Building a high power (100 watts) attenuator - Page 1
Forum section: electronics / projects

author: sundance
author_rank: Regular Contributor
timestamp: April 06, 2019, 05:08:30 pm
content: Hi everyone,I'm trying to build a dummy load using one of those 50 ohms/150 watts used flange mount termination resistors you'll get on the internet. The plan is to mount this resistor (G150N50W4B) on a CPU cooler (w/fan) capable to dispose some 100 watts. The resistor will sit in a small metal enclosure along with a BNC connector.Now I thought it would be very handy to have an additional output that is 30 dB below the input for monitoring / measurement purposes. I've calculated a PI attenuator and selected resistors from the E96 series. Since I can't use 52 ohms shut resistors, I'll have to be happy with a VSWR of 1.06. I'd like to use the dummy load up to 450 MHz, maybe 1.2 GHz if possible.tldr; -> and here's the challenge:Is it possible to build a 30 dB attenuator with only one PI network?Usually I'd split this into 3 x 10 dB to minimize the stray capacities and crosstalk over the series resistors. Unfortunately I can't build a 10 dB attenuator with a 50 ohms input shunt unless I'd tolerate a VSWR of 1.6.So how could a 30 dB att mechanically being built? Maybe it would help to to split the series resistor to 3 x 250 ohms and intruduce shielding chambers for each resistor?Any other ideas would greatly be appreciated...

author: German_EE
author_rank: Super Contributor
timestamp: April 07, 2019, 04:51:16 pm
content: The first thing that I would do is make the attenuator 40 dB, this means that 51 ohm resistors are required at either end and the VSWR is reduced. At 40 dB you will need a 2.5K ohm resistor for the middle leg, fabricate this using 5 x 500 ohm non-inductive resistors with some sort of screening between the sections and everything should be OK.Feed your 100W into this and the result will be 10dBm or 0.01 watts which means your test equipment won't even break into a sweat.There is however an alternative approach here using a power tap and the following link may be of interest:https://www.qsl.net/sz1a/download/build%20an%20rf%20power%20meter.pdf

author: bd139
author_rank: Super Contributor
timestamp: April 07, 2019, 05:12:01 pm
content: You can buy flange mount attenuators surplus on aliexpress. 4x 10dB steps should give a reasonable power distribution.

author: sundance
author_rank: Regular Contributor
timestamp: April 08, 2019, 08:28:22 am
content: @BD139 & German_EE:Thanks a lot for the valuable input.To be honest, the original plan was to have a 40 db attenuator but I thought this was not possible to achieve. But the design with seperated sections sounds feasible.And I didn't know these flange attenuators existed! Definitely trying to get some of those...

author: bd139
author_rank: Super Contributor
timestamp: April 08, 2019, 08:32:23 am
content: Typical example:https://www.aliexpress.com/item/ATTENUATOR-FLANGE-150W-20dB-50-ohms-RF-Termination-Microwave-Resistor-High-Power-Dummy-Load-150WATT-50OHM/32883203445.html

author: sundance
author_rank: Regular Contributor
timestamp: April 12, 2019, 10:18:02 am
content: And another example from the (German) magazine "Funkamateur":https://www.box73.de/file_dl/bausaetze/BX-140.pdf

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2019, 05:21:30 pm
content: Why not use more than one section starting with low attenuation so the power can be distributed across multiple devices?