On Sun, 2007-09-30 at 22:06 -0500, Bob McGraw - K4TAX wrote:
> >From a person that runs AM, a carrier is a carrier. AM is the modulation
> mode. CW is the modulation mode. Hence the FCC emission definition of A-3
> for AM and A-1 for CW.
>
> A radio rated at 100 watts CW or 100 watts SSB PEP both are the same power.
> A 100% modulated carrier, hence AM, has sideband power that is equal to 4
> times the carrier power. Thus for a 100 watt PEP rated radio to correctly
> run AM the carrier must be reduced to 25 watts. Then the PEP value is 100
> watts.
>
> As to the fold back issue, the Japanese radios typically use the SWR
> monitoring circuit to adjust the drive to the PA. Thus as SWR increases the
> drive decreases and output power goes down. Now with Tentec radios since
> the Paragon and Omni series, they monitor PA current. As PA current
> increased above a pre determined value the drive to the PA decreases. This
> is specifically effective when load Z is below the design value of 50 ohms.
> The smart part of this design is the fact the with supply voltage constant
> at 13.8 volts as load Z decreases, drive decreases but only where PA current
> exceeds a certain value. Therefore the input power, volts x amps, is a
> given and the output power remains almost constant regardless of load.
>
> Now what must be realized is the fact the SWR is not a simple indicator of
> load. If the load is 50 ohms the SWR is typically 1:1. If the load is 150
> ohms the SWR is typically 3:1. Now here's the kicker, if the load is 16.6
> ohms the SWR is also 3:1. By monitoring PA current the DC power into the PA
> is controlled and maintained a safe value, specially with low impedance
> loads. The system can still deliver rated power into SWR values of loads
> both higher than 50 ohms and lower than 50 ohms. Simply monitoring SWR
> won't allow this to happen thus the reason the Japanese radios fold back
> power with SWR values above a specified amount. Each radio company
> determines where the foldback point begins. Some choose 1.5:1 while others
> choose values like 3:1.
>
> 73
> Bob, K4TAX
>
The software in the FT-857 has settings in the alignment menu to set the
alarm and shut down thresholds. A complete reset sets those points to
1:1.
73, Jerry, K0CQ
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