With a 70 ohm load you should have around 20 watts reflected when reading
550 forward. Subtract the reflected power reading from the forward reading.
550 - 20 = 530 watts should be the true forward power.
Don't worry too much about the resistance of the old heathkit resistor in
the load. I have one around here that I cooked years ago that is no longer
50 ohms but still works fine for a general load. It doesn't get used much
any more as I now have better stuff but it served well for many years.
When those resistors are overloaded chunks of the material break away
causing the resistance to go up. It won't handle as much power but if you
are careful with it you will be ok.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of David C. Hallam
> Sent: Thursday, December 03, 2009 8:40 AM
> To: Dennis OConnor
> Cc: amps@contesting.com
> Subject: Re: [Amps] Dummy Loads & Wattmeters
>
> My original posting has garnered a lot of interesting discussion. To
> reduce everything to the simplest what I am trying to determine is;
>
> If I measure the output of an amp into a dummy load with a measured DC
> resistance of 69/70 ohm using a Bird 43 wattmeter and I read 550W, can I
> have confidence that this reading is within +/-10% of actual?
>
> David
> KW4DH
>
> Dennis OConnor wrote:
> >
> > The simplest way for a ham to accurately measure to within 5% power is
> to measure the RF voltage impressed across a known impedence...
> > There are a number of ways to refine your voltage measurement to be
> within 5% deviation from NBS, but quick and dirty works just fine for
> me...
> >
> > The very easiest is to get a dummy load... And get a 1% precision
> resistor that is near to 50 ohms - Mouser, et. al. about a $1.70...
> Use the resistor to measure your VOM and calculate a correction factor
> then measure the dummy load, apply the correction factor from the
> precision resistor and you know the DC resistance of your dummy with
> near 1% precision... (ya, I know DC and RF ain't the same - I said
> quick-n-dirty)
> > Once you know that, you apply the RF from the amp to the dummy load
> and measure the RF voltage... That measurement can be direct, with an
> oscilloscope, or by rectifying the RF to DC and measuring that...
> > If you are using a Fluke, or other quality VOM, you have a known
> precision factor for the DC measurement and can directly calculate your
> supposed precision value - just remember to add the 0.7 volt you lose
> across the diodes, to the measurement...
> > If you are using an oscilloscope you can take the RF voltage number at
> face value compared to the scope's built in calibrator - or there are
> other ways, just like the VOM + Precision resistor, to derive a
> correction factor... It would take a very long post to describe all the
> ways to do that - but i trust hams to come up with simple solutions...
> >
> > So for most hams, a dummy load, precison resistor, a couple of diodes
> and a cap, and a VOM, will get you into the ballpark of laboratory
> precision for a few bucks... Then this can be used to tweak your
> wattmeter to be right on (within your precison range) at the power
> output you normally run - or to make a correction card for the readings
> it currently gives (just like the compass on my boat)...
> >
> > denny / k8do
> >
> >
> >
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> >
> >
>
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