[Elecraft] OT: Now Balun effectiveness

[Don Wilhelm]

Darrell,

See comments below:

----- Original Message ----- 
From: "Darrell Bellerive" <ve7cla@shaw.ca>
To: "Don Wilhelm" <w3fpr@arrl.net>
Cc: "Elecraft Email List" <elecraft@mailman.qth.net>
Sent: Tuesday, March 23, 2004 10:35 PM
Subject: Re: [Elecraft] OT: Now Balun effectiveness


> Don,
>
> Thanks very much for your detailed replies! I would really like to
> understand this much better than I do.

-----------------------------------------
One just must 'hit the books' - the theory oriented sections of the ARRL
Handbook and the ARRL Antenna Book are good starting places - that was MY
initial education into a career in electronics.  It is confusing at first,
but gets better with detailed study - beware, this is not an easy subject -
as one instructor put it, the study of electromagnetic fields is one area
where one must handle 4 dimensions to place a point - the 3 dimensions
describing the point in space and the time of the occurance of that point -
the only other discipline requiring similar analysis is 3 phase AC motors.
---------------------------------------------
>
> Choke baluns seem to be the favoured solution, but I can't help but
> wonder about the losses in the coax cable due to the impedance mismatch.
> I can see where a choke balun of ferrite beads or from a coil of coax
> will keep the RF off the shield of coax feedline when used with a
> resonant dipole, but in multiband operation, I would expect the vast
> differences in reactance and impedance would create vast differences in
> losses. Particularly with long runs of coax.
----------------------
The input to output impedance of the balun is what keeps the RF off the
outer coax shield - imagine that single parameter as an RF choke on the
outer side of the shield.  The length of coax does not enter the picture
except when considering the loss of the coax due to its length and SWR.
Keep separate things separate - it makes understanding the system easier.
-------------------------
>
> As you have stated a doublet with open wire feeders can be built to have
> a finite and manageable impedance range over two or three bands, but
> getting a manageable impedance range on all 9 bands from 160 to 10
> metres is almost impossible to achieve.
------------------------
True, 'we pay our money and take our pick' <G>
------------------------

> Therefore I would believe that a non-resonant doublet connected at the
> feed point to a remote balanced-balanced tuner either directly or via
> ladder line would have less loss across all 9 bands from 160 to 10
> metres than would a coax fed doublet with a choke balun or current
> balun.
-----------------------------------
For most installations - ABSOLUTELY - but there may be situations where that
is not practical.  For another solution, Cecil Moore (ex W6RCA, now W5DPX)
described a system that switched feedline lengths in 1 ft increments to
achieve a match - a 130 ft. flattop antenna with a 450 ohm feedline variable
from 90 to 121 feet.  He dubbed it his 'Mystery Antenna', and it works fine
for 80 through 10.  I'm sorry, but I lost his website URL when he changed
callsigns.
-----------------------------------
>
> Certainly, from a practical standpoint using the KAT2 or KAT100 and a
> coax feedline to a choke balun or current balun fed doublet would be
> easier to design and construct than coax feedline and control cable to a
> remote tuner, however, I can't help but wonder what one looses in
> efficiency for the sake of convenience. Does the decrease in loss
> justify the complexity of the remote tuner? How much loss are we really
> dealing with?
------------------------------
Put the remote tuner at the antenna feedpoint or feed with low-loss parallel
line from the remote tuner to the antenna and the losses will likely be
small.  Bottom line - keep the SWR on the coax low and the losses will
likely also be low - high SWR lines need to be open wire or ladderline
balanced lines.  Keep in mind that it is not practical to go to extremes -
an SWR of 2:1 on most coax is quite tolerable when considering loss at HF
(VHF and UHF are quite different considerations - the loss becomes greater
as the frequency increases).
---------------------------------
>
> And finally yes, it is at saturation of a toroidal balun that harmonics
> are generated. The source of my understanding about harmonic generation
> is from an article by Richard L. Measures, AG6K where he states:
> "More turns means more ampere-turns of magnetic flux in the balun's
> core, and high magnetic flux densities can cause the ferrite-core to
> saturate. This distorts the RF waveform and creates harmonics. These
> harmonics extend well into the UHF TV band."
-------------------------------------
Rich Measures is quite correct, BUT, at 100 watt levels, this is not a
concern with balun cores of any reasonable size.  At 1500 watts, much more
care is required in sizing the cores. So the correct answer is 'It all
depends ...'.
------------------------------------
>
> Darrell VE7CLA K2 #1973
>
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Best 73,
Don W3FPR