Inline RF Attenuators - Page 1
Forum section: electronics / rf-microwave

author: German_EE
author_rank: Super Contributor
timestamp: May 27, 2019, 06:22:50 pm
content: I want to build an inline 4dB attenuator on an existing 50 ohm line. The resistors are no problem but I have no ideas about how to shield the thing AND connect to the coax braid at each end. Old metal pen body? copper pipe? any ideas?Note that this is low level RF about 17 dBm so heat dissipation will not be an issue.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 27, 2019, 06:38:35 pm
content: Construction's going to depend a lot on frequency. In lower bands almost anything shielded can do a decent job, but as you go up the size of the whole thing and associated parasitics become more of an issue, then beyond that the internal cavity of the shielding starts becoming a significant concern too.That said, a good starting point could be selecting some connectors and just trying to build them into as small an enclosure as possible, and in lieu of a good enclosure, you could try using copper clad FR4 or similar as the sides. Something like a copper pipe could be fine if your circuit is entirely floating, but you'd want good RF grounding between the circuit ground and the chassis ground, which shouldn't be a problem with the connectors, but grounding from the circuit in between could be tricky if the whole thing slides into the pipe.Especially if not really wide bandwidth, you could just layout a small PCB, leave the solder mask off, and leave a footprint on either side for your choice of coax and just solder everything in place and shield as necessary.

author: awallin
author_rank: Frequent Contributor
timestamp: May 28, 2019, 04:07:47 am
content: Here's some design inspiration:https://www.sv1afn.com/rf-attenuators-pcb-panel.htmlseems flat up to 3 GHz without much shielding - if we believe the plot..I guess the inside of a Mini-Circ PAT component is just suitably chosen resistors (and maybe some compensation?).Only problem with mini-circ for hobbyists is they usually have qty 20+ minimum order..

author: German_EE
author_rank: Super Contributor
timestamp: May 28, 2019, 04:36:02 am
content: The minimum order issue is why I am forced to build rather than buy. Right now my best option seems to be a small metal box about 25mm x 25mm x 10mm that I picked up at an electronics store some time ago but I would rather have something that has a cylinder form factor.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 28, 2019, 05:36:05 am
content: Quote from: awallin on May 28, 2019, 04:07:47 amhere is too...https://www.ebay.com.my/itm/Telonic-Berkeley-4dB-Fixed-Attenuator-DC-6GHz-2W-Model-8207-4-SMA/401358668318?hash=item5d72d7421e:g:5oYAAOSwA2hZqE0hif we believe the listing, flat up to 6 GHz...

author: ejeffrey
author_rank: Super Contributor
timestamp: May 28, 2019, 04:23:24 pm
content: Quote from: awallin on May 28, 2019, 04:07:47 amThe reason integrated chip attenuators work better is that they have the ground terminals on both sides. This helps them reduce the package inductance which limits the performance of discrete resistors or requires careful simulation driven layout to compensate.Also: Minicircuits makes the YAT+ attenuator series. The PAT series is made by susumu and is available from digikey in unit quantities and considerably cheaper. They are only rated to 10 GHz (vs 18), but IIRC they actually have slightly better mid-GHz performance than the mini-circuits ones.

author: AF6LJ
author_rank: Unknown
timestamp: May 28, 2019, 09:25:30 pm
content: Too many unanswered questions.What frequency range?I've used home made attenuators that work well to 1.8 GHZ with little more than the right carbon or non inductive mental film resistors. as for the housing a peqace of copper tubing with a pair of BNCs on each end.(yes they looked decent on the network analyzer at work.) Your performance may vary depending on how carefully you are at construction.

author: hagster
author_rank: Frequent Contributor
timestamp: May 28, 2019, 10:04:16 pm
content: If the whole thing is small (vs wavelength) and the attenuation is low to medium then I dont think shielding is very important. Just a simple PCB with a T or Pi network will do. Small SMT resistors help reduce inductance. Power handling may be something you need to consider too.

author: medical-nerd
author_rank: Regular Contributor
timestamp: May 29, 2019, 03:25:10 pm
content: HiyaAnother option for something more robust is enclosing your pcb in one of the rtl-sdr v3 cases available:https://www.rtl-sdr.com/product/spare-aluminum-metal-enclosure/

author: German_EE
author_rank: Super Contributor
timestamp: May 29, 2019, 05:05:22 pm
content: Ah!I don't have any RTL dongles handy but what I DO have is a dead USB stick that I was saving 'just in case'. It's now had its innards removed and the inside has been coated in conductive paint which is now drying.I knew that an idea would come along eventually, thanks medical-nerd!Oh yeah, Sue and others, working frequency is 10 MHz. My GPSDO puts out 17 dBm and one device has a 13 dBm max input.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2019, 08:10:52 pm
content: you can buy two SMA 6 GHz attenuators:1 dB:https://www.aliexpress.com/item/SMA-Fixed-Attenuator-5dB-DC-6-GHz-2W/32723128440.html3 dB:https://www.aliexpress.com/item/2W-SMA-DC-6GHz-Coaxial-Fixed-Attenuators-Frequency-6GHz-SMA-Fixed-Connectors/32956090844.htmlConnect them together, so you will get good 4 dB attenuator.Total price will be 8.25 USD, include shipping fee

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2019, 08:16:45 pm
content: Quote from: German_EE on May 29, 2019, 05:05:22 pmI don't recommend to put max power level which is at max level for your device input.Just buy 10 dB attenuator and it will be the best solution for your case:https://www.aliexpress.com/item/2W-SMA-DC-6GHz-Coaxial-Fixed-Attenuators-Frequency-6GHz-SMA-Fixed-Connectors/32956090844.htmlThe price is just 4 USD.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 03, 2019, 07:03:40 pm
content: for general purpose use, I've had success using 2 square flanged coax connectors, bolted together with threaded standoffs. have series resistors inbetween the connectors and just solder parallel resistors (as distributed as possible) to the inside of the flanges.The thing can be shielded by a piece of thin copper sheet rolled into a square tube around the flanges and soldered on. I've done it with tin can wall sheet too, just remember it's sometimes coated if it's for food.I agree with radiolistener though, unless you do it for fun or for some specialized purpose/non-standard values/impedances, it's better to buy them ready-made. They're rediculously cheap on ebay, you could probably get 3-4 for the cost of the two flanged coax connectors.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 03, 2019, 07:33:11 pm
content: at 10MHz and only 4dB attenuation almost every setup works; I'd try to integrate the pad into the target device, right after the input, 3 0207 resistor will just do fine; but I'd rather go with -6/7/8dB to be on the save side - unless you're feeding a 13dBm ring mixer.