Mike and I had a little discussion off reflector about the
spreaders for open line. I think this portion of it may be
useful enough that the reflector should be exposed to it. It is
a bit long and it contains some anecdotal information. So if you
aren't interested in the topic, please don't read it.
73, Eric n7CL
------- Start of forwarded message -------
To: <towertalk@contesting.com>
Date: Mon, 26 Oct 1998 09:28:51 -0700
From: Eric Gustafson <n7cl@mmsi.com>
To: n7ml@imt.net
In-reply-to: <36337CAB.20A03428@imt.net> (message from Mike Lamb on Sun, 25
Oct 1998 19:31:55 +0000)
Subject: Re: [TowerTalk] Open Wire Line
Reply-to: n7cl@mmsi.com
Hi Mike,
I don't think there is any real disparity between your
observations and what Tom and I said about the spreader
dielectric material.
In your antenna aplication, the material was at the open circuit
end of a self resonant system of reasonably high Q. It would be
easily possible to generate very high voltages across the
dielectric at this point in this circuit. If these high voltages
appeared across a small enough region of dielectric, the
dielectric strength of the material could be exceeded.
None of even the best dielectric materials are low loss once the
breakdown region has been exceeded. Note that polyethylene is
well known to be a low loss material when operated below
breakdown. It will probably even pass the oven test. In fact,
I'll wager that the loss tangent of the ceramic material that
worked OK was even higher than that of polyethylene. But
apparently its dielectric strength was considerably higher.
You are certainly correct that any heat generated represented
power that wasn't radiated as RF.
It is unlikely that voltages this high would be generated at such
low power levels on loaded open wire line which is used in
transmission line mode to transfer power (as opposed to an open
stub tuning element). Not even when operated at quite high SWR.
Even if enough power is postulated that high voltages are
present, the field gradient would not appear across a small
distance in the spreader but across nearly all of its length. It
would be hard to imagine generating stresses high enough to break
down almost any of the materials that would be suitable for
outdoor use in the first place. Of course the higher the power
under contemplation, the larger the line dimensions (both
conductor and insulator) should be to accomodate it.
<!!! WARNING - ANNECDOTAL MATERIAL TO FOLLOW!!!>
I can relate our local experience with homebrew open line. A few
years back, we built a full size 160 meter J-pole for use in one
of the winter contests. The 1/4 wave shorted open wire matching
section was made from #12 hard drawn 7 strand copper antenna
wire. The spreaders were 6 inch pieces of 1/2 inch white PVC
pipe. This stuff will NOT pass the microwave oven test.
The antenna played quite well but was a relatively high Q
structure. I think the 2:1 SWR bandwidth was a bit under 50 KHz.
This turned out not to be wide enough for N7DD in the contest.
So we tuned the system in the trailer with a tuner. Actually, it
was two tuners because we burn't one up (dielectric failure in an
insulating support at the hot end of the roller inductor - these
should have been ceramic).
Anyhow, with a robust tuner everything seemed to be going OK
until the system started acting a bit funny. We couldn't find
the problem in the trailer so we let the antenna down to examine
the open line feed system. All of the spreaders were OK but
there had been corona across the ceramic insulator (only about 2
inches long) at the open end of the feeder. The heat from the
corona had melted the copper wire loop through the hole in the
insulator enough to cause complete mechanical failure of the
wire. We put the system back together with two insulators in
series and had no more troubles for the remainder of the test.
There had been no damage to the ceramic insulator that the corona
appeared across. It had not dielectrically failed. There had
also not been any damage to the 6 inch plastic PVC spreader that
was within inches of the ceramic support. It was apparently
physically long enough that the field gradient it was exposed to
did not cause it to fail even though the voltage across this
element must have been nearly identical to that appearing across
the ceramic support.
73, Eric N7CL
To: <towertalk@contesting.com>
>Date: Sun, 25 Oct 1998 19:31:55 +0000
>From: Mike Lamb <n7ml@imt.net>
>
>Hello Eric: I may be opening up a hornets' nest, but here I go
>anyway....... in playing around with some loop antennas in the
>past (Hi-Q six meter "Halo" loops) I found that if I even used a
>nylon screw near the high voltage point, it would melt in no
>time with about 100 watts. If I ran some reasonably good
>polyethylene dielectric between the ends for additional
>capacitance, it would even heat up and start melting. I assume
>that that heat has to be robbing from the RF that would
>otherwise be radiated from the antenna???? By the way, I used
>ceramic standoffs to overcome the problem.
>
>73/Mike, N7ML
>
>Eric Gustafson wrote:
>
>> Hi Mike,
>>
Snip...
>> I have to agree with Tom that the dielectric characteristics
>> (beyond simply not being a conductor) of open wire spreaders
>> for use at HF are probably a lot less important than their
>> environment survival characteristics.
------------------------
------- End of forwarded message -------
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