Mosley Antenna Troubles
John Thornton
rotatable dipole option at my QTH. Care was taken to ensure that
everything was assembled as the instructions specify.
The antenna was installed about a month ago and performed very well. The
SWR curves were not quite as broadbanded as Mosley specifies but during a
phone call to Mosley (Kevin), I was told to set the reflector to the CW
setting and this would provide braoder bandwidth (not possible without
antenna removal). The Mosley manuals are not very well written and
should provide more technical detail.
There are some design and manufacturing flaws with the antenna; the boom
caps and element inside caps (made by Cap Plug) were designed for use on
sheet metal, not tubing (so they fall out on the ground easily) and two
of the element pieces were swedged incorrectly so wouldn't fit together
well (Mosley sent replacements). Kevin said that the reason the better
fitting cap plugs weren't used (like the ones that come with type N
connectors) is that the tuning of the antenna would be compromised (this,
he said was told to him by one of their engineers ;-).
Now the tuffy:
After using the antenna successfully for about 3 weeks with good results,
I installed my Kenwood TL-922 linear. I loaded up in a part of the band
that is < 1.2:1, and when the power got up to about 400 watts on 14.16
MHz, the SWR shot up to 4:1. The antenna covers 10,12,15,17,20, and 40
meters. The 20 and 40 meter bands have high SWR; the rest are ok.
Kevin at Mosley said to check two things. First, that the weep holes are
oriented downward (they are), and second that the orientation of the 40
meter trap is correct. The manual specifies that the color band on the
40 meter trap be installed at a different orientation than the others
(annoying since it can cause confusion, but I had it installed correctly).
Yesterday we removed the offending elements and discovered that the trap
cover on one of the 40 meter elements has holes burned into it, and the
other has some melted plastic. The coil form has a burn trail from the
tubing to the case of the trap. Clearly, there was major RF arc-over.
I inspected the traps carefully and noticed a slight bit of moisture in
the traps (it had been raining all morning- the IDEAL day for antenna
work!). What is appearant about the design of the trap covers is that
they do not provide much of a water seal. Water can easily run down the
length of the tubing during a storm and collect in the trap cover.
Here is what I am wondering from the net folks:
Have you experienced similar trouble with RF flash-over on a Mosley antenna?
Have any of your cap plugs fallen out or do different models use the
correct plug?
Any other information that you can provide would be useful. When I call
Mosley Technical Support on the phone, and speak to Kevin, he usually has
to "speak to one of the engineers" to find out any answers. He tells me
that the people who know more about this stuff and would normally answer
the phone are busy working on a military contract (which was supposed to
be complete before Dayton) but will be available in the near future.
Mosley had a nice display of antennas at Dayton and the engineers there
were obviously knowledgable. Their products are highly regarded- maybe I
received a Friday unit. (The 40 meter trap ends HAD been ground with a
grinder ;-)
Any input is appreciated.
73 de John WM6R in Berkeley, CA