[1000mp] Quadra Amp

[Adam Farson]

Hi guys,

I have been following this Quadra thread for a few days, and would like to
offer some comments. (My station consists of a Quadra with an IC-756Pro II
as exciter.)

There is a reflectometer between the ATU output and the antenna switching
relays. This reflectometer measures the reflected power from the load, and
will trigger lockout (and the VSWR > 3:1 error) for load VSWR > 3.1. Whilst
tuning, the Quadra switches a 20 dB attenuator into the signal path to
minimise QRM during the tuning cycle. 
 
The problem originally reported by Mark may be due to one of several causes.
Typically, these can be:

a degraded feedline or antenna component arcing over at high power. 

EMC issues such as RF feedback from an improperly-terminated antenna system
or incorrect grounding. 

These problems may not be apparent during the low-power tuning cycle, but
will manifest themselves at full output.

The best way to check for faults in the feedline and/or antenna system is to
sectionalise the system. 

First, check the amplifier with a 50-ohm termination connected to the output
via a short jumper and an external power/SWR meter. This test should include
a tuning cycle and a CW transmission at 1kW output. Then, move the
termination to the far end of the feedline (if convenient), and retest. If
all is OK so far, test with the antenna at low power (say 50W output), then
gradually increase drive and check for a sudden rise in load VSWR. This will
reveal breakdown-type failure modes in the antenna system. (I have
experienced this with wet traps arcing over in the BC rain.)

If the test fails at high power with the feedline terminated in 50 ohms, the
test described above can be performed by gradually increasing drive until
the breakdown-type behaviour occurs.

Note that if an external tuner is used with the Quadra, the internal
autotuner must be disabled. Also, as the Quadra input circuit is broadband
and flat, the exciter's autotuner (if fitted) should be disabled.

There is more information on my Quadra page:

http://www.qsl.net/ab4oj/quadra/quadra.html

In general, when dealing with such costly equipment, an "educated guess"
should always be backed up by cautious and well-planned testing.

Cheers for now, 73,
Adam VA7OJ/AB4OJ




-----Original Message-----
From: 1000mp-bounces at mailman.qth.net [mailto:1000mp-bounces at mailman.qth.net]
On Behalf Of Rick Stealey
Sent: 19 July 2004 11:48
To: 1000mp at mailman.qth.net
Subject: Re: [1000mp] Quadra Amp


Ok, good work.
Now .....  I forget if you said you had an antenna tuner.
We need to find out if the problem is the amp not liking a little swr, or if
there is pickup of rf (radiated from the antenna) getting into some lead. 
(When you used the dummy load you eliminated both causes, right?).

How can we determine this?  One way would be to use the antenna tuner
between the rig and dummy load to CAUSE a little SWR, instead of correct for
swr.

I said we would run controlled experiments, not guess.  But the temptation
to guess is sooooo great !!!!

Rick  K2XT

ok - I can't resist - my guess is the amp doesn't like hardly ANY SWR.  But
verify this, please!!!

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