How to safely use a SA to look at RF from ham radio - Page 1
Forum section: electronics / beginners

author: mcinque
author_rank: Unknown
timestamp: January 24, 2017, 10:52:52 am
content: Premise: I'm NOT an RF guyI've always used the SA for EMC testing and some minor RF stuff with extremely low power devices, well below the maximum input value of the frontend.I never measured a really powerful RF signal and I want to do it safely both for the SA and for the RF output stage.So I thought it would be a nice thing to have some advices from you.The first thing to do I guess is to have an idea of the output of the radio signal. I do not have a wattmeter, so I thought to use a fast multimeter on mV connected in parallel to the RF output stage and antenna/dummy load, and to write down the maximum reading obtained during TX + voice modulation.By converting that value in dBm I get a value around +46dBmAssuming the SA has a maximum input of +20dBm, I guess that two 20dB attenuators in series should do the job.My attenuators are rated for 0.5W so well below the RF output but since it's connected to the antenna/dummy load the most of the signal should go on the load. Is this correct? Shouldn't I consider also SWR? Should I purchase a proper dummy load to avoid any kind of variable introduced with an antenna?Does anyone would review my procedure/considerations and add his personal experience? Usually, I forgot always something

author: slurry
author_rank: Regular Contributor
timestamp: January 24, 2017, 11:13:48 am
content: If you have a dummyload that can handle your 40W then you can feed the dummy with a RG58 coax and use another piece of RG58 strapped along the first cable with zip-ties.The braid of the RG58 is leaky enough to couple some signal, the longer the cables are parallelled the more signal is coupled.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 24, 2017, 11:17:27 am
content: You're right using two 20dB attenuators in series, but ...Attenuators are impedance matched devices, you'd connect the first attenuator to the RF output, so it'll have to absorb most (9 / 10) of the TX power, outputting 1 / 10 of the input power.Connecting the attenuator parallel with the dummy load or antenna would result in a large mismatch, your transmitter is not supposed to be happy with that, even then the attenuator would have to absorb a significant higher level of power than 0.5W

author: dmills
author_rank: Super Contributor
timestamp: January 24, 2017, 11:24:59 am
content: Ah, no it doesn't really work like that, an RF attenuator is usually a matched impedance device, that is to say it is a Pi network 50 ohms in and out.If you put the attenuator in parallel with a dummy load you end up with each taking half the power and the rig seeing a 2:1 low Z VSWR at least until the attenuator melts.What you really want is a dummy load with a sample port (This is really a sort of attenuator, but one designed to handle big power on one port), they are usually 30dB or so. You follow this with one or both of your 20dB attenuators, no reason to be anywhere near the upper limit of the SAs power handling.A warning, high power attenuators (Same thing as a dummy load with a sample port) are often not really bidirectional, they tend to have the high power resistor only on one of the two ports, this is a trap for young players as they often have a N type on both ends (Who? Me, 30 odd years ago? Never)....You can build a sampling tap that would go between your rig and your existing dummy load, use a T piece on the front of the dummy load and follow it with a network consisting of a 2.5k resistor in series and a 51 ohm resistor as a shunt across the sample output, this will be good for pretty close to 40dB at least at HF, things get harder in UHF and microwave. Build this network with short leads on a bit of copper clad.+46dBm is 40W, most radios are designed for 100W PEP output, so I would assume 100W (the extra will not hurt as you will be using the SAs adjustable input pad anyway and usually want about -10dBm at the SAs first mixer).73 Dan.

author: mcinque
author_rank: Unknown
timestamp: January 24, 2017, 11:49:21 am
content: Quote from: slurry on January 24, 2017, 11:13:48 amThank you for the suggestion.In that way I guess I can't do any precise power related measurements, since the coupling is too variable (and I've seen RG-58 with different braids mesh depending on the manufacturer).Quote from: capt bullshot on January 24, 2017, 11:17:27 amRight, thank you for explaining that!Quote from: dmills on January 24, 2017, 11:24:59 amThank you for the technical details Dan!

author: mcinque
author_rank: Unknown
timestamp: January 24, 2017, 12:16:16 pm
content: Quote from: dmills on January 24, 2017, 11:24:59 amI found a similar solution on KO4BB page using a MFJ-264he suggest to use metal resistors for better frequency responsehttp://www.ko4bb.com/ham_radio/MFJ-264/

author: dmills
author_rank: Super Contributor
timestamp: January 24, 2017, 12:21:55 pm
content: R1 in that is your dummy load, it dissipates the power from the radio, hence why I suggested a T connector screwed to the front of the dummy load.73 Dan.

author: CJay
author_rank: Super Contributor
timestamp: January 24, 2017, 12:46:12 pm
content: Quote from: dmills on January 24, 2017, 11:24:59 amI built a sampling tap from the PDF that appears in the first link when googling for Wes Hayward AD8307, it's a trivial device to build and extremely handy when used with a dummy load.I did a simplistic measurement of mine and fed the sig gen output of my CMU200 through it, it's near as damn it -40dB to at least 150MHz (I got fed up thereand didn't go higher), while I'm sure my simple build isn't lab grade it's more than adequate for my needs.The one thing the OP may need to consider though is a DC block...

author: xrunner
author_rank: Super Contributor
timestamp: January 24, 2017, 01:27:08 pm
content: Quote from: mcinque on January 24, 2017, 10:52:52 amI know you don't have a wattmeter, but someday if you do get one here is how I do it. This is a Coaxial Dynamics power meter, and instead of a calibrated slug for a specific band you can insert a power tap that will tap off an adjustable amount of power, in this case I have it set for about -40 dB attenuated signal out the BNC. On the output you attach a dummy load. So if your input is, say, a 100W CW transmitter (+50 dBm), then you get out a +10 dBm sample, and if your SA has a Max input of +20 like mine, you are good to go.

author: mcinque
author_rank: Unknown
timestamp: January 24, 2017, 04:03:55 pm
content: Quote from: CJay on January 24, 2017, 12:46:12 pmI already have one from Mini Circuits, used when doing pre-compliance with a cheap open mini gtem cell, where PCBs can touch the metal case of the cell and DC can go in the SA.What's the reason for using also in this case? To avoid any possible damage caused by a malfunction of the RF output or because some transmitters can have still a small DC component on output?

author: slurry
author_rank: Regular Contributor
timestamp: January 24, 2017, 04:19:20 pm
content: Quote from: mcinque on January 24, 2017, 11:49:21 amCorrect, you cant measure absolute power but often relative measurements are needed for example 2nd and 3rd harmonic, as log as the dynamic range allows this method works well.But one have to keep in mind that the grade of coupling vary with frequency.If i where about to undertake that measurment i would have used a directional coupler and a dummy load, the SA connected to the coupler thru a 10 or20dB attenuator depending on the coupler.

author: mmagin
author_rank: Frequent Contributor
timestamp: January 24, 2017, 04:44:40 pm
content: I didn't see any frequency range mentioned in your post.If you want to be cautious, after any attenuation, I would suggest a diode limiter that is specified for the frequency of interest. I have a couple used HP ones around, but I think this part would be suitable:http://www.minicircuits.com/pdfs/VLM-33W-2W+.pdfI think many of these (at least the ones specified for 10 MHz and above) integrate DC blocking capacitors, but I'm not sure about this one. I would ask Minicircuits, but I'm cautious so I'd also check with an ohmmeterKeysight of course has the whole formerly-HP product line and more, but I do not sense you are looking at >3 GHz or have quite that much to spend on this project...http://www.keysight.com/en/pc-1000000225%3Aepsg%3Apgr/power-limiters?nid=-32700.0&cc=US&lc=eng

author: CJay
author_rank: Super Contributor
timestamp: January 24, 2017, 04:51:21 pm
content: Quote from: mcinque on January 24, 2017, 04:03:55 pmIn case of malfunction or accident (if you're repairing or adjusting it's not impossible to slip and put significant amounts of DC on the antenna socket), cheap insurance basically.I'd be surprised if there were any Ham transceivers with residual DC on the output as some antennae are a DC short.

author: mcinque
author_rank: Unknown
timestamp: January 24, 2017, 06:35:53 pm
content: Quote from: mmagin on January 24, 2017, 04:44:40 pmYou're right. Frequency range is from 27 to 433MHz covering all the radio cb/free/ham bands here.QuoteThat's a really a nice suggestion! Thank you.P.S. Keysight is not for me, despite having excellent products, is definitely out of my budget

author: mcinque
author_rank: Unknown
timestamp: January 24, 2017, 06:55:55 pm
content: Quote from: CJay on January 24, 2017, 04:51:21 pmdefinitely it's NOT impossible, I agree

author: w2aew
author_rank: Super Contributor
timestamp: January 26, 2017, 04:32:03 am
content: I have several videos on the subject of coupling transmitters to a scope or spectrum analyzer. Using some type of sampler is the easiest way to do it. Here are a few examples:

author: orin
author_rank: Frequent Contributor
timestamp: January 26, 2017, 06:44:54 am
content: Quote from: mcinque on January 24, 2017, 10:52:52 amA couple of ways:Attenuator. Something like this:http://www.ebay.com/itm/fast-shipping-50W-40dB-N-male-to-female-RF-fixed-Attenuator-DC-3GHz-/272146798124The only problem I see with it is that it probably heats up quickly, so do your testing fast. For 40W, I'd probably go with this! Check it at low power first - can't be too careful where SA inputs are concerned.Second way: Directional coupler, a dummy load and a low power attenuator.I did this with an Elecraft directional coupler kit, the CP1 from here:http://www.elecraft.com/mini_module_kits/mini_modules.htmbut unfortunately, that only works to 30MHz. If you can find a coupler that will pass the required power, then you connect the coupled port to the SA via a 20dB or so attenuator. If you coupling is 20dB, with the attenuator, you get 6dBm at the SA from a 46dBm signal. Dummy loads that can take 40W are a lot cheaper than attenuators that can take 40W. I wrote this before I found the above attenuator - I'd go for the attenuator - it's probably cheaper than any coupler you'd find and then you still need a dummy load.

author: mcinque
author_rank: Unknown
timestamp: January 26, 2017, 08:19:55 pm
content: Thanks Alan and Orin!

author: joeqsmith
author_rank: Super Contributor
timestamp: January 27, 2017, 12:12:05 am
content: I was attempting to do something like this a while back. Pictures in this thread show the load with the tap. I was looking for something DC to 100MHz in roughly the 40W range.https://www.eevblog.com/forum/projects/home-made-pa-and-load/msg639804/#msg639804

author: TimFox
author_rank: Super Contributor
timestamp: January 27, 2017, 05:15:37 pm
content: If you can find a high-power attenuator at a reasonable price, such as this onehttp://www.martinrfsupply.com/Bird-100-SA-MFN-30-BR-RF-Attenuator-100W-30dB_p_930.htmlit can be used as either a dummy load (with a reasonable-power 50 ohm load at the attenuator output) or as a useful way to sample the high-power RF at a normal 50-ohm device, such as a SA.