Re: R: Re: LF: TXing 2200m WSPR

[N1BUG <paul@n1bug.com>]

Hi Clemens,

OK, I understand about capacitive coupling in transformers. So I 
tried a simple common mode choke. At first 22 turns using RG-178 
coax on a FT-114-JC (yes, made a typo last time). No change for 
better or worse with the choke.
Maybe 22 turns is too much so I made it 18 turns. No change. 16... 
14... 12... 10 turns. No change. Is the core too small? I could try 
stacking a few of these.
I still have to try building a shielded box for the PA and LPF. This 
will take some days. Maybe it also needs shielding between input and 
output circuits of the PA.
BTW I forgot to mention during the night I also tried a CM choke on 
the DC power feed (16 turns, FT-114-JC). No change at all.
If I knew for sure that a different (properly built, proven design 
class D) PA would work here, I would not put any more effort into 
this PA. The trouble is I can't know that another PA would be OK.
Note: RG-178 on a coated core is very slippery! But I was able to 
keep the turns tight with some tape!
Paul N1BUG



On 12/29/2017 07:08 AM, Clemens Paul wrote:
> Hi Paul,
>
> > I can try common mode chokes instead of isolation transformers, but
> > will this really be better? Seems if the problem is common mode
> > current the iso xfmrs should kill it...??
> Basically yes.
> But we don't know how much capacitively coupling there is between primary and 
> secondary
> which might partially spoil the RF isolation.
> I would just give it a try:
> One chocke at the PA output is no big deal.
>
> > I still have a couple of ideas to try: Replace 50uH choke in PA with
> > different type core.
> In Spice there's only a ca. +/- 0.5Vp voltage swing across the choke.
> Hence the flux is less than 10% (3.7mT) of the max. flux for this core.
> So no problem with that choke.
>
> > Use completely separate bias supply.
> In Spice it makes no difference.
>
>
> 73
> Clemens
> DL4RAJ
>
>
>
> > -----Original Message-----
> > From: owner-rsgb_lf_group@blacksheep.org
> > [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of N1BUG
> > Sent: Friday, December 29, 2017 11:40 AM
> > To: rsgb_lf_group@blacksheep.org
> > Subject: Re: R: Re: LF: TXing 2200m WSPR
> >
> > During the night I made a few more tests...
> >
> > Iso xfmr at PA output. 14T:14T on FT-114-JC core. Made the problem
> > slightly WORSE.
> >
> > Iso xfmr at amp input. 4T:4T on 3 x BN73-202. Made the problem
> > slightly WORSE.
> >
> > Removed iso xfmr from amp output, left iso on PA input. Slightly
> > better than without iso anywhere, but still bad.
> >
> > Tried again all configurations of iso xfmrs (at antenna, at PA
> > output, at PA input) using HP 3325B to drive the PA. No difference
> > from using the Ultimate 3S.
> >
> > When I use an iso xfmr at the antenna the problem is WORSE than when
> > I have the coax connected to the antenna ground. It's not a big
> > change, but measurable and repeatable.
> >
> > Considered this might be an instrumentation problem with RF
> > affecting the HP 1980B oscilloscope, but I can see the signal
> > quality degrade in a receiver when the problem occurs so I don't
> > think so.
> >
> > I can try common mode chokes instead of isolation transformers, but
> > will this really be better? Seems if the problem is common mode
> > current the iso xfmrs should kill it...??
> >
> > I still have a couple of ideas to try: Replace 50uH choke in PA with
> > different type core. Use completely separate bias supply. However
> > since the PA works perfect into a dummy load I am not sure the PA is
> > the problem.
> >
> > I will try to make a shielded box for the PA. That will take some time.
> >
> > The Ultimate 3S is in the metal case sold for it. This is not great
> > because of the paint (or powder coat) at mating surfaces. I will try
> > to work on that but since using the HP 3325B as a PA driver does not
> > make any change I have doubts this will help.
> >
> > Paul N1BUG