[Elecraft] Balun on tuner input?

[Ian White GM3SEK]

Ross Primrose N4RP wrote:
>On 12/9/2011 6:09 PM, Jim Brown wrote:
>>
>>>    If it is an automatic antenna tuner, that is less of a problem.
>> Except that the automatic circuitry, and it's power supply must also be
>> isolated from ground. That part of the design problem seems non-trivial.
>>
>
>Seems fairly simple to me, use a transformer in the PS, and float the
>secondary side...
>

Nope - there will be a large capacitance between the transformer 
windings. In parallel with this will be the capacitance between the 
entire RF sub-chassis and the outer metal case. If the choke balun is 
connected on the input side of the ATU, these quite large parallel 
capacitances will partially unbalance the output.

Worse yet (as Alan points out), if you select a coaxial output with the 
SO-239 socket grounded to the case, the balun becomes short-circuited 
from end to end. Alpha are working on a very similar auto-ATU but their 
proposed schematic shows exactly how 'broken' this concept is:
<http://www.rfconcepts.com/PRODUCTS/New-Products/Alpha4040>

Elecraft have done well to avoid this trap, and to decide that the 
KAT500 will be all-coaxial - because there simply isn't a "one way fits 
all" solution for a balanced ATU. Different user installations will 
require different solutions on different bands, so the optimum solution 
will always involve some customization, swapping or switching between 
1:1 balun, 4:1 balun and 'no balun'.

Please don't be confused by some ATU manufacturers' efforts to sell us a 
"one way fits all" balanced ATU. They're only doing that because a fully 
versatile engineered solution would be too complex and expensive. So 
Engineering steps back and Marketing takes over instead - bending *your* 
perceptions to fit what they have to sell.

Again, Elecraft have done well to avoid this.

Some time ago it was suggested that an all-coaxial KAT500 should also 
include programmable band-switched relay control outputs (rather like 
DIGOUT0 and DIGOUT1 on the K3) so that users have the option to 
configure their own custom relay switching if they need it. Did anything 
happen to that suggestion?


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek