[Elecraft] [BL1] Seems lossy in all useful configurations

[David Woolley]

I tried using a BL1 (4:1) with my K2 and KAT2 to allow earth free 
operation using an improvised antenna in a first floor (US = second 
floor) flat, but I've come to the conclusion that under almost all 
circumstances, even though I'm getting reasonable SWRs, most of the 
power is going into heating balun.

I'm sure that this is partly through not originally understanding the 
limitations of baluns, in particular that they are only really usable on 
the equipment side of a balanced antenna coupler that presents the 
design impedance to the balun, or an antenna that does this without a 
coupler, so don't really make sense when impedance matching is done on 
the equipment side of the balun, as in the KAT2.

In particular, if the load is significantly reactive, with the 4:1 
configuration, the DC short that exists will also be a relative RF short.

However, some other concerns are:

- although the core type is not specified, so I cannot actually 
calculate the value, I'm concerned that two turns isn't going to be 
enough to get 10 times the load impedance across the whole frequency 
range (10 times is the typical guideline for transformers);

- flat line, rather than co-axial is used for the windings, which means 
that a significant part of the differential mode flux is carried in the 
core, and so the core acts a conventional transformer.  However, my 
understanding is that common mode choking is normally done in a 
frequency range at which the core is lossy, which means that some of the 
differential mode signal is subject to core losses;

- although the cores used are marketed as balun cores, their intended 
use as balun cores is as voltage baluns, with all windings going through 
both holes.  The BL1 configuration means that not all the flux around 
the winding is actually in the core, and that there will be some 
magnetic coupling between the two windings (which would, conventionally, 
be one two separate cores).

Can anyone:

- find flaws in the above;
- indicate the range of operating conditions over which most of the 
forward power actually reaches the load (you can get low SWRs without 
the load receiving power, if there are transmission losses) and over 
which there is also effective common mode rejection;
- suggest a source of a design or kit for an unbalanced to balanced 
antenna tuner that will work with the basic K2, and if not too much more 
expensive, will work with the 100W option.
-- 
David Woolley
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