Posted by
Don Wilhelm on
Jan 06, 2019; 9:40pm
URL: http://elecraft.85.s1.nabble.com/KAT-500-and-open-wire-tp7647776p7647783.html
All,
I think a bit of extended study on transmission lines and their
impedance transformation characteristics is in order - refer to any ARRL
antenna book, even those from the 1950 an 1960 era, not much new has
been added, other than baluns. The feedpoint impedance to the
transmission line can vary from quite low to quite high dependent on the
frequency, the length of line, and the load at the far end. It does NOT
depend on the characteristic impedance of the transmission line. The
ARRL TLW program can do a lot of computation for you.
You may be able to adjust the length of the transmission line so there
is a moderate shack end feedpoint impedance on all bands of interest and
be able to use a good current balun which has either a 1:1 or 4:1
transformation ratio. You have to measure the transmission line at the
point where you intend to install the balun to be sure which to use.
Yes, just hanging any balun on any length of parallel feedline connected
to any length of antenna and expecting it to work without problems is
just not going to happen in all but exceptional cases.
How many cases where you see "this antenna works great" do you see any
mention of how long the parallel feedline may be. Most hams simply
ignore that, but it is likely the most important part of how the antenna
works and what type of balun of you need to use.
Your experience may be very different with the "same" multiband antenna
than your local ham or that in the article you read unless the feedline
length and type is know. I do not see many antenna articles where the
feedline length is mentioned if it is parallel feedline.
With resonant antennas, the feedline length does not matter as long as
the feedpoint impedance of the radiator is the same as the
characteristic impedance of the feedline. In those cases, the feedline
length does not matter, but in all other cases, it matters a lot.
73,
Don W3FPR
On 1/6/2019 3:17 PM, K9MA wrote:
> Using a balun with balanced line is risky. Window line or open line can
> be used with reasonable losses up to at least a 10:1 SWR, but that means
> the impedance at the balun output could be anywhere from 30 to 3000 Ohms
> (for 300 Ohm line). The current balun should work if the impedance at
> the balun output is low, not much over 50 Ohms. This is hard to measure,
> but modeling should give a close enough value. On the other hand, the
> impedance could be thousands of Ohms, and that kind of balun will be
> ineffective. Transformer-type baluns have limitations at both high and
> low impedances.
>
> While a transformer type balun could overheat and/or generate harmonics,
> I suppose the worst thing a current balun could do is to upset the
> balance. I've you're not seeing a lot of "RF in the shack", you're
> probably good.
>
> The only really reliable way to feed unmatched balanced line is with a
> truly balanced tuner, a seriously endangered species. I'm still using
> the one I built almost 50 years ago.
>
> 73,
> Scott K9MA
>
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