RF Power Meter / SWR Meter Equivalent Circuit

[Mzzed]

I'm starting to get into RF electronics, and an upcoming project involves the use of a power meter / SWR meter. How do we model a power meter in terms of an equivalent circuit? i.e. is it just another impedance? or a transformer? or does it not effect the circuit at all, so we just don't include it in circuit simulations?

Thanks

 

[f95toli]

I am not quite sure I understand what you are asking, but from a circuit point of view it should just be a 50 ohm load.

Note that you can actually build a working power meter by measuring the increase in temperature of a 50 ohm termination as it warmed up by the incoming radiation. Hence, a good power meter should be a nearly ideal 50 ohm load (=a 50 ohm resistor).

 

[Averagesupernova]

A directional wattmeter or uncalibrated reflectometer uses directional couplers. Google should turn up some pretty good hits. There are several methods used to accomplish this. An ARRL handbook or antenna book would be a good start.

 

[berkeman]

I'm starting to get into RF electronics, and an upcoming project involves the use of a power meter / SWR meter. How do we model a power meter in terms of an equivalent circuit? i.e. is it just another impedance? or a transformer? or does it not effect the circuit at all, so we just don't include it in circuit simulations?

What is the nature of your transmissions? If they are RF packets and not CW, then a simple SWR meter will not be of use.

 

[Mzzed]

Thanks for all the answers. I'm planning on using a diamond SX100 power meter for measuring the power and SWR of a simple 13.56MHz sine wave, no data transfer or anything involved. There don't seem to be any datasheets on that power meter either. I more meant what is the equivalent circuit i should be using in my simulations - e.g. i know it should be 50 ohms impedance but does that look like a capacitor and inductor in parallel? or just a large capacitor in series? what is the equivalent circuit I should be using to simulate the power meter?

 

[f95toli]

It is still not quite clear what you are planning to do.

A "simple" power meter which measures the forward propagating power in a transmission line should ideally just be a 50 ohm load at all frequencies (i.e. purely resistive)

An SWR meter uses a directional coupler to measure the forward and reflected power using said power meter. Hence, the "power meter part" would once again just be a 50 ohm load. whereas the coupler would be just that.

Also, there is no such thing as the SWR of a sine wave. SWR is a property of a component; it is essentially just another way to describe the impedance mismatch at a given frequency.

If your simulation software does not include a coupler you would need to create circuit model of one. You should be able to find a few ways of doing this if you google.

 

[Averagesupernova]

So you have a transmission line delivering power to a load with the Diamond SX100 in-line. Ideally, ignore it. Yes, it will have some insertion loss, what are the specs? If you want to get technical you will have to check your network with a network analyzer. I would say what the manufacturer tells you is the best you have to go on.

 

[Tom.G]

The SWR/Power meters I've used are just a directional coupler constructed with a transmission line.

See for example: http://kambing.ui.ac.id/onnopurbo/orari-diklat/teknik/arrl/using-equipment/5904024.pdf

 

[Mzzed]

Thanks all, I think the correct way of thinking is to just to assume 1W will be lost due to the 0.1dB insertion lost, I was definitely overthinking it.

Cheers

 

[davenn]

I think the correct way of thinking is to just to assume 1W will be lost due to the 0.1dB insertion lost,

The amount lost depends on the input power, which you didn't state

you also still haven't answered the question about what you are trying to achieve with this 13.56 MHz signal

 

[Mzzed]

yeah I just checked the maximum insertion loss of the power meter and used that. Also it's for an inductively coupled plasma source :)

 

[sophiecentaur]

The SWR/Power meters I've used are just a directional coupler constructed with a transmission line.

See for example: http://kambing.ui.ac.id/onnopurbo/orari-diklat/teknik/arrl/using-equipment/5904024.pdf

There is an alternative approach for SWR meters, based on 'lumped components'. This link talks about them and has a wideband design. If it's for a limited frequency range then it could be tweaked to get very good directionality.