[Elecraft] Balun for KAT2

[Ron D' Eau Claire]

Don wrote:

> Jonathan,
> I may be opening a can of worms here because there are a lot of 'balun
> believers' out there, but I would start with no balun at all.  It
> WILL work,
> but there is a possibility that you may get some 'RF in the
> shack' - ...

I'll second Don's comments here. As a long-time user of open wire line, I
believe that the only way to be SURE you are getting all the benefits of the
line is to have balanced feed. If the currents are NOT in balance, the
feedline will radiate and pick up r-f.

But the fact is, as Don says, having great balance at the rig is not always
necessary and trying to ensure balance with a balun can create problems if
the balun can't handle the impedance it sees on the line.

Baluns are really intended to provide balanced feed to a line with low VSWR.
The main reason most of us use balanced line today is so we can run line
efficiently with high VSWR, such as in a G5RV or center fed doublet. If the
impedances are not too extreme, you can get away with it. But it's hard to
tell how much the balun is losing. Sure, running low power will stop the
balun core from getting hot. But that's not always because it is working
efficiently. It might be because you can put 10 watts into a large core and
it'll convert 9.9 watts of it into heat without feeling hot to the touch.

However, one clue that a balun has quit working efficiently is that it will
usually stop providing a balanced feed as well. If you want to use a balun
and wonder if it is ... er... 'balancing' the feed, all you have to do is
check.

To check the balance, all you have to do is to put two r-f ammeters in your
feeders and see if they read the same... Relax, you can buy a nice r-f
ammeter for less than a buck. It's called a small flashlight bulb.

FIRST, turn the power down to about .2 watts and transmit briefly so the
power control circuit has really turned the power down.

Next, put two identical bulbs in your balanced line (buy several, the K2's
power control system makes it easy to smoke them). Little 1.5 volt bulbs are
fine.

Transmit and watch the bulbs. Turn up the power so they glow dully. You can
actually see a very small difference (10% or so) by eye if you have them
glowing dimly. If they look pretty much the same, your balun is probably
working. I can't guarantee that it is working near top efficiency, but the
likelihood is that it is doing okay.

If the bulbs are very much different in brightness, you can be sure the
balun is not able to function properly and, at the very least, is not being
a balun. It might be making itself into a great dummy load. In any case
there is no reason to keep using that particular balun.

If you saw a bright flash from both bulbs, then nothing, you forgot to turn
down the power on the rig, or you forgot to transmit AFTER tuning down the
power  and before inserting the bulbs, so the rig hit the bulbs with the
full output.

IF the feedline presents a very high impedance at the rig, it may take all
the output of your rig to produce even the faintest glow. There is a high
voltage and very low current in that case, and it's the current that lights
the bulbs. At the other extreme, where your feeders present a low impedance
to the balun, you may find the bulbs quite bright running only a few hundred
milliwatts.

However bright or dull, may your bulbs glow evenly...

Ron AC7AC
K2 #1289