Resistors for high power RF attenuator - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: November 16, 2019, 03:53:02 pm
content: Hello,I want to build an attenuator to be able to check the output of an HF power amplifier with a spectrum analyser.Unfortunately, It seems very difficult to find resistors rated for more than 5W that are not a coil.What can I use? It would be nice to have a 100W rated attenuator, and not use a ton of resistors.Maybe it's easier to use a coupler?Thank you.

author: radiolistener
author_rank: Super Contributor
timestamp: November 16, 2019, 05:10:42 pm
content: you can buy refurbished 250W RF resistors from China. They are used but their price is just 1 usd.https://www.aliexpress.com/item/32982402240.htmlThe quality may be vary, but last time I got 32A1213F RF FLORIDA C0C (250W)Works very good.VSWR is (include 5 mm soldering wire):- 1.02 at 200 MHz- 1.05 at 400 MHz- 1.07 at 500 MHz- 1.18 at 800 MHzBut note, you're needs to use as short connections as possible. Ideally you're needs to solder it directly to RF connector.Also you're needs to attach a big heatsink.There are also RF attenuators in similar package:https://www.aliexpress.com/item/32963322489.htmlThe new dummy load will cost you about 100 usd.So these refurbished resistors are good deal

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2019, 09:02:12 pm
content: What you really need is a suitable dummy load and an RF sampler. A Google search for "RF sampler" will turn up many examples.Using an attenuator between an RF power amplifier and spectrum analyzer without a sampler is not generally done because if the attenuator fails, the expensive spectrum analyzer is toast.

author: TheSteve
author_rank: Unknown
timestamp: November 16, 2019, 09:29:48 pm
content: While a proper attenuator is the most ideal you can always use something like this:https://www.ebay.com/itm/223735673440It is a 250 watt dummy load with a sample port.You would need to characterize it to measure power accurately but if you're just looking for relative measurements it is great.

author: jujun
author_rank: Regular Contributor
timestamp: November 16, 2019, 10:00:24 pm
content: Thank you all, I will add a RF sample port on my dummy load, and maybe build a few high power attenuators with aliexpress modules.

author: German_EE
author_rank: Super Contributor
timestamp: November 17, 2019, 11:11:14 am
content: Whilst a proper sample port using either a resistive divider or a directional coupler is ideal there is another method.At the rear of my antenna tuner is a 'Monitor' port which is just a single BNC socket. Inside the tuner the center pin of the BNC is connected to 30 cm of PTFE wire with nothing connected at the other end. This is enough for me to pick up a sample of my transmitter output and feed it to the spectrum analyzer.Try this first, it's the easiest and cheapest option, but MAKE SURE that the end of the wire is well insulated.

author: geggi1
author_rank: Frequent Contributor
timestamp: November 17, 2019, 01:17:32 pm
content: Get a dummyload resistor off E bay they usualy can be bought at a reasonable price when bought form China.https://www.ebay.com/sch/i.html?_from=R40&_nkw=RF+Termination+Microwave+Resistor&_sacat=0&_sop=15Then build this power tap.http://www.ko4bb.com/ham_radio/MFJ-264/Get a decent size heatsink for the dummyload chip and if you think the HS is a bit small you can stick on a PC fan or two.As long as you are workig with HF and 6m this circuit is good enough. on higher bands you probbably need a better setup.

author: bd139
author_rank: Super Contributor
timestamp: November 17, 2019, 01:32:42 pm
content: You can use an inline tap and a normal dummy load. See the design in here:https://www.qsl.net/sz1a/download/build%20an%20rf%20power%20meter.pdfIt’s good enough for HF stuff