# How to choose RF attenuators

#### dukeman

Joined Feb 7, 2009
14
Hi,
I need to buy a RF attenuator to troubleshoot my amp. The amp output frequency is up to 300 MHz (pulse). The amp specs are 300 Watt pulse power with 30 Watt linear CW max. I need a RF attenuator in between the amp and my scope so that I won't fry my scope with maximum pulse power from the amp. The question I have is how to choose an attenuator with appropriate db attenuation and power rating in this case. Or in general, what are the rules of thumb on these two parameters?

Dukeman

#### Lestraveled

Joined May 19, 2014
1,946
Voltage is equal to the Square root of (watts times resistance), so 300 watts would produce 122.5 volts RMS at 50 ohms. A 40 db attenuator would drop that to .03 watts or 1.2 volts (RMS) at 50 ohms. This would be a good level for a scope. I suggest that you use two attenuators in series, a 10 db, 30 watt and, a 30 db, .5 watt. It might be cheaper than one 40 db 30W attenuator.

I suggest that you consider using a 30 watt dummy load and a coupler. This will probably be the cheapest route.

Edit: Oops, you said 30 watts average. Changing a few numbers.

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#### dukeman

Joined Feb 7, 2009
14
Hi,
Thanks for the reply. I kind of get the idea of how to choose based on your math. There is one question that I am still kind of confused, and that is the power rating of a RF attenuator. Out there, they sell a RF attenuator at a fixed db with different power ratings. My understanding is that power rating refers to the "average power" dissipation. Again, for 300 watts pulse power, would a power rating of it 1 Watt work for a 40dB attenuator?

#### Lestraveled

Joined May 19, 2014
1,946
....... Again, for 300 watts pulse power, would a power rating of it 1 Watt work for a 40dB attenuator?...............
No. A dummy load or an attenuator must be sized to dissipate the average power and withstand the peak power. If I read your text right, your transmitter can output 30 watts average and 300 watts peak. This implies a duty factor of .1 when in pulsed mode. Is this correct??

#### KL7AJ

Joined Nov 4, 2008
2,229
Hi,
I need to buy a RF attenuator to troubleshoot my amp. The amp output frequency is up to 300 MHz (pulse). The amp specs are 300 Watt pulse power with 30 Watt linear CW max. I need a RF attenuator in between the amp and my scope so that I won't fry my scope with maximum pulse power from the amp. The question I have is how to choose an attenuator with appropriate db attenuation and power rating in this case. Or in general, what are the rules of thumb on these two parameters?

Dukeman
If you want to spend a dollar or two, you can order precision attenuators for exactly the attenuation you need. These will be specified according to dB, which is POWER. Be sure to convert to voltage using E= sqrt (RP), the R being, presumably 50 ohms.

#### KL7AJ

Joined Nov 4, 2008
2,229
You can also order "pass through" terminators, which are basically dummy loads with an output voltage sampler.

#### SLK001

Joined Nov 29, 2011
1,549
If you want to spend a dollar or two, you can order precision attenuators for exactly the attenuation you need. These will be specified according to dB, which is POWER.
I think that you meant "dBm, which is POWER". A dB is just a ratio.

#### Lestraveled

Joined May 19, 2014
1,946
I think that you meant "dBm, which is POWER". A dB is just a ratio.
Correct.
db is a measure attenuation or gain, dbm is power relative to 1 milli-watt in db.

#### Lestraveled

Joined May 19, 2014
1,946
A more economic place to look for attenuators and loads in this frequency range is minicircuits.com

#### bertus

Joined Apr 5, 2008
22,276
Hello,

I have been using the attenuators from Radiall all the time.
They have a very good pulse power performance.
I have attached two PDF's, one with the cooled version and one of the basic version for the ordernumbers.

Bertus

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