Re: Procedure to measure rf watts

Jim Potter
 

Bob,

Good advice. A lot of people don't think through what they are doing. There's some distance between theory and "what works". Sometimes at HF a light bulb tells you all you need to know.

I once rebuilt a BC-610, 250TH in the final. The load was five 100 W incandescent bulbs in parallel. You could guesstimate the power by the brightness of the bulbs. On 10m you could see a standing wave. The bulbs were not all the same brightness.

Jim

On May 22, 2019 10:48:09 PM MDT, "Bob Albert via Groups.Io" <bob91343=yahoo.com@groups.io> wrote:
Richard I pretty much agree with your comments.  But I'd like to add
that, at HF, precision connectors and fancy adapters are overkill.  I
have a T connector, UHF type, in the line from the rig.  I plug into
that an adapter from UHF to BNC and another adapter from there to
binding posts.  I connect the HP 410B to the binding posts.
Now of course I am aware that this is not good practice.  But I suggest
that, for frequecies below 30 MHz, it's close enough.  There are no
discontinuities more than an inch or so, much less than what rule of
thumb (one tenth wavelength) says.  My readings are close to what I
expect.  And if there is an error, well it can't be great and I don't
think FCC will be breathing down my neck for running too much power.
Should I desire to work at 2 meters, then I might be more rigorous.  (I
am reminded of an old exam question for ham license that suggested
using an HF SWR meter at 2 meters if that's all you have, and it's
close enough.)
Bob K6DDX
On Wednesday, May 22, 2019, 6:46:39 PM PDT, Richard Knoppow
<dickburk@...> wrote:

    At communication frequencies I use a General Radio 1800A
mainly because I have the proper adapter for the probe. The 410B
is fine but its adapters for any of the more common coaxial
connectors are pretty scarce. Otherwise its fine. The 1800A takes
a  shell for a GR 874 connector which screws into the end of the
probe, the banana plug acting as the center conductor. I then use
an N type T with UHF adaptors on two ends and the probe in the
center. I have a couple of dummy loads which have been measured
on a GR RF bridge so I know their actual impedance at the
measurement frequency. In general a DC resistance measurement
comes close and they are pretty non-reactive. I can then
calculate the power with reasonable accuracy.
    Both the 1800A and  410B have single diode rectifiers for RF
and both have the usual characteristic of being square law at
small voltages transitioning to peak reading at high voltages.
The scales are calibrated in the RMS value of a sine wave and
both have different scales for low and high voltages as required
by the characteristic of the diode.  I have calibrated my MFJ
tuner/SWR meter/power meter using this arrangement. It seems to
be reasonably accurate. A scope could be used but as you say
requires some calculation because it reads peak-to-peak.
    Nothing is as simple as it seems or as you would prefer.
    OTOH, a scope can tell you an awful lot about what is going
on in the transmitter.

On 5/22/2019 5:39 PM, Bob Albert via Groups.Io wrote:
  I use my venerable HP410B.  The ac probe has wide bandwidth and the
most sensitive range of 1 V should suffice for all but the weakest
power from a transmitter.  Full scale corresponds to 20 mW.  Half
scale, or 0.5 Volt, 5 mW.
Sure you can use an oscilloscope.  You will have some calculation to
do.  Further, you would be able to ascertain whether you are working
with something close to a sine wave.
Ideally, use both instruments, one for checking waveform and the
other for a quantitative reading.  The load, of course, needs to be
purely resistive.  If not, your 'power' reading will be wrong.
In any case, an oscilloscope isn't the best tool for precise
measurements.  The trace width and screen nonlinearities need to be
considered.  A spectrum analyzer might be a good idea, since it would
show each frequency component and allow one to decide if any are large
enough to influence the result.  Most spectrum analyzers have 50 or 75
Ohm input already in place; the pitfall there is to make sure you don't
burn out the termination.  Ten Watts is too much, and most aren't safe
above 1 Watt or less.  A dummy load and attenuator might be a good
idea.
When I measure the power out of my ham transmitter I can get an
accurate reading with the HP voltmeter.  I use either a dummy load or
an antenna with close to 1:1 SWR.  Without the linear amplifier I get
around 65-70 V reading (100 W or so) and with the amplifier about
220-250 Volts (over 1 kW), depending on tuning and load quality.  The
HP pointer movement is so well damped that I can get peak readings when
operating pulsed, such as a series of CW dots.
I also have an oscilloscope to see the wave; I couple it to the
system with a loop pickup.  I connect a coaxial cable to the 'scope and
short the other end around one of my voltmeter leads.  Thus, inductive
pickup.
When I use audio modulation I can measure peak envelope power on the
voltmeter.  The 'scope will enable me to adjust the modulation such
that I don't get clipping.  I can also verify the performance of the
speech processor.
Bob
      On Wednesday, May 22, 2019, 4:01:03 PM PDT, Randy.AB9GO
<@AB9GO> wrote:
 
  Hello everyone,

I have a need to check the calibration on some low power watt meters
(10 watts or less, 21 megahertz or less) and was wondering if anybody
had a favorite procedure using their scope to measure peak-to-peak rf
voltage without letting out the magic smoke.  My thought is to use a t
connector, hook one side to the transmitter, one side to a dummy load
and then the center connector straight to the 10X oscilloscope probe.
Any other precautions I should take?

Thank you,
Randy.

--
Richard Knoppow
dickburk@...
WB6KBL




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