Posted by
P.B. Christensen on
Dec 08, 2011; 8:10pm
URL: http://elecraft.85.s1.nabble.com/KAT500-update-tp7072932p7075697.html
Vic,
Deja vu -- Seems like we had just discussed this on the AMPS list! In this
case, I'm with you on keeping the balun external to the tuner. That gives
the user an option of installing the balun of their choice immediately after
the tuner, or remotely through a short section of high-quality coax to join
the balanced line. It's useful in cases where the shack is adjacent to an
outside wall and the balun can be hung adjacent to a window or other
opening. A symmetrical tuner would still be preferred for balanced lines
where the balun *should* be placed at the tuner input.
It would be an interesting exercise to compare the production cost and
resulting sales price of a switched unbalanced "L" versus a symmetrical
tuner with a balun on the input but with switched, fixed components.
A few design-related problems: (1) ability to switch between balanced and
unbalanced lines with a symmetrical tuner while keeping efficiency high and
cost low; and (2) any commercial version would need to be offered so that it
could accommodate a line input Z range of not only 50-5K ohm but also Z
values less than 50 ohms for folks with short antennas. In the case of a
high-pass design, that requires switched reversal of the shunt C so that it
can be placed on either side of the balanced inductance. And, that's where
it could get messy with cost and component layout to keep efficiency high.
After building a motorized AG6K tuner, I realized that the complex vacuum
relay arrangement could have been omitted. If reasonably "full-size"
antennas are used, it's tough to conceive a condition when using 600-ohm
line where the input Z drops below 50-ohms - even with odd quarter-wave
multiples. After the tuner was built, I worked out several dipole and loop
models using 4Nec2 and TLW software. Change of bands, frequency, and line
length all resulted in reasonable input Z. Another factor when operating
into low Z line inputs -- the lower the line input Z, the greater the
importance of component Q to keep efficiency high, especially on the low
bands. So, now there's a bunch of expensive switching components that will
never be used since I don't operate with short antennas, at least not at
home. Many folks wouldn't have the choice -- and thus the need to offer a
"one size fits all" type of tuner. Probably the best approach is an
unbalanced tuner for coax and "output balun" feeds, and if there's enough
demand, a separate symmetrical tuner just for balanced lines.
It sure is nice being able to toss these ideas around and not worry about
any financial risk, unlike the manufacturers!
Paul, W9AC
----- Original Message -----
From: "Vic K2VCO" <
[hidden email]>
To: <
[hidden email]>
Sent: Thursday, December 08, 2011 12:22 PM
Subject: Re: [Elecraft] KAT500 update
>I t's been shown conclusively that an unbalanced 'floating' tuner with a
>balun on the
> input does not provide better balance than one with the balun on the
> output. Yes, I know
> that there is such a tuner in the ARRL handbook and Alpha is manufacturing
> one. But the
> math doesn't lie. So unless Elecraft wanted to use a balanced network --
> which would make
> the tuner MUCH more expensive -- I would prefer one without a balun. Then
> those who do not
> use balanced lines would not have to pay for the balun.
>
> On 12/8/2011 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
>>>
> --
> Vic, K2VCO
> Fresno CA
>
http://www.qsl.net/k2vco/>
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