As I've said before and not long ago, a 12 volt PA is very sensitive to
supply voltage and that affects its power output considerably. Even a
tenth of a volt will change the power significantly because the
available voltage swing is limited to supply - about 2 volts which is
only 11.8 volts for a 13.8 volt supply. And the output impedance match
is fixed, so if that load is set for 13.8 volts, the available power
with 13.9 volts will be 1.7% greater. If the supply gives 14.2 volts the
available power will be 6.9% greater. Enough to account for a
significant fraction of the differences reported on this forum. And if
the power supply dips to 13.2 from 13.8, the power will drop by 10%.
Really picky about voltage to make power.
73, Jerry, K0CQ
On 4/2/2011 6:22 PM, Don Jones wrote:
> Yea what Lee said! Perfect!
> I have three watt meters here and all three of them give me different
> readings.
> Bottom line, unless you are using a calibrated power meter, any power meter
> a ham uses or purchases is for relative indication purposes only.
> When I researched Ten Tec and found that the output of some of their rigs
> was rated at +/- 1dB, I made the same calculation Lee obviously did. When I
> purchased my OMNI VI Opt 3 the seller advised me he was only getting 80W out
> of the rig. I was not concerned and bought it anyway.
> When I got it here, I powered it up using a TT 961 power supply and got 100W
> out indication from my new VI on my "power meter".
> My guess is that my higher power supply has a slightly higher output voltage
> than the seller's Astron power supply resulting in a possibly higher power
> output indication on my un-calibrated power meter.
>
> 73,
> Don Jones
> Arlington, WA
>
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