"Antonio Vernucci" wrote in
:

If your BalUn has already done the bigger job of turning a High Z to
a modest one, the common logic for the need for twin line has also
been diminished.

Yes but my question regarded the advantage of twin-lead vs. coaxial in
that particular application where impedance is uncontrolled.

Tony, it depends on the details of your scenario, and may be different at
different frequencies.

Unless you believe in the myth that ladder line is *so* low in loss that
you *never* need to consider it, you need to calculate it out to really
know... it is not a no-brainer as we say, in fact it is a quite complex
problem to solve (mainly quantifying the loss and transformation in
transformers which both Roy and I have mentioned in this thread).

BTW, from time to time I see articles that recommend twin line for direct
feeding a ground mounted vertical (ie without using a balun at the feed
point). It is as insane as using a 4:1 voltage balun with coax at the
base of such a vertical, because both types of feed drive substantial
common mode current on the feed line. A review of such an article is at
http://vk1od.net/antenna/multibandun...tical/BSUM.htm , this one
using a magic ingredient, Belden 8222 twin feedline which Belden ceased
manufacturing. But... I am sure some hams have got the QSLs to prove that
it "works real good".

Owen

On Mon, 05 Oct 2009 21:30:06 GMT, Owen Duffy wrote:

Calorimetric measurments are problematic, they sound simple enough, but
latency my mean it takes hours to reach close to maximum operating
temperature. ...
and the case was quite warm near the balun after just minutes of testing.

Sounds like the BalUn was doing a superlative job.

Hi Owen,

What you describe (sans the problematic word latent) is specific heat
capacity. And just like any capacitor, charge/heat does not increase
after a source is removed. If it is not removed (which I presume was
the intent of both your statements), then the specific heat capacity
you describe is a design boon.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

....
What you describe (sans the problematic word latent) is specific heat
capacity.

I used the term latency in the context of time, there is a delay between
commencement of application of steady power to reaching substantially full
operating temperature. Yes, the effect can be predicted using the material
specific heat capacity, subject to the temperature variability of the
ferrite characteristics.

....
the intent of both your statements), then the specific heat capacity
you describe is a design boon.

Yes, but a trap if long term use is envisaged but tested only in the short
term.

Owen

To put some numbers around the problem, if one had a FT240 core which has
a mass of around 0.2kg, and specific heat capacity around 800J/kgK, the
energy to raise the core to a Curie point of 130°C to 300°C would be 17kJ
to 44kJ. If the core was well insulated (no heat loss) and dissipating
say 20W, that would take 15 to 40 minutes.

Of course, one would hope that the transformers do lose heat to the
environment, and that would substantially slow the rate of rise of
temperature.

Experience shows that a 5 minute test of a ferrite transformer does not
indicate continuous power handling capability.

Digressing slightly, but on this ferrite heat thing...

Martin questioned my article "A review of the Guanella 4:1 balun on a
shared magnetic circuit" at http://www.vk1od.net/balun/gsc/index.htm . In
particular, his issue was with my proposition that the extent to which
these things "work" is due to flux leakage on low µ cores, the lower the
µ, the more they resemble Guanella's balun.

Martin's inital experiments indicated that the thing did work, but on my
advice he tried prototypes on high and low µ cores and took thermal
pictures of the things after operation. The images showed non-uniform
distribution of heat in the cores which is either due to the main heat
source being the conductor losses, or that magnetic flux is a significant
contribution and not evenly distributed in the toroid. The flux
distribution is a credible explanation for the different patterns for
same winding on the different µ cores.

Again, this is one of those things that lots of hams have QSLs to prove
that they "work real good".

Owen

On Tue, 06 Oct 2009 06:25:53 GMT, Owen Duffy wrote:

Of course, one would hope that the transformers do lose heat to the
environment, and that would substantially slow the rate of rise of
temperature.

Hi Owen,

This opens another topic of my study with Thermal Resistance. One
paper that I have filed away that may aid you is W.E. Hord's "Recent
Developments In The Average Power Capacity Of Rotary-Field Ferrite
Phase Shifters." It may lack the specific application discussed here,
but it covers the math and interface relationships. Sorry, but I
don't have any publication details except for author/title.

Hord's work is with ferrites capable of sustaining RF power levels in
multiple KW. My first experience with ferrites (ca. 1972) was with RF
transmission line source/load isolation in the microwaves (the paper
is S-Band), a field that is wholly alien to discussion here.

73's
Richard Clark, KB7QHC

Owen Duffy wrote in
:

....
Martin questioned my article "A review of the Guanella 4:1 balun on a
shared magnetic circuit" at http://www.vk1od.net/balun/gsc/index.htm .

Ian (GM3SEK) kindly drew my attention to incorrect reference to two of the
figures in the above article. The error was misleading, it is fixed now. My
apologies to anyone who was confused by the error.

Thanks Ian.

Owen