[Test-Equipment] Bird Termaline Question

[Brian]

Time to get a few facts rather than armchair hypothesising, as entertaining 
as it may appear.

1. The only way to test a dummy load is with RF. A DC resistance measurement 
only speaks to connectivity.
2. The Bird 8135 is an oil-filled Termaline design rated at 150 W @ 50 Ohm. 
It has no meter. It uses a single, coaxial cylindrical resistor, usually 
made by Corning; the resistive material is fired into the outer surface of 
the ceramic cylinder.
3. The Bird Termaline design uses two negative exponential outer tubes, one 
over the load resistor and one over the tapered connection between the 
external connector and the resistor body, to ensure that a true, 
non-reactive resistive load appears at its termination.
4. The Heath Cantenna uses a similar resistor, but holds the load resistor 
in a parallel-sided coaxial tube. It is oil-filled. Its true RF performance 
is DC to DC only. And money in the bank for the Heath Company.
5. The MFJ terminating dummy load uses a resistor inside a perforated metal, 
square cross-section case. Its RF performance is similar to the Heath 
Cantenna. It relies on radiation through the surrounding air - it is not 
oil-filled.
6. The Marconi series of dummy loads use a variety of Corning-style 
cylindrical resistors. Many designs use tapered 'wings' surrounding the 
cylindrical resistor to achieve reactance-free impedance at RF. They are 
metered and air-cooled. Their RF performance approaches the Bird Termaline.
7. If the oil leaks out of the Bird or Heath loads, until actually used, the 
DC resistance should be close to 50 Ohm. However, the dielectric coefficient 
of the oil is part of the design, and affects the RF impedance. A suitable 
oil is the synthetic oil used in pole pigs - but do check the dielectric 
coefficient.
8. As soon as you use a load that was designed for oil but is now oil-free 
or oil-low, it will overheat and its DC resistance will increase. The solder 
to the N-type connector may also melt out, further increasing the DC 
resistance.
9. What Bird does not say is that the negative exponential tube is slotted 
to allow free oil flow. Most illustrations show Bird Termaline units 
horizontal. Heated oil loses some density and, on the Earth's surface, rises 
= convection. To get the longest life out of your Termaline, I suggest you 
use it vertical. The heat radiating fins on the outside don't really care 
about Earth's gravity.
10. If the internal resistor is 'cooked' , I'm sure Bird would supply a 
replacement. I have repaired Bird Termalines that have been slightly cooked 
by fitting a suitable resistor inside the ceramic tubular resistor; I use a 
safety factor of 2 for power rating of the added resistor; you can get away 
with such a low safety factor because most air-rated resistors will handle 
much more submerged in oil. But I wouldn't use this method with a '50 ohm' 
load measuring 73.5 Ohm.

For instance, if a '50 Ohm' load resistor measures 55 Ohm at RF, the 
parallel resistor needs to be 550 Ohm - you do the maths. If the load was 
specified for 100 W, then the 550 Ohm resistor will dissipate 9 W; I would 
use a 20 W solid-carbon-bodied resistor. If you never intend to load up to 
100 W, you could use a lower power rating.

73 de Brian, VK2GCE

<unresearched rubbish removed>
>
>> On Feb 4, 2017, at 14:17, Doug Hensley <w5jv at hotmail.com> wrote:
>>
>> I just received what was supposed to be and what certainly looks like a 
>> NOS (new old stock) Bird Termaline dummy load.  The label is marked 150 
>> Watt, 50 Ohm.
>>
>> When I put my Fluke meter on it, it measures 73.5 ohms.  Not the kind of 
>> response I get from my old Heathkit cantenna nor my OHR RFL-100 which is 
>> like 50.1 ohms.
>>
>> Has anyone seen this problem or heard about it?  Could Bird have made a 
>> mistake or could the vendor have swapped labels?
>>
>> Thank you,
>>
>> Doug W5JV