On Mon, 04 Jan 2010 11:57:33 -0600, Lostgallifreyan
wrote:

Richard Clark wrote in
:

At 1MHz, this 50 KOhms
would be a capacitance imbalance of 3 pF. Looking at the wire
dressing of the coils in the photo would suggest 3 times this easily
(and no attention has been paid to this at all).

That's the bit that seems most important, and I can also relate to it. It
doesn't take much to cause a few pF difference.

A few points occur to me though...

First, to get it out of the way.. if this was causing serious bother, which
it surely might if it is that bad at the lowest end of the SW range, what on
earth induces a designer to persist in thinking it's working? Either it
isn't, and he's deluded; or it is, so why?

That is answered rather simply: isolated experience. Having solved a
problem for single application does not make it a universal solution -
it is a good start however. For instance (as to this issue of
isolation), these designs were originally intended for lowfers where
this coupling would have been decimated (its impact reduced by 10).

Spinning out the same "successful" design to 10MHz without
consideration for this one issue I've pointed out would bring the user
to their knees: "I've tried everything and I still get this
interference!" Well, the user has not tried everything; it was simply
the right design in the wrong application which with a bit of effort
could be improved dramatically. Many of the layout and design
considerations for the lowfers are commendable and easily applicable
into the VHF. A VHF designer would have chosen other input topologies
is all.

In a sense, the VHF designer is also suspect for having an isolated
experience. Practical solutions are not always scaleable.

As I pointed out, very few changes are necessary and there is even a
simpler solution. You can shield the primaries from the secondaries
(kills the capacitive link). Unfortunately this brings a new problem:
where do you tie the new shield to? The wrong choice will actually
inflate the problem. As I said, this is not a simple field where
there are nesting layers of shields and broken shields with
non-contacting overlaps. Which one overlaps the other can bring
success or misery.

Second, I imagined balance to pretty much relate to symmetry. I hadn't seen
that file yet (even though I'd actually grabbed it with intent to), and the
looseness of the coil wiring isn't lost on me. My idea was to omit the amps
and just wind the toroids with neat symmetry to reduce obvious causes of
imbalance and take it from there. If it works, I use it, if not, I try
something else.

Well, here the topology you describe will carry the common mode
directly through (with some attenuation) as it is inherently out of
balance when the input primary's capacitive coupling is out of whack.
This is why I harp on "first principles." The amps have nothing to do
with the problem, they simply help illustrate the imbalance.

As to you "giving it a try" returns us to the heavily qualified
"success" of isolation. You could easily connect your antenna
barefoot to your receiver and experience no problems at all! This
does not constitute your "solution" as being universal. When you move
and try your "solution" again, it could easily fail for not observing
first principles.

However, this is not to impede you from simply getting on with it. The
hallmark of becoming successful is failing as many times as you can.

Given that I have more than once been told that I might
overdrive the input on the ATS-909 radio with a 18' whip in the back yard, I
decided that I might as well omit the amps as they only boost a few dB, and
instead rely initially on the built in attenuator for first efforts to see
what's out there, then consider building (or getting lucky with on eBay) a
preselecting filter on the input.

Ham radio was invented on a bread board. You need one coil
(selectable) and two variable capacitors that can be scavenged from
very, very old table top radios. You would spend more in shipping for
a "preselector" than what you would pay at the nearest junk store for
these three items. Someone who had to pinch pennies would do it with
one cap and one coil and a lot of alligator jumpers. Even this
solution can be made elegant and occupy a space no larger than a
pocket sized notebook. If you robbed these componets from very, very
old transistor radios, you could build it in a mint tin.

Third, the amount of effect a few pF has on a circuit would depend also on
the inductance, or resistance, or any delay in the circuit. I hadn't looked
closely at that (I'd want to see what happened with a simple test first), but
I am guessing that the simpler idea of a twin wire with transformers and no
amps would have a smaller risk of imbalanced signals, so a better common mode
rejection. Why not do the amplification after the second transformer where
balance is clearly irrelevant, if it has to be done at all?

As I said, the problem is in the topology, not the amplifiers.

I may still be missing something other than a grasp of quantities, but
whatever I do, it has to be something that aims to do what that scheme was
said to be able to do. I don't have a lot of space, and that radio ideally
needs a single antenna to cover all of its AM range, at least for initial
efforts. Ideally some scheme that can be improved rather than thrown out when
I need something better.

You have barely nicked the surface of possibility in discussion here.
Loop antennas (which can be truly balanced and passed through coax)
are a natural answer for a lot of the spectrum you want to listen to.
In fact, "shielded loops" are explicitly coaxial and extremely simple
to construct (although many web sources describe them incorrectly).
They exhibit very sharp nulls (when that matters), and you can lay
them over 90 degrees to make them vertically polarized (and then use
them in phased configurations for beam steering).

73's
Richard Clark, KB7QHC