In the past we used several beverages at a previous NR4M qth to good
advantage. Since moving to a farm about 15 miles away we expereinced
beverage problems that were a bit bewildering.
Last year we errected four two wire, two direction beverages. While they
were certainly helpful to some degree, performance when comparing each was
not at all consistant and f/b left something to be desired.
After getting them back up this fall the same thing was observed. Bill KC4D
had loaned us a K1FZ transformer and reflection transformer last fall. In
replacing the homebrew boxes on the NE/SW antenna we noticed a great
difference. Many folks have spoken highly of the K1FZ products, but I
questioned why they would be so much better than our homebrews that were
used so successfully by so many.
I measured SWR of the eight antennas at the feedpoint. The ones fed with
the K1FZ boxes had decent swr across 160, 80 and 40. The homebrews were all
over the place and quite high in many cases.
I did some research on transformers and posed a few questions to some of the
folks who frequent this and the top band reflector. Things just weren't
making sense.
I noticed that in one direction one of the beverages was much deader than
any of the others. I brought the matching transformer box for it in and
started looking for broken wires, shorted turns, or anything that would be a
problem.
Finding no visible issues, I decided to check continuity and resistance at
various connection points. The first thing I noticed was some amount of
resistance between the primary and secondary of one of the transformers. It
wasn't a dead short, but should have been completely open.
I ended up removing the transformer from the box. In that case the two
windings were isolated.
Next I checked resistance between opposite ends of the plastic box and was
amazed to find it was conductive. I walked around the antenna field and
found that all four boxes had varying degrees of resistance end to end.
Now with binding posts mounted through the plastic at the ends of the boxes
and SO239s with shield isolated from ground mounted in the plastic boxes,
one can only imagine how effective these transformers were. The varying
degrees of reistance in each box explains why performance varied so much.
We discovered that the boxes were actually designed to store electronic
components. There are black ones and red ones. The black ones were designed
to be conductive to prevent eletrostatic charge problems with sensitive
components. We checked the red ones and found that they had the normal
insulating properties of plastic.
After placing the transformers in red plastic boxes they have been working
great and display consistent low vswr across the three low bands similar to
what we saw with the K1FZ boxes.
The moral of the story is "use red boxes instead of black to house matching
and reflection transformers for beverage antennas";-).
While the odds of a plastic box being conductive are fairly small, our
expereince shows that it can obviously happen. It's entirely possible that
the pigment used to color plastic could be a conductive material such as
carbon!
73 de Larry K7SV
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|