rf dummy load tap - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: October 03, 2023, 11:18:17 pm
content: i would like to take a tap off of my dummy load to look at it on a scope etc,its hf,would anyone have a schematic for this i can use?,cheers+73.

author: langwadt
author_rank: Super Contributor
timestamp: October 03, 2023, 11:30:54 pm
content: a series resistor into a scope set to 50 Ohm input

author: p.larner
author_rank: Frequent Contributor
timestamp: October 04, 2023, 12:29:06 am
content: seems you have it wrong surley a sries blocking cap+ ndiode at least,also my scope is 1 meg input impeadance!.

author: langwadt
author_rank: Super Contributor
timestamp: October 04, 2023, 12:51:18 am
content: Quote from: p.larner on October 04, 2023, 12:29:06 amis your RF dc? use a 50R inline termination on the scope or add 50R resistorhttps://www.qsl.net/kp4md/qrpload.htm

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 04, 2023, 01:07:01 am
content: Depending on the frequency range and construction, modification of a dummy load while keeping it's parameters unchangen might be tricky. All the more as you might find BeO components inside.Of course, using an adjustable coupling probe (sniffer probe, akin to Radiall R435 170/270/470) before your dummy load would be an option. Another would be to substitute an attenuator of suitable ratings for the dummy load and then use a DC block and maybe some protective device at it's output.

author: fourfathom
author_rank: Super Contributor
timestamp: October 04, 2023, 03:41:39 am
content: HF? How much power, and what accuracy do you want?I've made a few resistive taps, and with careful layout they are flat within a dB or two up past 30 MHz. For a 100W power level you can make a -50dB tap using three 1W 2.7K resistors (not wirewound) in series, connected to a 1/4 W (or less) 51 Ohm resistor to ground. With a 50 Ohm load across that 51 Ohm resistor the attenuation will be -50.2 dB at DC, and close to that over the HF range. You can connect your 1M scope instead of a 50 Ohm load and the attenuation will be about 44 dB which will still be a safe level for a scope input.Most of my work is in the 10W or less region so a resistive tap can be made with 1/4W or surface-mount resistors, and probably -40 dB or -30dB is a useful amount of attenuation at the tap.If you want to run more than 100W then the transformer-coupled tap is a better way to go.

author: coppercone2
author_rank: Super Contributor
timestamp: October 04, 2023, 04:03:16 am
content: why not just use a coupler

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: October 04, 2023, 06:43:49 am
content: This article includes an easy to make resistive tap that is good to 70 cms and 100 Watts.http://on4khg.be/wordpress/wp-content/uploads/2015/02/Simple-RF-Power-measurement-W7ZOI-W7PUA.pdfSJ

author: coppercone2
author_rank: Super Contributor
timestamp: October 04, 2023, 06:57:31 am
content: I feel like the failure from a high power induction/magic/whatever coupler is not nearly as likely or glorious as a failure of a resistive divider (carbon comp)that must be something to have 1kw go into a scope. A coupler would need to totally carbon over, whereas the resistor does basically whatever it pleases. why do i think that there is a 3kw cb radio thats going to do thisgoing the extra mile to find a transformer to do this is IMO a brilliant decisionI feel like communications radio is so narrow band that transformer is fine. Some kind of more wideband? resistor seems useful with a really wide band oscillator that runs at low power levels. pDo wonder though if they have a gyrotron trashcan hooked up to a water cooled resistor divider plugged into a magic lecroy .1thz scope some where though. However, I did realize after writing this, that is exactly the setup they have in starfleet. explains everything. why is scotty undergoing skin replacement on his face? coupler response was deemed insufficient for the warp core telemetry.

author: fourfathom
author_rank: Super Contributor
timestamp: October 04, 2023, 03:59:10 pm
content: Yeah, why mess around with resistors??? I know!! Because it's fun and I have a junkbox full of crap.Here's an approach using a capacitive divider (40 dB) followed by a 20dB resistive divider, total attenuation = 60 dB. The tap capacitor is 10pF, which will have little effect on a 50 Ohm connection until the frequency exceeds 100 MHz. The low-freq response rolls off below 2 MHz. That 10pF cap needs to be able to withstand the voltage, 100W @ 50 Ohms = 100V Peak. At 1KW it's 316V Peak.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 04:02:25 pm
content: Quote

author: SeanB
author_rank: Super Contributor
timestamp: October 04, 2023, 06:21:24 pm
content: Quote from: joeqsmith on October 04, 2023, 04:02:25 pmThose you just tune for maximum anode current, the brighter red the anode the better.......

author: geggi1
author_rank: Frequent Contributor
timestamp: October 05, 2023, 09:43:43 am
content: The simplest is a directional coupler.There are a lot of descriptions on how to build one on the internet.One of the simples ones is probably the one by LA8AK (sk)https://www.noding.com/la8ak/m12.htmIt is basically just a piece of RG213 with a wire under the screen and a termination resistor.If you use high quality connectors it will have a pretty wide frequency range, maybe up to 1Ghzor higher.The coupling factor will change along the frequency range but if you have access to a spectrum analyzer with TG or a quality power meter you will be able to make a calibration table.