A few days ago I sent the following to the reflector:
In gusty winds, the SWR on my Force 12 C-3 fluctuates, as seen on the
transmitter SWR meter and confirmed on an MFJ-259. Condition appears to
be more evident on 15 and 10 than on 20.
I'm trying to determine whether it is the antenna or the feedline. I keep
looking at the three radiator elements on the C-3 and their close spacing
and wonder if their mutual coupling is particularly position (and thereby
wind) sensitive.
Would appreciate any comments.
73, Dennis, W1UF
___________________________________________________________________________
I received responses from nine C-3 and C-4 users, including two from Force
12 people and one from a Hy-Gain person. All but one responder experienced
similar SWR variations in the wind. Some comments are included below:
Dennis , I have same problem on C-4 , 20 & 40m ok. Pse post replys that
you receive. Tnx 73 Lanny N0AJZ
My C4 does this. The SWR fluctuates because of the coupling between the
driven elements. This is normal, and is even mentioned in the manual. I've
never known it to cause a problem, as the change is not particularly large.
73, Doug, KF4KL
Hi Dennis. We have a C4 at the Windwood Contest Station, WP2Z. The "open
sleeve" design of the C3/4 matching section insures the phenomena you are
observing. The 10/15 open sleeve component elements are constantly moving
in the wind, changing their position relative to the driven 20 meter
element, so the SWR changes too. At Windwood, note Wind....wood, the wind
blows at 20-40 mph most of the time, the SWR is always jumping around. It's
just a design "flaw" I guess but the antenna certainly seems to work well if
our scores are any indication.
Bottom line, we just ignore it. BTW, renters have actually climbed the
tower to fix the "loose" connection after seeing the SWR changes during wind
storms, then found, of course, nothing loose!
_________
The fluctuation is caused by the 10 and 15 mtr drivers moving at a different
rate in the wind than the 20. The open sleeve driver system is coupling
from the
20 to the adjacent drivers. We tried to make the elements move at the same
rate,
but that is tough, given the diference in lengths. One fellow made a spreader
using PVC and he liked it. It went from the 20 to the 15 and first 10 mtr.
The
second 10 mtr was not much of a factor.
We have looked at this from time to time. Appreciate your note and we will
continue to see what kind of remedy makes sense. At my location, it is not a
problem, even with the touchy 87A amplifier, but perhaps your winds are
different.
Thanks and please keep in touch.
73, Tom, N6BT
Force 12 Antennas and Systems
It is typical to see fluctuations in the VSWR when significant wind is
blowing. The open sleeve feed system we use to drive excite this antenna
relies on critical coupling geometries. The fluctuations are normal and as
I own and use three C3s, I experience the same thing on my windy hilltop
location. Typically, the movement of the elements is such that performance
is not affected, however, if the velocity is high enough, then you might
see some problems with the 15 meter driven element actually touching the 20
meter element. I have seen this only once in three years, however, it can
happen.
73s,
Natan W6XR/2
The C-3 uses a principle similar to the Hy-Gain Explorer-14 (designed in
1982), the open-sleeve dipole. We have always used insulators on the
Explorer-14 to keep the sleeve elements at a constant spacing so that the
SWR does not change in the wind. The C-3 would probably benefit from
these.
73 Roger WB0DGF (Hy-Gain)
Mine has done it since it was first put up. It's even mentioned in the
manual. It's clearly a function of the driven element cell. Just enjoy!
73, Pete Smith N4ZR
n4zr@contesting.com
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