Claton Cadmus said:
>Charles Kadesch wrote:
>
>: Jim:
>: A good way to use an unbalanced tuner with a balanced feed line
>: is to use a balun on the tuner input and "float" the RF ground
>: of the tuner.
>
>I would advise against this. Although it will provide "better" balance, an
>unbalanced T or L style tuner is just that, unbalanced. Doing this can put RF
>on the tuner controls and metal case.
>
>The best approach I have found for using unbalanced tuners with balanced
>feedlines is a coax choke balun after the tuner. This keeps the RF out of the
>shack, balances the feedline, is low loss and doesn't cost much.
>
>To make one get a piece of 3" to 5" OD PVC pipe. Close wind 30 feet of RG-58
>solid poly dielectric coax on the pipe(don't use foam dielectric). Put a coax
>connector at one end and a couple of connectors spaced for your balanced line
>on the other. That's it. Works great, and low loss on HF for reasonable
>SWR's. Good 80 to 10 meters probably OK on 160.
>
>Hope this helps.
>----
>73 de KA0GKC Claton Cadmus
>E-mail cla@spacestar.net
Ed Pacyna and others have raised a few questions about the whole question
of "balun on the input" or not. Here's my take...
It may be true that a balun on the input or output of an unbalanced tuner
will be problematic as far as perfect balance. However, it is also true
that any balun on the output will see variable impedances in the typical
multi-band "tuned line" approach. This means that if the line input
impedance is high, the balun's reactance may not be adequate to act
properly. Thus, in many circumstances, a balun on the output of an
unbalanced tuner is pretty dumb, becuase it just can't do its job. So if
you're gonna do it, put it on the input for sure.
I have also experienced the inadequacies of the coax choke balun described,
when broadcast band rejection is required. In this case, at AM broadcast
frequencies, the balun does nothing to keep these from riding right in and
causing problems even with the high-quality direct-conversion approach of
the R2. In these cases, a broadband transformer wound on a ferrite balun
core does a good job, as could one wound on a good-size ferrite toroid.
My homebrew implementation of this is with an unbalanced SPC circuit built
on a board (no hot chassis problems for me :-) ). The balun on the input
is a BN-43-3302 core with 3 turns each, primary and secondary. Rejection
of the AM broadcast problem is complete.
It is also complete with my more compactly-constructed Z-Match circuit,
which is closer to a true balanced setup. In practice, either one works
quite well with a 350-foot horizontal loop fed with 450 ohm ladder line.
Both show a substantial increase in line noise and broadcast interference
rectification when configured unbalanced (balun bypassed in the one case,
one side of output grounded in the Z-match case).
John Seboldt K0JD
http://www.pconline.com/~rohrwerk/k0jd/
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]