[Test-Equipment] [Boatanchors] Bird Termaline Question - Update (long)

[Richard Knoppow]

     The oil may be a PCB oil used in transformers for decades but now 
forbidden because its a carcinogen. There are modern transformer oils 
available, for instance, from MFJ. I don't know their dielectric 
coefficient.  Plain mineral oil covers a lot of territory but the kind I 
am familiar with is the kind sold for medical purposes. Its most 
economically available in the U.S. from shops that cater to horse owners 
since its used as a feed suppliment. Sold in gallon jugs. This is not 
the same as the transformer oil since it has a lower flash point and 
probably different dielectric coefficient.
    FWIW I have a paint can dummy load made by Dentron.  I measured its 
resistance and reactance using a good RF bridge. The resistor is a plain 
cartridge resistor and has gone high, about 55 ohms, but is not reactive 
up to about 30 Mhz. I will try this again in a day or so and make sure I 
am not mis-remembering it.  I think this is a simpler design than the 
Heathkit Cantenna. I filled it with horse type oil.
     If you are going to refill the Bird I suggest getting as close to 
the right oil as possible, suggestions about power company oil are good 
ones.

On 2/7/2017 11:21 PM, Brian wrote:
> 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


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL