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
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to k...@sdellington.us
--
Scott K9MA
k...@sdellington.us
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
