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Re: Pre-amp question



Dave:  You and Mike KA7HBB both had the same thoughts I did.  But the
problem at least *appears* to be more complex.  Here's why:

When I troubleshot this down-to-zero "SWR foldback," I got essentially the
same SWR reading as the antenna alone--about 1.2:1--no matter what length
of coax I used between another rig and the SWR bridge, and the SWR bridge
and the DCI filter.  It didn't even matter whether the DCI filter was in
the line or not.  So the SWR was not high enough to fluctuate significantly
with different line lengths.  Even if my first piece of coax to the SWR
bridge happened to be in a "node," I should have picked up a higher SWR
reading with a different length if the DCI was really out of tune.

I also measured the SWR curve of the DCI to a dummy load, and it was 1.1:1
across most of the 2m band, rising to about 1.4 or 1.5:1 at the high end.

Whatever the DCI filter is doing, my SWR bridge seems incapable of "seeing"
it.  The bridge is a  Maldol MR-2000 130-500 mHz power/SWR meter.  Is there
something a helical filter can "show" on a transmission line that a
standard VHF/UHF SWR bridge can't "see," even when measured with differing
lengths of coax?

On the other hand, the problem did *not* occur when I used an old ground
plane antenna that reads at about 1.8:1 at the lower end of 2 meters.  So
the rig can handle an SWR of nearly 2:1 without going berserk.  

Before I installed a sequencer, I did get a momentary shutdown during the
instant between my pressing PTT and the "brick" amp's transmit relay
closing.  This happened even on SSB, with the mic gain turned all the way
down.  A 1/2 wavelength of coax between the rig and the brick stopped this
(mostly), due to the "old CBer's myth" of the moving nodal point you
mentioned.

I told DCI about all this when I first got the filter.  They said that
given the evidence, they didn't think the filter was out of tune.  Yaesu's
silence was deafening.

What I'm getting at is that believe there is something hypersensitive and
quirky about the Yaesu 736R's SWR foldback.  I have read on amsat-bb that
Yaesu made it hypersensitive to avoid warrantee replacement of burned-out
final transistors.

One final question:  Is it possible to tune up the DCI filter with a dummy
load and wattmeter (by peaking the various sections), or is this something
that requires better test equipment.  If I can tune it up with what I've
got, I might try it, just to see if it solves the problem.

73,
--Peter, KD7MW

I said:

>> The DCI filter creates one little problem.  My Yaesu 736R's transmitter
>> does not like the 2m DCI filter.  The 736's SWR protection circuitry "sees"
>> the filter as a mismatch, and reduces output to almost nothing.  However,
>> an SWR bridge shows no significant increase in SWR over the antenna alone.
>> If I turn on the "brick" amp, it's quite happy to transmit.
>> 
>> I left message on Yaesu tech support's phone line about this, and they
>> never replied.  I tried several lengths of coax between the "barefoot"
>> Yaesu and the DCI, to no avail.  So now I always use the amp and sequencer
>> when operating 2m, turning the power down appropriately.


David WA0AUQ wrote:

>The problem is simple.  The filter is showing a high SWR to the 736.  The
"brick"
>amps protective circuitry is more tollerant of the SWR and so it does not
shut 
>down.  What it is really telling you is that the DCI filter needs retuning!
>
>Changing the length of the coax between the 736 and the filter will not and
>should not have any affect on the SWR.   "Trimming" coax for SWR is a old
>CBer myth that should have died a long time ago.  When you trim coax all you
>are doing is moving a nodal point along the line to the SWR bridge.  In other
>words, you are fooling your SWR bridge into thinking its seeing 1:1.
>
>This is why most of us use Bird wattmeters.  They are immune to this problem
>and give the same reading no matter where they are in the line.
>
>You could also try installing a simple impeadance matching network between
>the 736 and the filter.  A quarter wave stub tuner might work.
>
>--73-- David WA0AUQ
>
>

--- Peter
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