On 6/1/2010 6:29 PM, Jim Thomson wrote:
> ## I agree, if it's just for a 50 ohm choke balun.... just use teflon coax.
> like RG-393/400/303... and either
> wrap it around type 43/31 cores... or slide large diam type 43/31 beads
> over the coax,
> and be done with it.
>
To answer some of the questions:
-1:1 current balun is the goal, 3.7-30mhz. 5k ohms choke impedance
across the entire frequency span. I guess that's a basic question: Can
it be done with one design? I think the answer appears to be no, no
matter what you do?? I think it might be provably impossible with
current materials. But then I see material like nanocrystalline? is that
optimized for too low a frequency? I saw some curves and thought it
looked interesting...
If it can't be done with one design, then people either
-don't agree 5k ohms is needed:, or
-use tuned baluns, and maybe multiple different tuned or a multiband
feedline, or
-ignore the issue, use one balun which they can't measure accurately,
and argue it "works".
-I'd like it to be mechanically more stable than Jim's coax loops, and
smaller. (4x4x2 pvc box)
-I ordered some RG-400 to see what's that like. Found some at 1$/foot.
But all that double shielding? Why is all that extra metal good? Seems
like it would just cause more capacitance. Jim says all capacitance is
not bad capacitance. But then again, there's +-20% variance in the
behavior of the core material due to manufacturing. So I can't see how
there's capacitance tuning that you can rely on since its' interacting
with a ferrite that has +-20% variance.
And Jim says without the right gear, we can't measure it.
-I mistyped and said 18 awg. The wire I was going to use is teflon
insulated, silver plated copper, stranded, 16 awg. (RG-400 is < 18 awg
center conductor).
-It's all the same amount of work. I'm just trying to understand why I
would do one versus the other. It's all easy. The main question I don't
understand is whether bifilar windings give a more broadbanded response.
Jim's papers makes me think it does? So I'm thinking people don't do
bifilar only because they think it's harder, or they worry about the SWR
bump when they shouldn't. And that threading more teflon over it because
you're worried about voltage is wrong (for me) since it makes the
bifilar impedance worse?
-I was just playing measuring a small number of turns (1-3) on varying
numbers of cores. It does seem like using less ferrite and more turns is
the way to go.
-Are you saying that you think the performance of a coax-turned balun is
equivalent or better than a bifilar-turned balun, assuming there is no
voltage breakdown issue, and the impedance discontinuity isn't a big
deal (short length relative to the overall feedline)?
Maybe it's clear to everyone what's "best"..but it's still not to me. It
seems like the answer is to have tuned baluns. But I can't believe
everyone out there has tuned baluns.
We started this saying the baluns at balundesigns could be outdone. I've
not seen the one true answer that makes me believe that yet.
What I think i see is a little bit of a cheat: tuning baluns so you say
they're "better" for restricted ranges within 3.7-30mhz?
Am I missing something? I know this is just a stupid little thing. But
if so, how come there's not one true answer for what's best, today? It
reminds me of discussions about winding plate chokes for amps. Jim
suggests his cookbook is best, but I don't want to do that (plus it's
the multiple baluns across the band solution)
So what am I missing? I bought some baluns from balundesigns. I still
can't tell that was reasonable or not reasonable.
-kevin
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