I remember convincing myself that Stauff black polyamide (PA) clamps were ok
when I constructed my 40M yagi. I dug up the data again to refresh my
memory. This is probably more than anyone wants to know, but for the record
or in case you have nothing else to read:
First, regarding W5FH's question, the green clamps are a polypropylene
copolymer, not polyamide. They will be somewhat less mechanically robust
than polyamide.
Next, to address the electrical characteristics, PAs have dielectric
constants in the range of 2.5-3.6, (similar to mylar and PVC) with volume
resistivity better than 10E12 ohms-cm (some as high as 10E16).
The complex permittivity for PA-6 in film form is fairly flat at about 3.7
up past 1MHz then increases exponentially to about 4.5 at 10MHz and keeps
ging. The imaginary part is nearly zero up past 1MHz then increases
exponentially to about 0.7 at 10MHz. Still low in both cases over LF and MF
ranges, and low is good for insulators like element clamps. The exponential
trend could be trouble at higher frequencies. But, calculating AC
conductivity from complex permittivity data requires more math than I care
to do before I retire.
NBS data from 1961 for 6,6 nylon (a typical polyamide) show dielectric
contstants of 3.3 at 1MHz and 3.2 at 10MHz at 23 deg C., increasing a little
bit with temperature. The loss tangent (aka dissipation factor) is well
below 0.1 at LF and MF over temperatures to which antennas would be subject.
Those data are for neat PA. I could not find any published details of
Stauff's PA material composition (I'm waiting to see if they respond to a
recent request), so we don't know what material they use for colorant.
Additives could impact the data and conductivity. For example some published
data show resistivity decreasing rapidly once the volume fraction of carbon
black in 66 nylon exceeds about 2%.
All that said, I chose the black PA for its dimensional stability and
environmental robustness. I have not been disappointed, and have not noticed
any impact on tuning my 40M yagi (it matches EZNEC prediction). No clamps
melted when operated at legal limit. However, I would not be inclined to use
them for VHF element clamps. I could be wrong.
73, John AI6LY
John KK9A wrote:
My 40m OptiBeams use black clamps, I am not sure of the manufacture but
they look like Stauff clamps. I wonder who is saying >that the black clamps
have a carbon issue? Can you buy one and test it?
Byron W5FH wrote:
Hello- I am a little confused about the UV and electrical
properties of the polyamide insulator clamps (hydraulic and tubing line
support clamps) used on the VHF LFA yagis. I have viewed >commercially
produced VHF LFA yagis that utilized green colored ones to support 13mm
tubing elements. I have read the black >ones are better in regards to UV
resistance. However, some antenna manufacturers state the black colored
clamps have additional >carbon in them and the
antenna element lengths will need adjusting. My head is nearly
spinning after trying to research this. I have tried to look through the
thousands of polyamide clamps in the Stauff catalog. Can >anyone shed any
light on whether the green colored polyamide clamps, such as used on an
E-Antenna 6 meter 7 element LFA yagi, >are inferior in regards to UV
resistance? I see where a competitor (making mostly HF
yagis) states the black clamp assemblies they use are better in regards to
UV resistance of the green ones that E-Antenna uses, but >I wonder if the
black clamps have identical electrical properties or would corrections need
to be made to elements? I see where >G0KSC, an authority on the LFA and
their construction, offers the green colored
clamps. Thanks, Byron W5FH
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