Posted by
ke9uw on
Dec 08, 2011; 9:54pm
URL: http://elecraft.85.s1.nabble.com/KAT500-update-tp7072932p7076043.html
Hear's a link to Roy Lewallen's paper on the ineffective input balun. He mentions Tom Rausch, W8JI who got to this conclusion first.
http://www.eznec.com/misc/ibalbrf.txtChuck, KE9UW
aka Jack, BMW Motorcycles
________________________________________
From:
[hidden email] [
[hidden email]] on behalf of Wayne Burdick [
[hidden email]]
Sent: Thursday, December 08, 2011 11:34 AM
To: Oliver Johns
Cc: Elecraft Reflector;
[hidden email]
Subject: [Elecraft_K3] Re: [Elecraft] KAT500 update
Hi Oliver,
Sorry to disappoint you. Here's what happened:
We originally planned a floating L-network with an input balun, and
that's how we wired the prototypes. But recently we studied this
configuration in depth, doing both a theoretical analysis and lab
measurements. We confirmed the findings of other researchers that the
input balun has no advantage over an output balun (more on this
below). Taking the balun out in favor of a third antenna jack will
reduce the cost of the ATU significantly and enhance utility, and
those who need a balanced feed can still use an external balun.
Now for the theory:
When a balun is located at the output, between the tuner and a
balanced line, one side of the balun transformer is connected to
ground at the tuner and the other is connected to one side of the
balanced line. The full RF voltage is then applied to the the balun
transformer. For a given balun design, if that voltage is high enough
then the balun transformer will overheat.
When the balun is located at the input, between the balanced feedline
and an unbalanced network, one side of the balun transformer is still
connected to ground, and the other is still connected to one side of
the balanced feedline. So the voltage applied to the balun is exactly
the same. Therefore there is no advantage to floating the network.
Doing so also greatly complicates the tuner and increases cost.
If anyone is interested in more detail, we could post a drawing, as
well as references.
73,
Wayne
N6KR
On Dec 8, 2011, at 8:58 AM, Oliver Johns wrote:
> Wayne,
>
> When I talked to you briefly at the 2010 Pacificon, you seemed to
> say that the KAT500 would be a "floating-L" design. That is, it
> would contain an unbalanced L network, but that network and its
> circuit board would be floating and not connected to the metal
> enclosure. You said a balun would be put at the 50 ohm end, feeding
> a SO239 connector for the coax to the rig. The floating-L could
> then be connected to balanced lines, or to unbalanced ones by tying
> one side of it to the case at the antenna end.
>
> Now it seems that you are talking about a totally unbalanced tuner,
> with no provision for balanced antenna feeders. Do you mean to add
> an external balun on the *antenna* side of the tuner? Surely not.
> That seems an inherently bad idea for many balanced lines. The
> impedance there will be unpredictable, hence the need for the
> tuner. Not a good place for a balun IMHO. But if you put the balun
> on the 50 ohm end where it belongs, then the whole enclosure will be
> tied to one side of the "balanced" line, which will likely unbalance
> it.
>
> What happened? I had hoped to put the KAT500 in a weatherproof box
> of my own fabrication and use it between a balanced line to the
> antenna and a coax feed back to the rig. It could live out near the
> antenna, where it belongs.
>
> 73,
>
> Oliver Johns
> W6ODJ
>
>
> On Dec 7, 2011, at 5:22 PM, Wayne Burdick wrote:
>
>> We originally had two antenna jacks on the KAT500, with one of them
>> switchable between balanced and unbalanced.
>>
>> Recently we decided that a third antenna jack would provide greater
>> utility overall. (In my case, I'll be keeping a dummy load on the
>> third jack.) All three jacks will be unbalanced (SO239).
>>
>> We're hoping to add a high-power, high-performance balun to our
>> product line sometime next year that will serve the needs of those
>> using balanced lines.
>>
>> 73,
>> Wayne
>> N6KR
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