I shredded a few carbon resistors and noticed that many have the
carbon layer laid out in a spiral pattern around the cylindrical core,
up to several turns.

I also shredded a few metal-film ones, but could not figure is the
metal was on a single layer or not, and how it was connected to the
leads.


I wonder which best approximates a non-inductive resistance.



Do "spiral" carbon resistors have a significant inductance?

Do all metal-film resistors have a single layer, or are there some
multilayered or (G_d forbid!!) spiral-layered, with plentiful
capacitive bypass???



I have found a set of 16 x 100 ohm / 2W metal-film resistors which I
am ready to assemble them into a 400 ohm / 15W (derated) terminator
for a T2FD, but I'll wait for advice...


tnx de n1jpr/i2
filippo
http://filippo.ru.ru

"SpamHog" wrote in message
om...
I shredded a few carbon resistors and noticed that many have the
carbon layer laid out in a spiral pattern around the cylindrical core,
up to several turns.

I also shredded a few metal-film ones, but could not figure is the
metal was on a single layer or not, and how it was connected to the
leads.


I wonder which best approximates a non-inductive resistance.

I am no expert on this, but what you describe as carbon is perhaps carbon
film
Your best bet for low inductance would probably be the carbon composition
type. These are found in radios from the 60's-80's- Mouser is still carrying
these.
In appearance they look like a perfect cylinder- no bumps on the ends.
Dale W4OP

Metal-film and metal-oxide resistors are commonly done as a spiral
like the carbon film ones you describe. It's possible to find RF
metal film resistors which are relatively non-inductive, but clearly
the spiral ones will have an inductance about the same as a coil with
the same dimensions and number of turns. OTOH, that may not be a
problem--see example below. I've put a couple 100-ohm 2-W metal-oxide
resistors in parallel, with very short leads, and tested the
combination for return loss and found it to be better than 20dB r.l.
(1.22:1 SWR) out to beyond 150MHz. As they say, your results may
vary. But in any event, if you can use the resistors you have to make
a 50-ohm load and have a 50 ohm SWR meter you trust, you can get at
least an idea of their performance at RF.

Cheers,
Tom

(Example: say you have a 100 ohm metal-oxide resistor which is a
four-turn spiral 0.25 inches in diameter and 0.5 inches long. Its
inductance will be about
40 nanohenries, which less than j8 ohms at 30MHz. As a non-precision
load, that probably won't be an issue at all.)

(SpamHog) wrote in message . com...
I shredded a few carbon resistors and noticed that many have the
carbon layer laid out in a spiral pattern around the cylindrical core,
up to several turns.

I also shredded a few metal-film ones, but could not figure is the
metal was on a single layer or not, and how it was connected to the
leads.


I wonder which best approximates a non-inductive resistance.



Do "spiral" carbon resistors have a significant inductance?

Do all metal-film resistors have a single layer, or are there some
multilayered or (G_d forbid!!) spiral-layered, with plentiful
capacitive bypass???



I have found a set of 16 x 100 ohm / 2W metal-film resistors which I
am ready to assemble them into a 400 ohm / 15W (derated) terminator
for a T2FD, but I'll wait for advice...


tnx de n1jpr/i2
filippo
http://filippo.ru.ru

Lots of useful replies - thanks to you all !


Dale W4OP:
Your best bet for low inductance would probably be the carbon
composition
...they look like a perfect cylinder- no bumps on the ends.

Understood! I remember popping resistors that contained a blackish
compound throughout the cylindrical body: I assume those were cabon
comp.

Also, correct me if I err, they tended to be LARGER that similarly
powered resistors that had their heat-generating portion right under
the skin.


Dave Platt )
Or use one of the low-inductance metal-film/metal-foil types.
and Tom Bruhns )
Metal-film and metal-oxide resistors are commonly done
as a spiral like the carbon film ones you describe.

I am satisfied that metal film/foil types do not come in
rolled-up-foil geometry like some capacitors. This is good - no
capacitive bypass. Also, avoids the same temperature gradient / heat
dispersal issues of carbon comp.

OTOH, still no guarantee that a metal film resistor is non-inductive.


Cecil Moore )
I have some non-inductive wirewound resistors. They
simply reverse the direction of the winding every
so often to obtain canceling fields.

I knew they existed, and saw them in catalogs, but never used them.

I have been tempted to use a high power, low resistamce inductive WW
resistor as a coil in a regen receiver, just to see how well one can
compensate for losses.... I bet both regeneration and tuning will be
VERY smooth


Tom Bruhns )
I've put a couple 100-ohm 2-W metal-oxide resistors
in parallel, with very short leads, and tested the
combination for return loss and found it to be better
than 20dB r.l. (1.22:1 SWR) out to beyond 150MHz.

That pretty much cuts it!

I can easily test my 16 x 100 ohm resistors as a couple of 50 ohm
dummy loads on HF. After all, a decent HF match is all I really need,
and chances that a low "random" SWR will result if great reactance is
added atop resistance is quite modest. Thank you for pointing out the
obvious to my obviously obnubilated mind!

Filippo
N1JPR/I2
http://filippo.ru.ru

It was easy!

I just slammed a couple of 100 ohm 2W carbon film resistors in
parallel on the output SO239 of a SWR meter, and powered it from a CB
transceiver. Center contact by plugging crumpled, twisted leads into
hole. Grounding with help of a tiewrap. Extremely low SWR. The 2x
2-watt resistors became quite hot at 3W RF carrier after 10 min.
keydown (well... let's sat 3 min.
"mike-PTT-kept-pressed-by-rubberband").

I made the T2FD 400 ohm resistor as 4 "squares" of 4 x 100 ohm
resistors, soldered as a series of diamonds. Painted in high
temperature engine black paint. Will go inside a generous ABS project
box, weather sealed but with weephole. Will hang asymetrically, to
point weephole down. Ball of crumpled alu foil should help keep bugs
out, yet allow dripping.

Nominally, that would be 16x2=32W. Derated to 1/3 (say, 10W) should be
still OK with up to 30W RF out @ 100% duty cycle, down to the minimum
frequency.

It probably could take more at low duty cycle or higher frequencies,
but I won't put in anything more, as the balun I made includes 60V
neons between feedpoints and ground, to provide paranoid clipping of
pulses from nearby lightning strikes.

If matching is fair, they'd clip at about 70W, as impedance referred
to ground is 50 ohm. OK for QRP or for typical surplus 20W manpack
operation.