It started when the dummyload was too small for a job. It's for 40 mtrs.
But making it is barely possible. It should have a Zo of 50 ohms ? And what
about the Q factor ? Looking on my swr meter it gets better when i put my
hands near it and at a very small C rate but still in the red.
Anyone ?

On Sat, 21 Nov 2009 18:56:07 +0100, "Ronald" wrote:


It started when the dummyload was too small for a job. It's for 40 mtrs.
But making it is barely possible. It should have a Zo of 50 ohms ? And what
about the Q factor ? Looking on my swr meter it gets better when i put my
hands near it and at a very small C rate but still in the red.
Anyone ?

Hi Ronald,

If you mean by "dummy load" something with a 50 Ohm match and
resistive (somewhat redundant to the 50 Ohms) then it has to be a
resistor (department of triple redundancy).

Finding a resistor that is sufficiently power rated is not a simple
(cheap) chore. Making your own will take time and ingenuity.

Consult:
Caddock Electronics, Inc.
17271 North Umpqua Hwy
http://www.caddock.com

for resistors that will fill the bill - once you build the heatsink.

73's
Richard Clark, KB7QHC

It started when the dummyload was too small for a job. It's for 40 mtrs.
But making it is barely possible. It should have a Zo of 50 ohms ? And what
about the Q factor ? Looking on my swr meter it gets better when i put my
hands near it and at a very small C rate but still in the red.
Anyone ?

An LC circuit won't make a good dummy load.

A "pure" LC circuit is lossless... both the L and the C are pure
reactances, and neither will (or can) dissipate any power.

In practice, you could make an LC series circuit which would be 50
ohms resistive at a single frequency... you'd use a really lousy
inductor (one wound with many turns of small wire) which has a pretty
significant DC resistance. The series C would be just enough to tune
out (resonate) the inductance at the frequency you're going to be
using it with. The inductor's ESR at this frequency is going to be
higher than its DC resistance, due to skin effect and other loss
phenomena.

The resulting circuit would *not* be 50 ohms resistive at other
frequencies. It'll have a capacitive reactance at lower frequencies
and an inductive reactance at higher frequencies.

Seems like an expensive and annoying way to make a dummy load. Unless
you've got other requirements you haven't mentioned, I'd suggest just
making a halfway-decent dummy load from a bunch of noninductive
resistors (e.g. in series-parallel) mounted on some sort of heatsink.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Dave Platt" wrote to us
It started when the dummyload was too small for a job. It's for 40 mtrs.
But making it is barely possible. It should have a Zo of 50 ohms ? And what
about the Q factor ? Looking on my swr meter it gets better when i put my
hands near it and at a very small C rate but still in the red.
Anyone ?
A "pure" LC circuit is lossless... both the L and the C are pure
reactances, and neither will (or can) dissipate any power.

Not dissipate but surely radiate in a closed box!


In practice, you could make an LC series circuit which would be 50
ohms resistive at a single frequency...

Yes, it's for a single frequency. Since an antenne is also a LC-circuit
that doesn't get warm but radiates in fields.

I had it all open built so i suspect mantelwaves etc. But on paper a series
LC circuit forms a loading of a particular Zo. This statement is still right?
Lets say an input on a semiconductor p.a. stage with an LC series circuit..

Suppose we feed 20 watts in a p.a. but don't want to use a dummyload
in the p.a. Come on this is done all the time.

On Mon, 23 Nov 2009 19:18:37 +0100, "Ronald" wrote:


Suppose we feed 20 watts in a p.a. but don't want to use a dummyload
in the p.a. Come on this is done all the time.


Ronold,

Your discussion gives every impression that you don't know
1. what a dummy load is
2. what it is for
3. why it is principally resistive.

Your statement above is true on the face of it, but has no meaning
beyond a tautology: you don't use a dummy load IN a p.a. because you
don't use a dummy load in a p.a. So what?

So, the direct answer to the question in the Subject line:
How can one make a dummyload from a LC circuit ?
is:
You don't

You seem to be hedging around another purpose but don't want to tip
your hand. Obfuscation does not help find answers.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 23 Nov 2009 19:18:37 +0100, "Ronald" wrote:


Suppose we feed 20 watts in a p.a. but don't want to use a dummyload
in the p.a. Come on this is done all the time.


Ronold,

Your discussion gives every impression that you don't know
1. what a dummy load is
2. what it is for
3. why it is principally resistive.

Your statement above is true on the face of it, but has no meaning
beyond a tautology: you don't use a dummy load IN a p.a. because you
don't use a dummy load in a p.a. So what?

So, the direct answer to the question in the Subject line:
How can one make a dummyload from a LC circuit ?
is:
You don't

You seem to be hedging around another purpose but don't want to tip
your hand. Obfuscation does not help find answers.

73's
Richard Clark, KB7QHC


I'd like to know more about the 'mantelwaves'!

Chris

On Mon, 23 Nov 2009 19:18:37 +0100, "Ronald" wrote:

Suppose we feed 20 watts in a p.a. but don't want to use a dummyload
in the p.a. Come on this is done all the time.

The only reason that works is that the transmitter has a protection
circuit that detects high VSWR and either reduces power or shuts down.
The idea is to not destroy the output xisitors. Into an infinite VSWR
load, you can easily find twice the collector voltage across the
output stages, which on many radios, will destroy the xsistors. With
other type of loads, the output stages could easily draw twice their
rated current, this time destroying them with too much dissipation.

Judging by your wording, I don't think you have a clue what a dummy
load does or where to install it. It's not "in" the PA stage. It's
after the PA, after the low pass filter, after the T/R switch, and in
place of the antenna. You use a dummy load for testing and
measurement so that you don't pollute the airwaves or fry your radio
tuning into an unknown load.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

A "pure" LC circuit is lossless... both the L and the C are pure
reactances, and neither will (or can) dissipate any power.

Not dissipate but surely radiate in a closed box!

It will not radiate efficiently.

The radiation resistance of a physically-small coil is going to be
very small... a small fraction of 1 ohm. What your transmitter is
going to "see" will consist of:

- The radiation resistance of the circuit (probably less than 1 ohm),

in series with

- The loss resistance (mostly in the coil)

in series with

- Whatever reactance remains after the series L and C resonate against
one another.

If the coil is "perfect" (lossless) and the L and C reactances are
equal and opposite, then all you'll be left with is the radiation
resistance.

If it's in a closed (shielded) box, matters get *worse*. The presence
of the reflections from the walls will reduce the radiation resistance
even further.

Will your transmitter be happy with a load which looks almost exactly
like a short circuit to ground.

Yes, it's for a single frequency. Since an antenne is also a LC-circuit
that doesn't get warm but radiates in fields.

And it radiates with reasonable efficiency, precisely because it's
physically large - it has a substantial radiation resistance.

This will not be the case for a physically-small LC circuit.

I had it all open built so i suspect mantelwaves etc. But on paper a series
LC circuit forms a loading of a particular Zo. This statement is still right?

No, I don't think so... at least, not in the way that you think.

If you show us the specific circuit you're thinking of (components and
their values, wires, and desired operating frequency) we can analyze it.

Lets say an input on a semiconductor p.a. stage with an LC series circuit..

A series LC circuit is not, by itself, going to present a resistive
impedance.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Suppose we feed 20 watts in a p.a. but don't want to use a dummyload
in the p.a. Come on this is done all the time.

Sure. If one does not want to have a dummy load in the PA, one uses
an *external* dummy load.

A dummy load, almost by definition, is something which accepts RF, and
dissipates it as heat. An ideal dummy load doesn't radiate at all. If
it does radiate significantly, we call it an "antenna".

One *cannot* magically make those 20 watts disappear. They either
have to be radiated (out into space) or converted to heat and
dissipated.

A small LC circuit will not do a good job of either of these things...
won't radiate efficiently and won't have enough resistance to act as a
comfortable dummy load (e.g. Zo near 50+0j).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!