http://elecraft.85.s1.nabble.com/KAT-500-and-open-wire-tp7647776p7647785.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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