What do you do for resistors if you can't get carbon composition in the
ratings you need? I built a MFJ-1020A out of surplus and scrap before
realizing that the metal film resistors I was using had inductive reactance
when used in an RF circuit. My local parts depot mostly just has metal film
resistors, not the carbon composition devices I need. Seems I heard
somewhere that there is such a thing as non-inductive metal film, but if
there is, how do I find it? Otherwise, where can I get carbon composition
resistors these days?

Any help would be appreciated. My home-built active antenna (for shortwave)
puts out such a terrible signal it is virtually useless.

Thanks,

Dave (an RF newbie)

"Dave" wrote in message
...
What do you do for resistors if you can't get carbon composition in the
ratings you need? I built a MFJ-1020A out of surplus and scrap before

I can't imagine that the resistors have enough inductance to
make any difference at HF. Plus, the reactive component
is in series with the resistance, which would increase the
impedance of the resistance slightly--in most cases that
would be more of a benefit than a hinderance; since I'd
bet most of the resistors are being used for biasing active
components, or to provide signal isolation..

Does anyone have any evidence that it makes a hill of
beans difference at HF?

Pete

On Apr 7, 7:57 pm, "Uncle Peter" wrote:
"Dave" wrote in message

...

What do you do for resistors if you can't get carbon composition in the
ratings you need? I built a MFJ-1020A out of surplus and scrap before

I can't imagine that the resistors have enough inductance to
make any difference at HF. Plus, the reactive component
is in series with the resistance, which would increase the
impedance of the resistance slightly--in most cases that
would be more of a benefit than a hinderance; since I'd
bet most of the resistors are being used for biasing active
components, or to provide signal isolation..

Does anyone have any evidence that it makes a hill of
beans difference at HF?

Lots of stuff that works should be ample evidence.

There are some numbers in Experimental Methods for
RF Design that suggest such resisters are fine to low UHF.
It's really a non-issue.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89lg pied a terre..."
ICBM: 49 16.57 N 123 0.24 W - Hospital/Shafte

"laura halliday" wrote in message
oups.com...
On Apr 7, 7:57 pm, "Uncle Peter" wrote:
"Dave" wrote in message

...

What do you do for resistors if you can't get carbon composition in the
ratings you need? I built a MFJ-1020A out of surplus and scrap before

I can't imagine that the resistors have enough inductance to
make any difference at HF. Plus, the reactive component
is in series with the resistance, which would increase the
impedance of the resistance slightly--in most cases that
would be more of a benefit than a hinderance; since I'd
bet most of the resistors are being used for biasing active
components, or to provide signal isolation..

Does anyone have any evidence that it makes a hill of
beans difference at HF?

Lots of stuff that works should be ample evidence.

There are some numbers in Experimental Methods for
RF Design that suggest such resisters are fine to low UHF.
It's really a non-issue.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89lg pied a terre..."
ICBM: 49 16.57 N 123 0.24 W - Hospital/Shafte



Hmmm. Okay, well, could the problem be the monolithic ceramic caps I used?
Something is creating a boatload of harmonics, and that's the only other
component besides the transistors themselves.

Thanks much for the input...

Dave

"Dave" wrote in message
...
Hmmm. Okay, well, could the problem be the monolithic ceramic caps I
used? Something is creating a boatload of harmonics, and that's the only
other component besides the transistors themselves.

Thanks much for the input...

Dave


Hi Dave,
This post mentions something that I did not perceive from your first post.
You mention a 'boatload of harmonics". As I understand it, the MFJ-1020A is
an active antenna circuit, but I don't have any details.
How are you noticing these "harmonics"? If you are hearing them from the
receiver this circuit is feeding, my guess is that either the active antenna
or the receiver front end is being overloaded and driven into non-linearity.
I would suspect the active antenna.
Can you provide more information? Where are you located, urban or rural?
What kind of system is this circuit a part of?

Bob

"Bob Liesenfeld" wrote in message
...

"Dave" wrote in message
...
Hmmm. Okay, well, could the problem be the monolithic ceramic caps I
used? Something is creating a boatload of harmonics, and that's the only
other component besides the transistors themselves.

Thanks much for the input...

Dave


Hi Dave,
This post mentions something that I did not perceive from your first
post. You mention a 'boatload of harmonics". As I understand it, the
MFJ-1020A is an active antenna circuit, but I don't have any details.
How are you noticing these "harmonics"? If you are hearing them from the
receiver this circuit is feeding, my guess is that either the active
antenna or the receiver front end is being overloaded and driven into
non-linearity. I would suspect the active antenna.
Can you provide more information? Where are you located, urban or rural?
What kind of system is this circuit a part of?

Bob


Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on my
workbench. I mention harmonics because the sinewave goes from nice and
clean to "blurry" and looking "smeared" across the screen of my O-scope. I
may *be* overloading it, but I thought that would result in clipping of the
waveform. I have the signal generator set to attenuate the signal severely,
and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not overloading the
device.

Thanks,

Dave

Wrong resistor value = bad biasing = harmonics? Just a S.W.A.G?

Dave wrote:

"laura halliday" wrote in message
roups.com...


On Apr 7, 7:57 pm, "Uncle Peter" wrote:


"Dave" wrote in message

...



What do you do for resistors if you can't get carbon composition in the
ratings you need? I built a MFJ-1020A out of surplus and scrap before


I can't imagine that the resistors have enough inductance to
make any difference at HF. Plus, the reactive component
is in series with the resistance, which would increase the
impedance of the resistance slightly--in most cases that
would be more of a benefit than a hinderance; since I'd
bet most of the resistors are being used for biasing active
components, or to provide signal isolation..

Does anyone have any evidence that it makes a hill of
beans difference at HF?


Lots of stuff that works should be ample evidence.

There are some numbers in Experimental Methods for
RF Design that suggest such resisters are fine to low UHF.
It's really a non-issue.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89lg pied a terre..."
ICBM: 49 16.57 N 123 0.24 W - Hospital/Shafte





Hmmm. Okay, well, could the problem be the monolithic ceramic caps I used?
Something is creating a boatload of harmonics, and that's the only other
component besides the transistors themselves.

Thanks much for the input...

Dave





--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

"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! )


where can I get carbon composition resistors these days?

They are still available. Typically, Farnell offer 220R to 4K7 at 1W - but
they are quite expensive.


John A

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.


where can I get carbon composition resistors these days?

They are still available. Typically, Farnell offer 220R to 4K7 at 1W - but
they are quite expensive.

And obsolete, because for every application there is now a better
alternative - most often, metal film.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

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