[Elecraft] KAT 500 and open wire

[K9MA]

Well said, Don!

73,
Scott K9MA

On 1/6/2019 15:40, Don Wilhelm wrote:
> 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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-- 
Scott  K9MA

k9ma at sdellington.us