From: Ian White GM3SEK on Sun, 8 Apr 2007
12:03:18 +0100

John A wrote:
"Dave" wrote in message

What do you do for resistors if you can't get carbon composition in the
ratings you need?

Do what the professionals do - use film resistors. Concerns about inductive
effect at HF are greatly exaggerated. There are rarely as many "turns" as
often suggested. ( cf. Radcom Jan 2007, p58, fig 1! )

As the person who wrote that article, I strongly agree.

Hello Ian,

I don't get a chance to read Radio Communications often and didn't
see your article. As a professional in the design end, I'll offer
a few comments:

There is still (needless) confusion in amateurism as to metal
film resistors' "inductance" in comparison to wire-wound
resistors which DO have considerable self-inductance.

While there IS some self-inductance in metal film resistors
(due to laser-trimming and patterns of film on the usually
ceramic substrate), it is difficult as @#$%!!! to measure and
easier (but still grudge work) to model as a conductive strip
spiral-wound on the same physical dimensions. For nearly all
amateur applications up to and including 6m, that won't be
noticeable. With some caveats, of course.

Self-inductance of metal-film resistors will vary depending on
the manufacturer and their methods. So will construction which
adds varying self-capacitance from the end-caps (metal) holding
the wire leads. Self-capacitance is easier to measure on a Q-
Meter but is seldom over a half a pFd. That results in an
equivalent of a resistor in series with self-inductance, the
whole in parallel with self-capacitance. The effect on a
circuit depends on WHERE it is placed in the circuit. I've
found that carbon-composition resistors - in general - have a
slightly higher self-capacitance...but that depends on who
made them and what internal structures were involved (has to
be broken and observed if no X-Ray machine is handy).

As a dummy load consisting of many smaller resistors in series-
parallel, one can estimate the total capacitance and inductance
based on individual resistor models arranged in whatever
combination is planned. Offhand, I'd say that rarely does
that affect the dummy load's VSWR beyond 1.3 at 6m. In
arranging a series-parallel combination, there will probably
be more effect from whatever conductors' shape are in doing
the interconnects...less so if on a PCB, probably more if by
wires. A good rule-of-thumb is simply "make all connections
as short as possible, consistent with allowing air flow to
dissipate heat."

The only place to get paranoid about effects of self-inductance
and self-capacitance is in metrology. Metrology NEEDS to have
a minimum of each and to have accurate resistance values at
the rated frequencies. Everyday dummy loads for amateur radio
are far from lab-quality metrological stuff and don't need to
be in that precision range.

73, Len AF6AY