On Sun, 2006-09-24 at 14:06 -0700, Jim Brown wrote:
> I'm encountering a strange problem with my Titan. If I follow
> standard practice and achieve a 1:1 match with the exciter, then
> take the amp out of standby, the amp typically shows mismatch on
> the order of 1.5 or 2:1.
>
> If I then re-tune the antenna for minimum reflected power on the
> Titan, the VSWR at the tuner, and also at a Bird inserted right
> after the Titan, shows a mismatch of 1.5 - 2:1. The mismatch seems
> to be greater at higher frequencies.
>
The directional coupler in the Titan is out of calibration or is not
capable of proper calibration because it has too poor a directivity. So
that it shows a match for a mismatched load.
> The bridge in the tuner and the Bird are consistent -- that is,
> when the bridge in the tuner shows a match, so does the Bird, 12
> inches of RG-8 from the Titan's output connector.
>
> I have three different tuners -- a Ten Tec 229B, a Ten Tec 238A,
> and Drake MN2000. It happens with all of them. I've traded cables,
> made them as short as possible, changed the switch that switches
> between the tuners (from a B&W to an Alpha Delta). The exciter is
> going through the Titan to get to the antenna for tuneup. I've
> traded Birds. I've eliminated the Bird. All of the cables are RG-8,
> all are short.
>
> So you say, go with the bridges. BUT: Grid current in the Titan
> tends to be much lower when the Titan bridge indicates zero
> reflected power, and rises rapidly when I don't. And grid current
> is the big NO-NO in 3CX800A7's, the output tubes in the Titan. I am
> careful to keep grid current down to an indicated 30 mA for the
> pair (120 mA is absolute max). I can't get close rated power
> without excessive grid current unless I tune the antenna for zero
> reflected indicated on the Titan.
You should NOT tune the amplifier by currents, but tune for maximum
output to the matched load. It is quite possible to get respectable grid
and plate currents and very little RF gain. I saw that in a Henry 2K3 in
a MARS station back about 1968. Tuned for currents only it had maybe 200
watts out with 100 watts of drive (KWM-2A). This MARS station ran two
shifts of operators running phone patches. One evening, something was
wrong with the S-line I'd been using working my dad, and the propagation
to Nam was down so I took over the new rig. When I finished tuning up I
had to back off the drive to get DOWN to a KW out and I had a good
contact with my dad. The military ops (I was military that year too)
wouldn't let me leave until I made them a new tuning chart tuned my way.
They reported Nam said the next day, "I see you finally got the final
turned on."
>
> The SWR indicator in the exciter (TS850) also agrees with the Bird
> and the bridge in the tuner -- IF I bypass the amp's RF circuitry
> and connect the radio direct to the tuner (or through the
> switches). This points to the RF path within the Titan between the
> SO-239 output connector and the final output stage. Has anyone
> encountered this before? Any fixes?
>
> 73,
>
> Jim Brown K9YC
The fix may be to get that good dummy load and try to adjust the
reflected power meter in the Titan for minimum reflected while driving
the dummy load. According to the designers of the Collins wattmeter many
years ago, it should be possible to get better than 30 dB directivity
with careful construction and adjustment.
One other phenomena may be some influence besides maladjustment or poor
directivity. That's diode drop. Showing a few watts reflected on a meter
built to carry a KW+ can get the RF voltage on the diode fairly small
and with 0.6 volts barrier voltage in the silicon diode that may show
the reflected power of a few watts as zero when it really is more, so
the lower power wattmeters may be showing a truer SWR than the high
power meter used at low power.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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