It sounds like a couple of things might be going on. First off, SWR meters
are notoriously inaccurate due to anomalies in their construction, and
calibration. If there is reflected power on the line, there will be certain
places on the line where the meter will indicate a lower SWR than other
places, so it's possible to see 1:1 on one meter, and 1.5:1 on another. If
adding a few feet of coax changes the readings, that's an indication that
things are not matched. The best test is to run the system into a dummy
load and they should all read 1:1 if the dummy load is good (don't bet the
farm that it is...I've seen 50 Ohm loads that measure 44 Ohms!).
Adjusting the tuner for lowest screen current isn't necessarily the best
match, but if you're not making 1500 Watts out with about 55 Watts in and
screen current < 30 ma, your finals could be bad, your plate voltage too
low, or something wrong in the input circuit. The Pi network should match a
fairly wide range of impedance, and should give an efficiency of 65 to 70%
for most combinations of R and X that don't deviate to far away from 50 +j0.
Ron N6AHA
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "Ten Tec List" <tentec@contesting.com>
Sent: Sunday, September 24, 2006 2:06 PM
Subject: [TenTec] Titan 425 Reflected Power
> 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 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.
>
> 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
>
>
>
>
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>
>
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