> If you remove the slit PVC tubes you will be subjecting the
> aluminum elements to stresses that they were not designed to
> receive. The original U-clamps will be of the wrong diameter
> and will not fit around the elements correctly, compressing
> from the top only.
>An even larger problem is that changing the elements of an
>F12 antenna from insulated mounting to grounded with the
>relatively large boom to element plate will detune the antenna.
>The element mounting plates will change the effective length
>of the elements and you will effectively have a different
>antenna. How much that changes the performance of the antenna
>is hard to say but it's a big risk unless you know exactly
>what you're doing.
I recently went through this- on top of my 100 foot tower that I shunt feed for
160 is a Force 12 EF 420/240 (4 element 20 + 2 shortened 40m elements). On 160
I was not getting any flashover of the insulators, but there was a big problem
with the ferrite choke balun on the 40m yagi. When running high power on 160,
the 40m balun heated up quite a bit and the 160m swr changed noticeably after
transmitting a short time. The ferrites got hot enough to partially melt the
PVC plastic tube containing the balun.
I modified the F12 beam in two ways:
1) replaced the 40m ferrite balun with a large air-core coax balun on a plastic
sewer pipe.
2) grounded the center of each parasitic element (20m and 40m) through a small
air-wound coil of #12 wire. I left the plastic element insulators in place.
Now the 160 shunt feed is completely stable at high power. Judging by how I had
to retune the shunt feed, the top-loading increased. I have not noticed any
problems with the F12. There was a slight SWR shift on the 40m yagi, but I
attribute this to a slightly different lead length on the air-core balun.
Tor
N4OGW
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|