Hi voltage RF probe - Page 1
Forum section: electronics / rf-microwave

author: Flump
author_rank: Frequent Contributor
timestamp: September 27, 2016, 09:41:15 pm
content: I have a fluke RF Probe, one of the things I use it foris to use it as a reference and set up my RF power meters to itas the probe is reasonably accurate but it only works up to30v which is 18W @ 50R and I would like to have a RF probethat works up to 100W HF up to UHFIs this possible ? , maybe it's something I can make ?http://en-us.fluke.com/products/all-accessories/fluke-85rf.html

author: tggzzz
author_rank: Super Contributor
timestamp: September 27, 2016, 10:05:33 pm
content: How close to high power RF do youreallywant to get?Personally I'd insert a 20dB directional coupler (appropriate power and frequency), and measure the output of that.

author: Flump
author_rank: Frequent Contributor
timestamp: September 29, 2016, 08:31:54 am
content: Thanks tggzzz ,i will look into getting an Attenuator but they are expensive new

author: conander
author_rank: Contributor
timestamp: September 29, 2016, 09:53:15 am
content: This probe is mostly what I was looking for.... looks quite nice.A DIY schematic to make one would be quite interesting.

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2016, 10:50:04 am
content: Quote from: Flump on September 29, 2016, 08:31:54 amDirectional couplers and attenuators are very different things. One is designed to dissipate power, the other is designednotto dissipate power.They often have such things at hamfests; the largest is this weekend:http://www.nationalhamfest.org.uk/or seehttp://rsgb.org/main/news/rallies/

author: Flump
author_rank: Frequent Contributor
timestamp: September 29, 2016, 12:19:36 pm
content: I was thinking of using an Attenuator to reduce the amount of RF that reaches the dummy load.RF Source 100w-----RF attenuator 10db-----Dummy Load 10wso where as before my RF Probe can measure up to 18w now it could measure up to 180w

author: tggzzz
author_rank: Super Contributor
timestamp: September 29, 2016, 02:43:56 pm
content: Quote from: Flump on September 29, 2016, 12:19:36 pmIt will come down to cost and flexibility.If using a switched attenuator, be sure you understand what happens as you switch, and the effect that would have on the instrument and the power source.

author: KJDS
author_rank: Super Contributor
timestamp: October 01, 2016, 01:17:55 pm
content: I've got a couple of 100W 30dB attenuators available, £40 each, plus shipping at cost

author: Flump
author_rank: Frequent Contributor
timestamp: October 01, 2016, 06:45:02 pm
content: KJDS Sent you a PM about them.

author: conander
author_rank: Contributor
timestamp: October 02, 2016, 03:38:18 pm
content: The 85RF probe seems not very difficult to make according to the manufacturer datasheet. All the values are shown except the diodes. I guess the magic should be there....Research: CR1 BAT41ZFILM and CR2 1N4448W

author: AF6LJ
author_rank: Unknown
timestamp: October 25, 2016, 03:58:55 pm
content: Quote from: tggzzz on September 27, 2016, 10:05:33 pmThat is not a bad idea, as long as you expect accuracy within a DB.Better yet, get and use a decent scope, that will work fine as long as you are not trying to measure microwave power.

author: Earendil
author_rank: Regular Contributor
timestamp: October 26, 2016, 01:44:27 pm
content: Quote from: AF6LJ on October 25, 2016, 03:58:55 pmYou need to be very careful when using an oscilloscope with high voltage RF signals. You need to check the oscilloscope's specifications thoroughly before trying to measure those signals.Tektronix MDO3000 has the following maximum input voltage specification for 1MOhm input for example:"The maximum input voltage at the BNC, 300 VRMS.Installation Category II.De-rate at 20 dB/decade between 4.5 MHz and 45 MHz, De-rate 14 db between 45 MHz and450 MHz.Above 450 MHz, 5 VRMS."In 50Ohms mode it's far worse. It can only handle 5 VRMS only at any frequency.

author: tggzzz
author_rank: Super Contributor
timestamp: October 26, 2016, 02:06:56 pm
content: Quote from: Earendil on October 26, 2016, 01:44:27 pmThe probes also have such limitations, so a *10 probe doesn't help as much as might be thought. Butyouknow that.And RF burns can be quite nasty, I'm lead to believe. One result from google:https://www.reddit.com/r/amateurradio/comments/2yrts7/rf_burns/

author: AF6LJ
author_rank: Unknown
timestamp: October 26, 2016, 02:51:52 pm
content: Quote from: tggzzz on October 26, 2016, 02:06:56 pmThis is quite true, one must consult the equipment's manual before attempting said measurement.As far as RF burn goes, Yes care must be taken.High energy burns tend to leave tissue bleached and desiccated...Other than that, I won't understate or overstate the issue, RF can be very hazardous especially from the mid VHF on up. I've had my share of RF burns, most of them minor, a few not so much.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 15, 2018, 09:52:04 pm
content: Why dont you use a resistive tap ?https://electronicprojectsforfun.wordpress.com/rf-module-gallery/the-rf-splitter-and-combiner-gallery/a-resistive-tap-attenuator-for-rf-measurements/no burnt fingers with this one

author: vk6zgo
author_rank: Super Contributor
timestamp: August 16, 2018, 12:10:14 am
content: Quote from: tggzzz on September 27, 2016, 10:05:33 pm100watts is hardly high power!The voltage needed for that power level in 50ohms is 70.71 volts RMS.The directional coupler is an elegant solution, but has the disadvantage that you are unlikely to get one that works satisfactorily from HF to UHF.An attenuator is a reasonable idea, but the OP needs to remember that is the device where most of the power is dissipated, with the test load only handling "what's left".A power attenuator, (what we used to call a "through load") will work well, as will a test load with a tapping or built up from individual resistors to do the same thing.