SatFinder



		*A cheap 23cms signal strength meter*


		Model: SF-95LDB Freq: 950-2150MHz
		These came round at Lidl last year for as I remember £7 and they were back again a year later for even less, just four pounds. It's designed for aligning satellite dishes, giving a means of showing the signal strength at the dish end whilst the on screen signal quality display is out of view.



		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 OP amp IC2 - KA2284 5 segment 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 I guess


		*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 goes to ground, then through L1, C19 again 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 is in at the LNB socket then through C21 to the attenuator switch. Power to supply the unit is taken off between L3 and L4. I cut a track and put a jumper in between L3 and L4 so that the DC feed was interrupted to the LNB socket but the signal path was still intact to the metering circuit.

		One feed via R1 feeds Z2 as a zener shunt regulator giving 5v and another runs current via R3 through Z1 to give a 12v rail. Also R2 feeds the common anodes of the signal meters LED array.

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


		On the sig gen it now registers signals above 16mV. Not spectacular but we'll see how it works in practice once I've buttoned it back up.

A cheap 23cms signal strength meter

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

On the sig gen it now registers signals above 16mV. Not spectacular but we'll see how it works in practice once I've buttoned it back up.

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