Topband: Inverted L

[RT Clay]

> 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