[Boatanchors] Bird Termaline Question - Update (long)

[Brian]

Bonjour Jacques,

Your first paragraph is spot on.

However, your conjecture about the oil is not so good. The original oil had 
a dielectric coefficient of 2.3. The conic section around the tubular 
resistor was designed for that dielectric coefficient. Mineral oil has no 
specified dielectric coefficient; further, mineral oils produced for 
internal combustion engines contain soap compounds that will not be 
beneficial to the load resistor or the conic sections - some soaps 
flocculate and don't settle back to a liquid state for some time. The 
bubbles will give rise to local hot spots that may have the resistive 
coating peeling off the ceramic substrate.

Be very careful of the armchair theorists who are 'contributing' to this 
stream. Few have opened a Bird 8135, and even fewer have a clue about the 
conic sections inside the better dummy loads.

73 de Brian, VK2GCE.

On Wednesday, February 08, 2017 11:44 AM , you said:

Robert,

The reactance component is created by the resistance value that shifted up.
The way a Termaline is done, it behave as a tapering impedance transmission
line from the feed point to it's end, where the outer shield touches the
center element, bringing the impedance value to zero.
This also means that at the center of the resistive element, the resistive
value should be 25 ohms, and that the impedance formed by the log-tapered
shield should also give a 25 ohms impedance related to the center resistor
diameter at that point.
Changing the center conductor resistive value WRT the line impedance creates
the reactive component measured, because it mismatches the internal line
impedance (meaning: it is no longer the proper resistive value along the
structure of the load).

About the oil: who tell us that it is the original (and the proper) one ??
Not sure that substituting the oil originally used by Bird for, say, 10W30
motor oil does not give the results I theorized about....
And a change of oil may also change the dielectric constant of the internal
transmission line, so even if the resistive element is OK, DC wise, the SWR
(and the global impedance) can be quite off up in frequency.

A lot of bad things can happen to a dummy load resistor, depending on who
used it.
I saved some from local "Army Surplus" stores in the '80s, but when the
resistive DC value was not good to start with, I just leaved those there.

73, Jacques, VE2JFE