Non reactivity of reesistors for use as loads for measurement? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: September 18, 2020, 03:59:55 pm
content: Hi,I want to buy a bunch of the least reactive resistors I can for general use but also for loads to test things like baluns for loss i.e. use as reference loads. I figure that the next time I buy resistors I want them to be ones that can do double duty.What am I dealing with as far as the tradeoffs involved with this issue? I know I can just put individual parts on my VNA but it would be nice to start out with resistors that were fairly decent in this respect.I'm not really needing any real power handling capacity now but it would also be useful to know what components I'd want to use to make decent, non-reactive dummy loads for the future. I mostly just want non-reactive resistors to help me learn about impedance, reactance, etc, in the real world.In addition to small dummy loads just to use as known impedances + impedance bridges for example..

author: coppercone2
author_rank: Super Contributor
timestamp: September 18, 2020, 04:03:10 pm
content: what frequency range? I think if you limit yourself to lower frequencies, things are interesting, otherwise just expensive components

author: cdev
author_rank: Super Contributor
timestamp: September 18, 2020, 04:10:20 pm
content: Well, today I'm just testing out different mini-baluns I am winding on FT-50 size toroid, binocular cores and a couple of Mini-Circuits parts that I have to see how lossy they are.Really my main interest is deciphering the confusing wealth of opinions on HF ununs and baluns by testing them myself. I tend to retain a lot more of whatever I actually do than just read.Right now I am just using axial resistors.But ultimately I want to make some dummy loads that are precise enough to just deddicate to this in common ratios to 50 ohms, so 50 ohms, 200 ohms, 450 ohms and so on.Quote from: coppercone2 on September 18, 2020, 04:03:10 pm

author: fourfathom
author_rank: Super Contributor
timestamp: September 18, 2020, 05:13:35 pm
content: FT-50? So you're probably interested in sub-100MHz work, right?I use my stock of standard SMT resistors, 1205, 0805, and 0603, depending on my power requirements and whether I've made a regular PCB, or hand-cutting traces/lands on a blank board. A pair of 100-Ohm 1% resistors makes a good 50 Ohm termination up to a couple hundred MHz.

author: cdev
author_rank: Super Contributor
timestamp: September 18, 2020, 07:20:11 pm
content: I am pretty lazy and so I only make a PCB when I am pretty sure I am not going to make any further changes. I am trying to get a better feel for using the right size traces, where. For example, with a resonant circuit, parts of some filters effectuate an impedance transform, despite it staring me in the face, it took getting a VNA for me to get that the geometry of the filter traces really shoud change when its working under the assumption the impedance is higher. Of course that matters a lot less and doesnt seem to matter at HF frequencies.I need to make up a bunch of small boards that can just be everything and be done with itI also need to find some thinner copper tape. The narrowest I have is 3 mm. There is a nifty program called ATLC that allows you to approximate the dimensions required for any arbitrary cross section of transmission line using a graphics program, like GIMP.I'm using the toroids Ive got right now because I have basic things I want to get a feel for and there they are, Ive already got the small ones.If one is talking about size of toroids vs bandwidth, definitely the small multicore ones using #61 material can work up into the gigahertz range.Ive never possessed any of those big ones, but I plan to get at least one for use in an upgrade to make an end fed vertical all purpose ham antenna- for HF.Nothing fancy.Quote from: fourfathom on September 18, 2020, 05:13:35 pmAre differentSMTresistors brands all roughly of comparable similarity in terms of being mostly resistive, not inductive/capacitive?

author: TimFox
author_rank: Super Contributor
timestamp: September 18, 2020, 07:30:49 pm
content: Probably not much difference near 50 ohms, but for higher resistance parts the reactance will depend on the resistive material in the SMT resistors. Thin film resistors (with better DC performance) may be serpentine (due to lower resistivity), and the capacitance between "serps" may degrade the performance at high frequencies. Similarly, the glass in the mixture for thick-film resistors may have a similar effect, capacitively shorting out the resistive material at higher frequencies.

author: geggi1
author_rank: Frequent Contributor
timestamp: September 18, 2020, 07:37:10 pm
content: For low power stuff on HF regular SMD resistors are good enough. You can even use normal TTH resistors.

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2020, 07:16:20 pm
content: For leaded parts if you do not want to use carbon composition resistors, then ceramic composition resistors are non-inductive but values are limited and they are relatively expensive.There are non-inductive leaded thick film resistors, some of which are available in very high power packages. These might be desirable where high precision is required.

author: coppercone2
author_rank: Super Contributor
timestamp: September 20, 2020, 09:48:49 pm
content: Quote from: geggi1 on September 18, 2020, 07:37:10 pmI think it helps to use lower wattage parts too, i.e. 1/6 or 1/8 through hole resistors, to aid in dead bugging, but have better signal integrityWould be nice if someone setup with a Impedance analyzer can test this

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2020, 11:08:58 pm
content: Quote from: coppercone2 on September 20, 2020, 09:48:49 pmIn the past, the smallest low value carbon composition resistors also provided overload protection. Film resistors, including surface mount ones, usually have a lower peak voltage and power rating.

author: tkamiya
author_rank: Super Contributor
timestamp: September 21, 2020, 04:22:04 am
content: For HF use, anything other than wire-wound registers will be just fine. I used to have a dummy load (home made) that used ordinary carbon power resisters. These are the type where resistivity element was coated and cut spirally to make up the specified resistance. Of course I didn't have a VNA back then. (back in 1980s) Worked just fine as a dummy load.For just about the same amount of money, you can buy a strip line power resister. You'll need a heat sink though.https://www.ebay.com/itm/Celwave-100w-50ohm-Dummy-Load-EMC-6040980-Resistor-UHF-N-Type-BNC-connector/124342849441?hash=item1cf3694fa1:g:tjUAAOSwZbRdQdPNLook at the 6th image. It will easily go to GHz range, and 100 watts. You can buy just the element. If I can find it, I'll give you one of mine....<edit>https://www.ebay.com/itm/5Pcs-100w-50ohm-dummy-load-RF-resistor-RFP-100-50-TW-power-terminator-MFOOH-sg/143668130273?_trkparms=ispr%3D1&hash=item217349b1e1:g:qkcAAOSwO8pcEdRK&amdata=enc%3AAQAFAAACcBaobrjLl8XobRIiIML1V4Imu%252Fn%252BzU5L90Z278x5ickk7d4nremBkvNKtcC0ZwqXDE2asyxoC9kseqcCtxH6%252FJfVnHEGfvZFMXAEasNIIqnNU7Y3XMcMsMx8Vvom48VYZv78TgpbbIanlvvcXUHiUTB2%252BMh3vEz1IOwlT0eWBLsmQi4H4KS%252B2FEl0e31MtuNDDD6XwsiTZ71qrwkmCH2H5YX9rgrd%252Fvym%252BNww2asBxX0WOZ0R9cyBDprvUK68AZ4vVkbIPNgEgvSThesqSJt9QZliWn1ylrssPuC%252FIzwRFDdz4hmQcvXDQYc%252FSTXUJG1NFnurWzGCFrY%252BndTaRHa%252FY3GGfJRELuwXZ0qJ47QMcb7%252B0XE%252FpuPy0xZJ%252BoCi2AUVON1loemZHrOt1uugbMB%252FBr9OtFFKwzySAAVfNh3h%252BJNWZjWZGJZnY2OLNRx0S8TPsFoPhQHv2RTwTDB6piqru2Kh3VxtuyqfAj3i8BH8GGHkgFZ3vgoW%252FO%252B1N89za4i42RrvEWsNBQN5K41PyXmWA2ebtc1%252FYXn8mSm%252BEy3N4OjNZYQ5oPNgXk3k9El6Cej%252BiZvfYcY9qrVpuXUQSD1gDTJxrmqUh6Go9LFGT9K0Fps82LULWgIGy7uEtWv8a5loG941y0TEUvscXDzaaAcHFmCnctEkQnR0bsj2xChOOBFW2WoG4%252Fv48ioR3r%252BRHuxaSFLwiFqE53%252BKAX2xBZ57JaJ2kRFacuzyU9XlgUyztc3CNyePAbHbXzs7l8ZpDyCqQkEyTWA0sNG6uBEWUp8kHeoadisZ2ju5b8McgP%252B2g0JlZlJuJQWy8LNWA1vo1dmKg%253D%253D%7Ccksum%3A14366813027362efa8c6b8ef4665b5c9f287685c42ee%7Campid%3APL_CLK%7Cclp%3A2334524

author: Kalvin
author_rank: Super Contributor
timestamp: September 21, 2020, 09:39:13 am
content: 50 ohm / 250W termination resistor up to 3 GHz (this was just the first hit from eBay):https://www.ebay.com/itm/RF-Termination-Microwave-Resistor-Dummy-Load-RFP-250N50-250W-50ohms-DC-3GHz/283987804175Of course, one needs to useverygood heat sink at higher power levels, and the 3 GHz may be a bit hard to meet. But these kind of resistors should be usable for the OP's applications for the dummy load rated more than few watts.ps. The the other end of the resistor is connected to the metal body, so the series/parallel connection is somewhat challenging. One can buy 100 ohm and 200 ohm resistors for easier parallel connection for improved power rating / power dissipation. Also, there are 50 ohm resistors with the both contacts insulated from the metallic body.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 21, 2020, 11:53:02 am
content: Agreehttps://www.eevblog.com/forum/projects/home-made-pa-and-load/msg635226/#msg635226

author: syau
author_rank: Frequent Contributor
timestamp: September 22, 2020, 11:24:49 pm
content: Achieved very good result using thishttps://www.vishay.com/docs/60093/fcseries.pdfThe way i use is using sma connectors design for semi rigid cable.