I use a TT model 253 tuner with my OMNI-VII or CORSAIR-II and TITAN-425E (when
required) usually to extend the usable BW of my top loaded HF-2V w/160 meter
kit. On 160M the top loaded HF-2V w/160M has a very narrow 15 to 35 kHz 2:1
VSWR BW. The TT-253 requires about 45 W input to start tuning on 160M and as
much as 85 W on 10M. The maximum power for tune up of the TT-253 is 150W. The
MAX power AFTER tuning is 2 KW. The tuning accuracy of the TT-253 is 1.5:1 VSWR
maximum AFTER tuning and 2.5:1 VSWR maximum DURING tuning according to the
manual. During tuning a resistive pad is switched in line.
Given the preceding, I have seen MOMENTARY OMNI-VII TX METER (SWR) readings of
3:1 or greater during TT-253 tuning (at 50 W) and OMNI-VII HIGH CURRENT
warnings if I forgot to reduce the OMNI-VII POWER from 100W before cycling the
OMNI-VII TUNE PWR to start the TT-253 tuning cycle. I have seen longer periods
of HIGH CURRENT warnings when I have failed to retune the antenna after a QSY
and for that reason I now keep the OMNI-VII TX METER in SWR so that can monitor
the SWR seen by the radio.
I have routed the TX EN / TX OUT loop through the TT-253 and TITAN-425E so that
if the TT-253 cannot find a match and goes into FAULT, the keying loop opens to
shut down the transmitter.
Over the past thirty plus years, TenTec transmitter protection circuits
(magnetic breakers or current limiting circuits) have kept me out of trouble
when I switched to the wrong antenna, no antenna, or an unsuitable match for
the output transistors of the OMNI-C, CORSAIR-II, PARAGON, OMNI-VI+, and now
the OMNI-VII (and the old CORSAIR-II is still on the desk). I do not believe I
have been lucky to have not damaged a radio, rather I feel TenTec design
engineers knew mistakes happen during operating, or antennas fail during
operating and knowing that they went about protecting the PA by limiting
overcurrent to the PA. Having tested the TT power supplies’ built in PA
protection has always given me confidence to share my radios during Field Day.
73 ES DX,
Gary -- AB9M
I am an Ambassador for TenTec and may receive compensation as the result of my
recommendations of new TenTec products.
From: Bob McGraw - K4TAX
Sent: Saturday, January 04, 2014 2:38 PM
To: Ten-Tec-Omni-VII@yahoogroups.com
Subject: Re: [Ten-Tec-Omni-VII] Omni VII shut down at high current message?
In general I find that many radios monitor SWR and, at some predetermined
value, use the reflected value to establish some amount of power reduction.
Now with that said, the Tentec series of radios, being later models that I'm
familiar with, Paragons and later, do use current monitoring to establish power
cutback values.
In looking at SWR, as it was pointed out earlier in another message, identical
SWR values can be derived from different impedances.
The example:
Load = 50 ohms SWR = 1:1
Load = 100 ohms SWR = 2:1
Load =150 ohms SWR = 3:1
Load = 25 ohms SWR = 2:1
Load = 16.6 ohms SWR = 3:1
As you can see from the above, a 3:1 SWR can be the result of a 150 ohm load
or a 16.6 ohm load. A person needs a proper antenna analyzer to determine just
what impedance value is causing the 3:1 SWR. It is the low impedance load that
demands higher values of current and not the higher impedance load.
As Doug commented earlier; "Another alternative would be to fold back power
based not on SWR but on current to the finals. IOW, simply do not let the
finals cook themselves." This is the Tentec method in that the current of
the PA is monitored and used to reduce drive thus using the current reduction
method. Of course there is the "current trip limit" that does exist in the
Tentec radios and this does shut down the entire radio.
Yes, as in any designed system there are most likely ways of doing things
better or faster or more efficiently. But sometimes at much greater cost and
complexity.
Regarding the automatic tuners, most of the current models, via a series of
relays, switch values of C and values of L in various combinations to attain
the desired impedance match. While I do not physically have one of these in
residence to confirm this, I can see from the schematic of one of the more
popular tuners that the interval between the switching of relays can
momentarily short the input to ground. Thus at this point a near zero
impedance is seen by the transmitter. Ideally the sequence should be a
make-before-break combination in place of, what appears to be, a
break-before-make combination. If indeed the tuner does momentarily short the
input for an instant, then the PA current would accelerate to some maximum
value and thus the current limit circuit would react as designed and terminate
operation of the radio. This of course presumes one is tuning at rated power
output, i.e. 100 watts, as opposed to suggested much lower values of "tune"
power.
And yes, all tuners have some amount of insertion loss. Typically the greater
loss occurs when the tuner is called on to match a low or very low impedance as
opposed to a higher impedance. To this end, most 1/2 wave dipole antennas fed
with coax and at nominal heights will most likely have an radiation resistance
of less than 50 ohms. From many accepted tables, a horizontal 1/2 wave
antenna that is less than ~0.2 wavelength above ground will have a center
impedance somewhat less than 50 ohms depending on ground loss. In fact a
horizontal 1/2 wave antenna radiation resistance can vary between 10 ohm to
near 100 ohms depending on height above ground and accumulated ground loss. If
the antenna is over 1/2 wavelength in height AGL the impedance begins to
normalize at about 70 ohms with further variations in height having the affect
on center impedance between 60 ohms and 80 ohms. This confirms an earlier post
by Rob that a 75 ohm feedline would be more desireable than a 50 ohm feed line.
73
Bob, K4TAX
----- Original Message -----
From: Doug Smith
To: Ten-Tec-Omni-VII@yahoogroups.com
Sent: Friday, January 03, 2014 10:59 AM
Subject: RE: [Ten-Tec-Omni-VII] Omni VII shut down at high current message?
A couple of points:
>> 2 ways to do it; which one is better?
Actually, I would much prefer having the final amplifier stage shut down NOT
the entire radio. Of course, that’s yet another feature that would have driven
the cost of the radio higher. But, how cool would it be if an over-current
episode caused the 12 VDC circuit to the final amplifier to open and have the
entire screen start flashing to alert the operator. But, that would require a
“soft” reset button and probably a different sort of interrupter circuit.
Anyway, I think it’s a little harsh to shut the radio completely down.
Another alternative would be to fold back power based not on SWR but on
current to the finals. IOW, simply do not let the finals cook themselves.
Point being, there are more elegant solutions to either shutting the radio
off or folding back in the face of 1.9 to 1 SWR.
Unfortunately, elegance is not free.
>>. (BTW, all internal ATUs have a loss of about 10w).
I think it’s good, even important, to point out that there are losses in
antenna couplers. But, the above statement is not accurate. Coupler losses
are very dependent upon the load they’re matching, the configuration of the
coupler circuit and the components used. In many scenarios losses (assuming
100 watts into the coupler) can be much less than 10 watts and in some cases
more like 50 watts. Yes, there are losses but they’re all over the map.
73,
Doug, W7KF
http://www.w7kf.com
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