for even less. It's designed for aligning satellite dishes, giving a means of showing the signal strength at the dish end whilst the satellite decoder's on screen signal quality display is out of view.

Model: SF-95LDB
Frequency: 950-2150MHz

Now this thing has a five LED bar graph to indicate signal strength and takes it's power as you may suspect from the satellite receiver it is plugged into, and passes it on to the LNB.

I wanted to know if this cheap little device could be used for sniffing 23cms signals. The first problem is the fact that it feeds 13-18v out of the 'F' socket which would normally connect to the LNB. If I were to connect this to my yagi, it would probably blow a fuse in the satellite receiver as my sandpiper antennas are a DC short. I can't even plug it into my signal generator without some sort of DC blocking.

What's inside?
Actually quite a lot given the unit's low cost. On the double layered board are three chips, one tranny (to drive the beeper), a couple of pots, a -6bB attenuator switch, a bleeper, a row of LEDs and a handful of discretes. The chips are ...

IC1 - 4558D operational amplifier
IC2 - KA2284 5 Dot LED level meter driver. ID = 15 mA in 9-pin SIP package.
IC5 - SMD 6 pin device, way too small to ID but an RF amp which drives the detector circuit. This is normally hidden under a small piece of shielding.

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Component side showing the five rectangular LEDs
Sure enough, a DC path is evident from one 'F' socket to the other. From the receiver socket, C20 straps to ground, then DC passes through L1, C19 again straps to ground, then through L2 which is a ferrite bead, C18 again to ground, L3 (another ferrite) C17 to ground, L4 then goes to the LNB socket. The signal path to the metering circuit is directly in at the LNB socket then through C21 to the attenuator switch. The signal from LNB to satellite decoder follows the DC path previously described but of course in the opposite direction.

One power feed via R1 feeds Z2 as a zener shunt regulator giving 5v for IC5 the RF amp, and another runs current from between L3 and L4 via R3 through Z1 to give a 12v rail. Also R2 feeds the common anodes of the signal meter's LED array.

I cut a track and put a jumper in between L3 and L4 so that DC was interrupted to the LNB socket but the signal path was still intact to the metering circuitry as well as the existing power rails remaining in place.
Instead of cutting a track, L4 could simply have been removed but this of course closes off the possibility of using the sat finder for its original purpose or indeed any other application where DC is needed at the masthead.

A capacitor could be added accross the cut in the track so that the signal path is restored and the device could be used as an in line measuring device.

IC5 has a small inductor L6 associated with it which could perhaps be tweaked to take the frequency response high enough to cover 2.4GHz transmitters.

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Note the jumper I've added top right by the LNB socket  

On my signal generator the unit registers signals above 16mV. Not spectacular but we'll see how it works in practice with an antenna once I've buttoned it back up.

Tony G7TXU 21st October 2005