I was able to make a few more tests without getting frostbite. :)
I disconnected the coax from the matching transformer (at the
antenna) and put the coax into a dummy load. All OK, exactly the
same as when the dummy load is in the shack. I guess the coax is OK.
Next I connected the coax as usual to the primary of the matching
transformer. I put the dummy load on the transformer secondary and
set the transformer taps for 1:1 ratio. All OK, amp was completely
happy. I did see a very small phase error between current and
voltage on the scopematch, but nothing more than a couple of degrees.
It seems the problem must be RF getting into something, RF current
where it does not belong, but only when feeding the antenna.
An important note: There is no such thing as complete isolation of
antenna ground from shack AC power ground here. Both towers are
grounded for lightning protection, one has an extensive radial field
because I shunt feed the tower on 160m. All of the 160m - HF - VHF
coax cables are grounded to antennas and towers, so the antenna
ground is connected in the shack to other equipment. Of course that
equipment is connected to the power ground through 3 wire cords.
What can I do?
73,
Paul N1BUG
On 12/28/2017 03:19 PM, N1BUG wrote:
Hi Clemens,
Thank you for taking the time to run these simulations!
My Ultimate 3S does have a LPF before the feed to the PA. I
understand some PAs prefer a square wave. Should I remove the LPF? I
have a function generator which can make the same power as the
Ultimate 3S with either sine or square wave. I could try that as a
test...
Today I made some more tests. I can run a little more power before
trouble begins when I have the primary (coax) and secondary
(antenna) grounds tied together at the antenna! When I isolate the
grounds, the problem is worse! What does this tell us?
I'm not sure how to make an effective common mode choke for this
frequency. I can try another transformer at the shack if I can find
a low power version that uses an inexpensive core. At this point I
don't want to buy 3 x FT-240-77 cores for a <20W test. Any
suggestions? I have some FT-144-JC cores. Any chance these could be
used for a test?
Could I have some mistakes in my isolation transformer? I have
primary turns spread around the whole core, with the secondary turns
around almost the whole core placed between primary turns. But the
primary and secondary windings do not start at the same position on
the core. I thought this wouldn't matter. Maybe it does?
73,
Paul
On 12/28/2017 08:00 AM, Clemens Paul wrote:
Hi Paul,
I already have plans to build the big T type LPF.
Out of curiosity I've done some spice simulations of your PA.
They indicate that a T type LPF won't help at all.
This was to be expected because your PA is not a class D voltage mode
push pull PA like W1VD's or IK1HSS's or 2E0ILY 's.
Indeed the T type LPF will cause considerably higher voltage spikes at the
FET'S drain.
In the attached screenshot you can see three simulations,each with a different
output filter.
A shunt C LPF,a series L LPF and even a diplexer filter.
The red traces represent voltage at the drain,green is DS current.
With the diplexer voltage spikes are lowest.
But I obviously the filter is not your problem.
It seems that the near field of the antenna is causing the trouble.
I would suggest to try a second isolation transformer or a good
common mode choke *at the shack* end of your coax run instead of building a
T-type LPF.
P.S. Has your Ultimate 3S a LPF installed before you feed it into the PA?
73
Clemens
DL4RAJ
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of N1BUG
Sent: Monday, December 25, 2017 1:15 PM
To: [email protected]
Subject: Re: R: Re: LF: TXing 2200m WSPR
Hi Marco,
I don't detect any heat in the LPF but at this low power level it
could be difficult to detect it.
I already have plans to build the big T type LPF. I have the
T-225A-2 cores and metal to make a compartmented box. Capacitors are
on the way, could arrive around 29 December. I still need to order
the wire and some kind of material for mounting the big inductors.
I'm making that LPF because eventually I will also build the big
amplifier. I'm not sure when that will happen, probably a bit late
for this LF season.
The most curious thing is what seems to be some measurement error or
impedance transformation but the cause remains elusive. I hope to
find the reason so I can be more confident about measurements when I
go go high power.
73,
Paul N1BUG
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