How to measure LNBF Voltage "loaded"

[FaT Air]

We'll use OHM's LAW:
E(voltage)= I(current) x R(resistance)
P(Power)= I(current) x E (voltage)

The best way to measure the voltage being supplied to the LNBF/LNB is with a load. As can be seen in the following, only supply voltage can be measured
with the load (LNBF) disconnected.

12v---^^^^1k^^^---(12v)----^^10Mohm^^--GND effectivly an Open circuit
12v ---^^^1k^^^---(6v)--^^1k^^^--GND A "Loaded" circuit

The voltmeter presents too high a resistance to effectivlely 'load' the circuit. The supply will see many megaohms as a load.

WHY?

Your LNBF normally draws 120ma (.12 amp) @ 18v.
It's resistance/impedance equivalent is 18/.12= 150 ohms
Say your coax has corrosion in a connector that represents 10 ohms.
With the 10 ohms in series with the 150 ohms of the LNBF the total circuit resistance is 160 ohms. With 18v supplied, and 160 ohms the circuit current is now only 18/160= .1125 amp (.1125ma).
The LNBF voltage is now what?
150 ohms X .1125 = 16.875 Volts.
That may not be high enough to switch your LNBF to horizontal polarity.

So how do we easily 'load' our LNBF system to test?
Parts Required:
Top:
1) 150 ohm - 5 watt power resistor
Bottom left to right
1) F-81 barrel connector
1) electrical 'ring connector'
1) F connector nut

Fasten the ring terminal onto the the F-81 with the nut:

Insert one lead of the resistor into the F-81. Other lead goes to the ring terminal.

Testing: Voltmeter positive lead to F-81 side of resistor, Negative lead to the 'ring' lead.

NOTE: one could use a lower value resistor.
100 ohm would draw 180ma (PLL LNBF equivalent?) in a 'good' system @ 18v.

Power rating of the resistor is determined by increasing the system voltage 10%: 18x1.1= 19.8 round to 20.
System current (@ design resistance 150 ohm) also increased by 10%
18/150=.12 x 1.1 = .132
E x I = P: 20 x .132 = 2.64watts A 5 watt rated resistor will stay relatively cool.
and standard values are 1 and 5, so 5 it is.

Happy troubleshooting!

 

[migold]

Accurate and well written instruction!!

 

[KE4EST]

Very good write up there FaT!!

 

[FaT Air]

Tried to adhere to the KISS principle, to make this tutorial simple. Hope I succeeded.
But the question: what's your LNBF voltage(?) has been raised quite a bit over the last couple months.

 

[jorgek]

Why not just measure the real world voltage at the LNBF rather than into a "dummy load"?

 

[Cham]

I have a piece of RG6 somewhere with the shielding and dielectric stripped away to expose the center conductor, and a wire soldered to the seperated shielding to attach test leads to. It can be connected in series with the LNBF/LNB so you have the actual working voltages in the circuit. I'd attach a picture but I can't find it.. hope it didn't get thrown out... likely put it in the wrong tool box. Good diagnostic tool. Can even use an automotive test light (24 volt capable one) instead of a VOM if you haven't got one around. You just don't want to short out the center and shield while testing!

 

[Titanium]

I use the same type of coax jumpers with stripped back braid and open center conductor. Great to test with the actual hardware.

 

[Magic Static]

I have an adapter with banana plugs for the fluke meter that taps you into a coax. Got that back in the 80's when I was digging holes for C-Band dishes. One of the sat installers left it behind. I never knew what to do with it until many years later.

 

[FaT Air]

Why not just measure the real world voltage at the LNBF rather than into a "dummy load"?

easier said than done. But possible if you make an adapter:
2 chassis F connectors mounted on an aluminium box. Bare copper wire between the two. I have a couple of them.

 

[Titanium]

Sometime we make things too difficult.

 

[Babadem]

Never knew these methods and options were available.

 

[jorgek]

All excellent testing methods.
I just modified a dollar store TV splitter. Connectors 1 and 3 for the voltage test, note jumper between 1 and 3. Apply test leads to jumper and body of splitter.
For the MA test, connectors 2 and 3 ( note no jumper) and apply test leads to the centers pins on connectors 2 and 3.

 

[polgyver]

I have a piece of RG6 somewhere

I know very well what you mean...
Took quite a while to find some of temporary hook-ups.
Have to modify the metal splitter according to Jorgek's idea - for measuring current (mA).
When urgently needed, it was faster to make another one than search for old splitter.
Tail with marrette was used for current measurement

 

[Cham]

Just curious, when you are testing for current using your VOM, do you still get a readable signal on your satellite receiver? The tester would insert a rather inconsistant impedence into the circuit, especially at L-band frequencies. Shouldn't hurt anything though other than lowering the IF signal and Q readings the receiver sees; the measured amperage should be close enough to find a problem in the circuit if there is one.. I've never bothered to check for current (I) in these curcuits, the receivers are only capable of a few hundred mA so any short would show up as low voltage (E), but the advantage of this test would show if there was an open where a voltage only test really does not indicate a problem.

 

[polgyver]

The tester would insert a rather inconsistant impedence into the circuit

I was curious, too, so, for good measure, added in series with miliAmpmeter also 10 Ohm wire-wound resistor, and an unknown inductive coil (salvaged from speaker enclosure).
This system seems very tolerant to any added elements. See photos below.

 

[lost_mesa]

Interesting polgyver, I never would have expected your results.

 

[Cham]

Cool!