[Elecraft] Ferrite transformer losses, 43-foot vertical and the K3

[Sandy]

Absolutely, Don!

My main message was to watch the baluns for heating.  During a contest or a 
long winded transmission, you can really screw up a nice ferrite balun 
QUICKER than one thinks.

73,

Sandy W5TVW

----- Original Message ----- 
From: "Don Wilhelm" <w3fpr at embarqmail.com>
To: <elecraft at mailman.qth.net>
Sent: Saturday, September 20, 2008 8:59 PM
Subject: Re: [Elecraft] Ferrite transformer losses, 43-foot vertical and the 
K3


> Sandy,
>
> You are correct that using a balun (either 4:1 or 1:1) is not
> necessarily the most efficient solution.  The 'old fashioned' link
> coupled tuner will most always be more efficient.
>
> When the feedline input impedance (and that has nothing to do with the
> characteristic impedance of the feedline) is close to the output
> impedance of the balun, the balun will be just almost as efficient as
> the link coupled balanced tuner, but that rarely happens in practice.
>
> Yes, using a balun following an unbalanced transmatch is a compromise.
> It lends itself to easy bandswitching and its associated convenience.
> If one is searching for the most efficient antenna tuning mechanism,
> then either dedicated resonant antennas are required, or one must accept
> the inconveniences of changing coils in a simple balanced link coupled
> tuner or accept the compromises of an easy bandswitching system.  The
> old Johnson Matchbox was an effort to provide bandswitching convenience
> with a link coupled balanced tuner, but even it has limited matching
> range compared to the simple single-band tuner designs.
>
> Bottom line, one must either accept the compromises dictated by the
> conveniences of bandswitching or accept the inconveniences of using the
> most efficient tuners that can be constructed.  There is no 'best of all
> worlds'.
>
> 73,
> Don W3FPR
>
> Sandy wrote:
>> When a high VSWR exists with a toroid ferrite balun due to a high
>> inductive or capacitive reactance and the toroid begins to heat, the
>> losses will rise to quite unacceptable losses and can possibly destroy
>> the balun itself, even tough the tuner used appears to have reduced
>> the VSWR on the input side of the circuit to a very low value.
>>
>> Generally a 4:1 or 1:1 transformer type balun should be preferably
>> used for just a resistance transformation, not where there is a
>> highly  reactive component on the output side of the balun.  Lots of
>> people "get away" with this situation, but it isn't a very good idea.
>> I commonly did this for years with very large ferrite cores and a
>> vanilla high pass "T" network tuner (like the many MFJ and other "T"
>> network tuners)
>>
>> For the last 15 years I have used nothing but the common Parallel type
>> balanced line link coupled tuner configuration and had extremely good
>> results with the old fashioned and sometimes very cranky to get setup
>> right circuit.
>>
>> "choke" type baluns (the ones that traditionally use a large number of
>> ferrite beads on a length of coaxial cable) are much less troublesome
>> than the transformer type.  If your "balun" setup runs cool, then you
>> probably have hit upon a length of feeder that is "just right" and you
>> are "OK".  If it is running warm then you are treading on dangerous
>> ground and things may be getting ready to surprise you one day with a
>> catastrophic failure, especially when you run the legal limit!
>>
>> This no matter what the VSWR meter says between the tuner and the
>> ferrite balun in question.
>>
>> 73,
>>
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