[Elecraft] Balun for KAT2

[Charles Greene]

At 12:52 AM 1/19/2002 -0500, Don Wilhelm wrote:
Here's a question that occurred to me.  With a 1:1 coaxial or a W1JR balun, 
will that disturb balance of the initially balance transmission line?  Take 
a case where the balun has an impedance of 1000 ohms to ground and the 
transmission line at point where the balun is inserted also has an 
impedance of 1000 ohms to ground, you are shunting one side of the 
transmission line to ground, but the inside is shunted to Zo which is 50 
ohms.  This would cause a small unbalance it seems.

>Bill and all,
>
>It is true that the antenna half the antenna voltage is shunted across the
>choking impedance to ground.
>This will be on the side of the antenna that the shield (or its extension
>through the balun) is connected.
>In reality, this should be an effective RF choke, and SHOULD have enough
>inductance to be negligible - otherwise it is not working as a proper balun.
>
>I think the real point here is that bad baluns exist and sadly, some have
>been marketed commercially.  A good balun does have that sufficient
>impedance, and will work just fine - provided it is not used outside its
>design limits (now if we just knew what those limits were - and we often do
>not).
>
>Here is the big BUT in all this IMHO - if the choking impedance is not
>adequate, the presence of a balun can upset antenna balance just as much a
>not having a balun in the first place.  And it is all due to placing that RF
>choke from one side of the antenna to ground and finding out that the choke
>is too small to do the job properly.
>
>I believe the original question did have to do with using a balun to feed
>450 ohm line and a multiband antenna, so I pose this retorical question:
>
>If you were designing an RF circuit having an impedance of 4000 ohms - would
>you attempt to put an RF choke of 1000 ohms reactance to ground in that
>circuit?  Certainly not - because it would shunt a bunch of your signal
>straight to ground.
>
>4000 ohms is not unusual in a multiband doublet antenna situation, and many
>baluns designed around a 50 ohm environment have much less than 1000 ohms
>inductive reactance (on the lower bands especially).  So without a lot of
>thought and calculation, one never knows if a particular balun will be a
>benefit or not.  One could address the situation by using a balun with a lot
>of inductance on the low frequency bands and one with fewer turns (or beads,
>etc.) on the higher bands, but then we have come to expect all baluns to be
>broadband (at least 1.8 thru 30 MHz).  So again in the old words of Sportin'
>Life: "It ain't necessarily so!"
>
>I prefer to make the solution match the problem and I never presume that
>same solution will address all problems.  If that were true we could fix all
>the world's problems with just a 'spoonful of sugar'.
>
><g> If one wants to use the balun with very low losses - use the quarter
>wave line balun (look at baluns for VHF if you need to find a description).
>It is very effective and has been around for ages, but only works on the
>band for which it is designed - and at low frequencies, it gets quite long.
><g>
>
>73,
>Don W3FPR
>----- Original Message -----
> >
> > However, you have to account for the fact that the choking impedance (as
> > it true in all current balun designs), is shunted across half of the
> > antenna element to ground (via the coax sheild). This circuit faces
> > potentially half of the antenna voltage (provided the antenna is
> > balanced), and so some current will flow through the choke.
> >
> > With sufficient inductance, this current ought to be small, but if the
> > balun was designed for resistive loads and is suddenly faced with higher
> > voltages due to reactive loads, failures can occur.
> >
>
>
>_______________________________________________
>Elecraft mailing list: Elecraft@mailman.qth.net
>You must be a list member to post to the list.
>Postings must be plain text (no HTML or attachments).
>See: http://mailman.qth.net/mailman/listinfo/elecraft
>Elecraft Web Page: http://www.elecraft.com

73, Chas, W1CG