[Elecraft] SGC-239 vs KAT2

[Charles Greene]

At 09:26 PM 3/4/2002 +0000, Julian, G4ILO wrote:

Julian,

If you are concerned about RF on the outside of the coax, try a 1:1 W2DU 
type balun, or put some beads on the outside of your RG8x at the feed 
point.  Actually, the balun won't do much for the SWR but it will help keep 
the RF off the outside of the coax.  You might want to check the power by 
measuring voltage on loop at the feed point with and without the balun to 
see if the balun is causing too much loss.  I's say see if it gets hot, but 
with the K2, it probably won't.  If your loss is high, try a coiled coax 
type balun.  Just keep the coils in a cylinder, ie don't jumble wind 
them.  RG 8 is the best for the coax type balun, and the wire in the RG8X 
may migrate to the outside of the coil through the foam insulation.  That's 
the theory; you are always free to try it.  10 turns of RG58 wound on a 
FT240-77 toroidal core works pretty good too.


>My antenna is approx. 80ft of wire looped around my loft. It is fed in the
>center of one side by a SGC-239 autocoupler, which is fed using a few feet
>of RG-58 co-ax to my K2 in my office on the floor below. This antenna works
>better than I expected. However, I'm not over-impressed with the SGC-239,
>which sometimes refuses to tune up, or jumps out of tune in the middle of a
>QSO, and needs a carrier to be applied in order to know what band you're
>on. The reason for this arrangement was so I could feed the antenna with
>co-ax. Open wire feeder would be unacceptable in this environment.
>
>With the SGC-239 not powered up, so that the RF goes straight through, I
>find that the KAT2 can produce a good match on all bands. However, the SWR
>reported by the K2 in CAL S mode on most bands is >9.9:1, and of the order
>of 8:1 in the best cases, so I presume that feeder loss would be a bit
>steep even though the co-ax is fairly short, about 18 feet. Also I suppose
>that there will be radiation from the shield, due to feeding a balanced
>antenna with an unbalanced line, though I guess that winding the co-ax
>through a couple of ferrite rings a few times might fix that.
>
>I'm wondering if it would be possible to feed this loop successfully using
>co-ax and doing the tuning at the K2 end using the KAT2 which is redundant
>in the current configuration. Is there a way to reduce feeder losses to an
>acceptable minimum? Would putting a 4:1 or a 6:1 balun at the antenna feed
>point reduce the SWR on the co-ax to a level where feeder losses would be
>negligible? Would losses in the balun itself be significant? It would be
>operationally more convenient to use the KAT2 with its ability to remember
>the tuning position for each band, but as I'm running low power to a small
>antenna I don't want to sacrifice more power to achieve that, so perhaps
>I'd be better off sticking with the '239 and its idiosyncracies.

73, Chas, W1CG