Phil,
I just took a look at the PowerMaster's specifications on p. 25 of the most
recent on-line manual. For both the 3KW and 10KW couplers, its rated
accuracy is specified down to 1-watt. It's a good thing that published
accuracy is cut off at 1-watt because accuracy greatly suffers below that
level where the LP-100 and Alpha 4510 stay in agreement (Power & SWR) with
each other and the scope into a precision load down to 100 mW.
However, the PowerMaster's SWR function is not specified as a function of
power. Like you, I found that the SWR function does not work down to QRP
levels. For most folks, it's not an issue, but for me, it tells me that
when the competing products can accurately measure power from QRO down to
QRPp (e.g., 100 mW), that they probably offer a superior overall design.
Even when normally running QRO, having the ability to look at QRPp levels
can make a difference when trying to find the cause of some problems. For
example, several months ago, I had an intermittent balun feeding a Force12
Sigma 40. At powers of roughly 100W, I would see the SWR change. I could
blast the antenna with 1500-watts and all was fine for a few days, until the
erratic behavior started again. I finally ended up lowering power in
increments to see if I could better assess the situation. As I brought
power down below 2-3 watts, SWR would always jump very high, almost to an
open. But, bring the power back up to +5 watts and the problem would
disappear or become intermittent. With the Alpha 4510 and LP-100, I could
see this action. The PowerMaster was useless for this testing since it does
not measure power accurately at QRPp levels, nor does it display SWR at QRP
levels where this type of testing is valuable. The root cause? Enough
aluminum oxidation had occurred in the last several years to where a
high-resistance had built up at the antenna feed. The blasting of RF at QRO
levels was enough to re-establish conductivity for just a while.
Shame on me for not conducting preventive maintenance with Penetrox or
Noalox compound. I took the easy way out and just assembled the hardware,
not preparing for the long-term.
Paul, W9AC
page (24----- Original Message -----
From: "Phil & Debbie Salas" <dpsalas@tx.rr.com>
To: <amps@contesting.com>
Sent: Tuesday, December 08, 2009 9:58 AM
Subject: [Amps] FW: Decent HF directional coupler for RF power meas.
> My PowerMaster tracks very nicely down to 2.5 watts (within a few percent
> of my TekTronics TDS-2022). 2.5 watts is the lowest power I can get out
> of the IC-706MKII that I'm using for a source. Somewhere around here I
> have a power 6dB attenuator. If I can find that, I'll look at lower
> powers. The PowerMaster does stop computing SWR when you get below 4.5
> watts though. So I would definately not consider it a QRP SWR meter. The
> PowerMaster does have diode drop compensation (I got a chance to look at
> the schematic).
>
> Phil - AD5X
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> Amps@contesting.com
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>
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