One of the nice things and one that is quite different than many other
radios is the fact that Tentec uses current monitoring of the PA stage.
Thus low current values caused by higher than 50 ohm nominal loads do not
cause the power output to "fold back". At the same time, a higher PA
current caused by a lower than 50 ohm nominal load will be controlled by
reduced driver power thus lower drive to the PA hence PA current is
controlled. In any event, the available power to the PA remains as a
constant and again does not fold back. In either case, the volts remain
constant from the power supply and the current is controlled to a max value
thus the power input remains the same regardless of load.
Yes, the input of the low pass filter and SWR at that point is one that we
don't see. In some cases the PA transistor combined Z is something near 0.9
ohms. Thus the low pass filter has a job transforming this very low Z of
0.9 ohms to 50 ohms nominal. Its transfer characteristic will vary with
load and more specifically the reactive components of the load.
Now regarding SWR, I have encountered cases where a component in the low
pass filter has failed more specifically as a result of trying to operate a
75M antenna on 160M without any type of external matching network. Those
voltage values can get nasty and can exceed the component ratings in the low
pass filter.
All in all, err on the side of caution and double check antennas and loads
at a reduced power level. And most definitely I agree with Jerry on the
fuses and the brands. All fuses are not created equal, regardless of the
printing on the case or package. My solution is a good quality switching
power supply with both current limiting and over voltage protection. For a
play on words, when all else fails, let it be the fuse that fails.
73
Bob, K4TAX
----- Original Message -----
From: "Dr Gerald N Johnson" <geraldj@storm.weather.net>
To: <tentec@contesting.com>
Sent: Saturday, September 22, 2007 9:36 AM
Subject: Re: [TenTec] (no subject)
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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