Any specific consideration for the load resistor of a RF PA?

Hello! I am trying to make my own test PCB for my RF power amplifier operating at 433MHz. Since the RF PA will directly drive an antenna of 50 ohms in reality, I will use a 50-ohm resistor instead on the PCB as my people suggested. The output power delivered at the output of the PA is about 10 mW at most.

So here is my question. Do I need to use any special or specific resistor in the application? Someone told me that I need to use a power resistor of 50 ohms. I am not familiar with the power resistor. So does anyone have any idea about this? Thank you.

For a resistor to actually behave like a 50 ohm resistor at 433 MHz, it must be made in a special way. Take for instance a carbon film resistor, it will have a resistive coating on a ceramic tube, to get it to its proper value a spiral groove would have been cut in it. So at 433 MHz it looks more like an inductance then a resistor. A proper carbon block resistor is required, 47 Ohms is near enough. If you are doing precision work then a precision RF load is required. I would think its impossible to buy a load with a power rating as low as 10mW so you will have to buy a bigger one (1W?). One way of getting a good load is to connect two bits of coax to your output, one is 1/4 wave length, the other 1/2 wave length. adjust their lengths on a transmission test set at your operating frequency.
Frank

At this relative low frequency and low power, even a standard 0603 SMD resistor will do the job:

SMD / SMT Resistor :: Radio-Electronics.Com

To minimize the parasitic inductance of the resistor (and also to increase the dissipated power) you can place in parallel 3 x 150 ohms SMD 0603 resistors.

Could you please tell me why you need to increase the dissipated power?

Does it mean I can just use a regular resistor of 50 ohms since 433MHz is relatively low in RF?

Above what frequency roughly, I need to consider using a special resistor for the test?

What are the main factors I should consider as far as choosing the right resistor is concerned for the test? Precision of resistance? Parasitics?

Thanks.

---------- Post added at 01:05 ---------- Previous post was at 00:54 ----------

Thanks for you reply. Sounds like the precision of the resistor is an important factor to consider. Am I getting it right? I am trying to understand the main factors to consider for choosing the right resistor for the test.

For the power rating value, if there is no such a resistor with power rating as low as 10mW, I will just need to find a lowest one possible right?

About using two bits of coax connected to the output, I don't quite understand what it tries to accomplish. Decreasing signal reflection? It seems has to do with transmission line. If the traces on the PCB were already done in transmission lines, then how do the two bits of coax come into play?

Could you explain a little more?

Also, since 433MHz is relatively low in RF range, do I really need to use transmission line rather than discrete components?

Thank you very much.

No its not critical ! at the freq and power level you are using, a standard 1/4 W 47 Ohm carbon resistor will be just fine you could use a metal film 50 Ohm resistor just for that bit of extra resistance accuracy. Just use as short as possible leads on the standard wire lead resistors. Its really not all that critical.
As Vfone said you could use some surface mount types he suggested using 3 x 150 Ohm ones in parallel for better power dissapation, just to ensure those physically very small resistors are not dissapating too much power individually

Dave

Thank you Dave. Now it makes sense to me. I will use surface mount resistor instead of through hole ones.

Another 2 questions I have are about connecting my power amplifier to a spectrum analyzer or a network analyzer. Since the connector of the two analyzers are 50 ohms termination, do I still need to connect the resistor(representing an antenna) in parallel between the output of PA and the analyzers? Or the 50-ohm connector termination already represents the resistance value of the antenna in testing? Thank you.

ok you prob wont need that load resistor for going into the Spec An. Most Spec An's can take 50 - 100mW (15 - 20dBm) input. look by the input connector on the analyser it will have a max power input notice there. Depending on the make and model of the analyser it will be either 50 Ohm or 75 Ohm input so again assuming its a 50 Ohm input analyser, you again dont need a terminating resistor.
The input impedance should be noted beside the max power indication.

cheers
Dave

---------- Post added at 05:58 ---------- Previous post was at 05:49 ----------

once you are connected up to the spec an. you will be able to tune for max power out whilest keeping a clean signal, free of spurii's and harmonics :)

Dave

By the way, if I connect the output of the power amp to a mixed-signal oscilloscope, I still need the 50-ohm resister, right? Thanks.