Guys, thanks for the very detailed test results, especially you joeqsmith! I've made a summary of the results so far:
poster mfr/series construction value measurement reactance?
mag_therm Vishay PR02 metal film 4.7k S11 strongly inductive
mag_therm Multicomp 1206 unknown 4.7k S11 moderately inductive
joeqsmith unknown carbon comp 390/0.5W S21 not significant
joeqsmith unknown carbon film 390/0.5W S21 not significant
joeqsmith unknown carbon comp 750/0.5W S21 not significant
joeqsmith unknown carbon film 750/0.5W S21 not significant
joeqsmith unknown carbon comp 24k/0.5W S21 weakly capacitive
joeqsmith unknown carbon film 24k/0.5W S21 weakly capacitive
joeqsmith unknown carbon comp 220k/0.5W S21 moderately capacitive
joeqsmith unknown carbon film 220k/0.5W S21 moderately capacitive
joeqsmith unknown carbon comp 1k/1W S21 not significant
joeqsmith unknown carbon comp 1.5k/1W S21 not significant
joeqsmith unknown carbon comp 1Meg/1W S21 strongly capacitive/fixture effects
joeqsmith unknown carbon comp 3.9k/2W S21 moderately capacitive
joeqsmith unknown carbon comp 47/2W S21 not significant
joeqsmith unknown wirewound in phenolic 56/1W S21 strongly inductive
joeqsmith unknown carbon comp 87/1W S21 not significant
joeqsmith unknown carbon comp 240/2W S21 not significant
joeqsmith unknown carbon comp 270/1W S21 not significant
joeqsmith unknown metal film 1k/0.25W S21 not significant
joeqsmith unknown metal film 82/0.25W S21 not significant
joeqsmith Ohmite red devil carbon comp(?) 82/0.25W S21 not significant
joeqsmith Ohmite red devil carbon comp(?) 82/2W S21 not significant
joeqsmith RCD 160 wirewound 47/5W S21 severely inductive
I see some key take-aways:
- Just because a resistor looks like a classic carbon composition resistor doesn't mean it's non-inductive! Those results on that 56 ohm / 1 watt resistor are alarming. I wonder how many folks over the years tried to use those instead of 51 ohm ones in a quick and dirty QRP dummy load and had frustrating results.
- The carbon film resistors so far look to have insigificant reactance.
- Any resistor over a couple hundred kilo ohms looks difficult to quantify (fixture effects, capacitance starts to dominate)
- And this - ultimately one might have no choice but to measure every candidate resistor before using it in an RF circuit to know with certainty whether its reactance will be minimal or not.
I've got my own measurements to do this week. And just for grins, I sent an email to tech support at Vishay, asking if they have lab data on the PR02 series for reactance. Won't hold my breath for a response...
There is a range of carbon resistors at https://www.surplussales.com/ of Nebraska
I buy tubes etc and metal film there, but I wouldn't buy carbon.
Yep, I recently ordered some carbon comps from them and several had drifted more than 10%. I was not surprised and expected that because they are NOS and who knows how old they are. I will use some of them for my VNA comparisons.
If inductance really matters it's probably time to get acquainted with SMT...
My desperation plan is exactly that - make a tiny bit of PCB to hold a 1206 SMT resistor (or possibly two, one on each side in parallel for higher wattage), then solder wire leads to it to make a cobbled-up low-inductance resistor replacement.