[Boatanchors] Bird Termaline Question - Update (long)

Jacques Fortin jacques.f at videotron.ca
Tue Feb 7 19:44:10 EST 2017


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

-----Message d'origine-----
De : Boatanchors [mailto:boatanchors-bounces at mailman.qth.net] De la part de
WA5CAB--- via Boatanchors
Envoyé : 7 février 2017 18:36
À : test-equipment at mailman.qth.net; boatanchors at mailman.qth.net
Objet : Re: [Boatanchors] Bird Termaline Question - Update (long)

It appears that a fair number of units changed resistance from the same
value to the same incorrect value.  Which would indicate either a design
error or a manufacturing error.  I think that the possibility that the oil
is dissolving the resistance element can be eliminated.  Else the older the
unit, the higher the resistance would be.  Or, the dissolving action is
selective to the magic portion of the resistance that has a positive
reactance component and happens quickly after filling.  :-)

In a message dated 02/07/2017 13:51:31 PM Central Standard Time,
jacques.f at videotron.ca writes: 
> Hi Doug,
> It was my first thought that the resistor inside was overheated, but 
> it can be something else..
> What if it is the cooling oil slowly dissolving the carbon layer of 
> the resistor ?
> Could it be a moisture contamination of the oil giving that result ?
> If ever you dismantle everything and extract the resistor, it should 
> measure the same 72.6 ohms you got.
> The other proof that the resistor is damaged, in your case, is that 
> you got a capacitive component of the impedance at most frequencies: 
> it should not be any reactance showing up to the GHz range.
> I have a 6156 Thermaline that measures 51.8 ohms, and I believe it is 
> still good, but in the future, who knows ?
> 
> 73, Jacques, VE2JFE
> 
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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