# How do you measure TX and RX power of a comm device?

Discussion in 'General Electronics Chat' started by EngIntoHW, Sep 25, 2010.

1. ### EngIntoHW Thread Starter Member

Apr 24, 2010
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I know that you should measure it in the antenna section, I just don't know where exactly and how it's done.

Thank you.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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In Tx mode to measure the antenna input power - which is related to the Tx power - one can interpose a directional coupler between the transmitter output stage and the antenna input. The coupler sensing point can then be connected to a calibrated RF power meter to monitor the antenna input power.

See this on couplers ..

http://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

To measure received power is rather more difficult when the received signal is of very low power level.

Radio amateurs often make use of an 'S' meter to monitor the received signal strength.

http://en.wikipedia.org/wiki/S_meter

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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For that u need dummy loads and RF power & SWR meters

4. ### EngIntoHW Thread Starter Member

Apr 24, 2010
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Thank you t_n_k!

5. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
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Hi,

What do you mean by dummy loads?

6. ### beenthere Retired Moderator

Apr 20, 2004
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Use Google. Basically you substitute a resistor for the antenna.

7. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
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Oh I see,

The resistor's resistance should be the equal the antenna's resistance as the transmission line see it?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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The problem with using a dummy load is that you don't know how much power in Tx mode will go to the actual antenna being used. The assumption is that the antenna models exactly the same as the dummy load - which is probably not the case in practice.

While the directional coupler approach won't be exact either it probably gives a better measurement of the antenna input power.

9. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
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Yes I see, as antennas stated to be 50-ohms, aren't exactly 50 ohms rated.

The directional coupler is connected in parallel with the transmission line, right?

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Not in parallel.

The input side of the directional coupler is connected to the transmitter output and the output side of the direction coupler is connected to the antenna. The sensing (coupled) point or output connects to a power measuring device.

The sensing (coupled) point output power is usually a fraction of the main transmission path through the coupler. The power ratio between the main path and the coupled sensing path is known and usually specified as a calibrated dB value.

11. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
0
Hi t_n_k.
Thanks again.

If the input of the coupler is connected to the input of the transmission line (which is the transmitter's output) and the output of of the coupler is connected to the output of the transmission line (input of antenna), how come they aren't connected in parallel?

12. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Depends what you mean by "in parallel".

The main power transmission path through the directional coupler is continuous between the input and output. So the main power passes along a single continuous path.

Some of the input energy is tapped off at the coupled point(s) - whether this be a fraction of the forwards (incident) or backwards (reflected) energy. This is electromagnetically coupled from the main transmission path. See Fig 1 "Directional Coupler" at the Wikipedia link.

I guess the coupled path looks something like a parallel connection - indeed it is diverting some energy from the main path to enable the coupled power to be monitored (in this proposed scenario) by a RF power measuring device.

Suppose this is a 30dB coupler. If the input power from the transmitter is 100W then the coupled power (under correctly matched conditions at all points on the coupler) would be 100mW. Meaning that - in the absence of transmission losses in the coupler itself - of the 100W of input power 99.9W of power passes to the load side. This diversion of power from the main transmission path would be taken into account in the calculation of the case of antenna input power measurement. From this perspective you might consider this low power path to be "in parallel" with the high power path - albeit with the important qualification that it is not a direct galvanic connection. Low frequency HF couplers can be made using discrete components such as ferrite cored transformers as the coupling elements.

Last edited: Sep 26, 2010
13. ### gootee Senior Member

Apr 24, 2007
447
50
No. The output of the coupler is connected to the input of the transmission line, not its output.

The coupler is inserted in series with the line, between the transmitter output and the transmission line.

Directional couplers are usually available as a small box, with a coaxial connector on each end, for the "through" connections. The coupled "pick off" port(s) is(are) usually on the side(s) of the box. You just insert the coupler in series with the signal path and connect your measurement equipment to the pick-off port. (Note: Make absolutely sure that the power you expect from the pick-off port is lower than the maximum allowed at the input of your equipment! Otherwise, insert an RF attenuator!)

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But what are you really trying to measure? Do you want the power that is actually radiated by the antenna. i.e. the "effective radiated power" (ERP)? Or do you want the power that is presented to the antenna's input (as was assumed)?

And what about the received power you want to measure? Are you wanting to measure the RSL (received signal level) at the receive antenna's location? Or are you wanting to know the power received inside the receiver, just after its antenna?

Last edited: Sep 26, 2010