On Sat, 2007-09-22 at 00:07 -0500, Bob McGraw - K4TAX wrote:
> Folks there's two things to consider with regard to SWR. One being a load
> the is much below 50 ohms and the 2nd is a load the is much above 50 ohms.
> Now, should the load be greater than 50 ohms, say 250 ohms, then the SWR
> 5:1. This condition will not cause the PA transistors to draw excessive
> current thus likelihood of damage is nil. On the other side, if the load is
> say 10 ohms then the SWR again would be 5:1 but the PA current would likely
> be quite high Here Tentec uses a current monitor circuit, unlike other
> radios that use a SWR monitor circuit, and a reduction in current to the PA
> will occur. This is the one reason that Tentec radios do not fold back
> power with high SWR values.
>
> While it is not advisable to operate the radio with a SWR of 3:1 or greater,
> one must know if the SWR is caused by a very low impedance load or a high
> impedance load. One causes no reason for concern with the other will simply
> require the radio to reduce current to protect the PA devices. That is
> handled by the ALC circuit in the driver stage.
>
> To prove my point, I just turned my Omni VII on with no antenna connected.
> In Tune mode transmit the SWR was basically infinity to 1. The supply shows
> current was about 8 amps. With the output shorted, the current was about 12
> amps and again SWR was basically infinity to 1. PLEASE NOTE: Don't try
> this with you non-USA manufactured radio unless you obtain a RA from the
> service depot first.
>
> Regardless of the circumstances, it is always advisable to provide some
> external current limiting method, be it a very fast acting circuit breaker
> or a power supply with internal current limits.
>
> 73
> Bob, K4TAX
>
>
That's all true neglecting the impedance transformations of the low pass
filter. I'm not willing to bet that the load Z at the output of the low
pass filter will be the same at the input of the low pass filter. In the
extreme it might act as a lumped transmission line a quarter or 3/4 wave
long just exactly swapping the effects you report.
However the effects on the PA you report are correct when applied to the
SWR as seen AT the PA.
Tentec PAs tend to be conservatively designed so that mismatches don't
fry them, but the PAs are assured longer life if the power supply
current is limited or the current is interrupted by a fast acting
circuit breaker. Having checked the "fast" acting circuit breaker's
curve, I use an automotive fuse of the miniature tag style rated at 15
amps. Buss type ATM-15. Fuses will carry 125% of nameplate current
forever so it will carry 20 amps but gets increasingly sensitive to
currents above that. So far I've blown 3 with my Corsair II while tuning
an antenna tuner at full power. I've had no effect of PA performance.
Its important to use only fuses from a reputable manufacturer. Its been
recently reported that some Chinese automotive fuses don't open even if
the wires connected to the fuse holder have burned open.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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