[TowerTalk] Force 12 C-4

[force12@interserv.com]

Good morning, Dave.

Appreciate the e-mail this morning over the reflector. I scanned back 
through the entire e-mail file to mid-November and have not received 
anything from you. Also checked the waste basket, just in case it 
accidentally got deleted from the active listing. Nothing there, either.

A general comment for those who are using any kind of center loaded dipole 
(or base loaded vertical): 
	The center of a full size dipole is theoretically maximum current 
and zero volts. The tip has maximum voltage and zero current. If the antenna 
is initially tuned higher in the band and then operated lower, using a tuner 
or by adding more loading to the center, the voltage at the center will be 
higher than at the original frequency. If the dipole has been working just 
fine at the original frequency, it does not mean the insulating method will 
be adequate for all conditions when more loading is applied at the center, 
or when a tuner is used to operate at a lower frequency. It might happen 
suddenly, or slowly over time, as the high voltage slowly makes a path 
somewhere on the insulator.


So, this is the situation on the 40 mtr dipole:

In this case, the addition of a tuner to move the resonant frequency down 
shifts the higher voltage closer to the center.

The PVC insulator has arced over to the U-bolt holding the element. The slot 
in the PVC should be aligned with the gap between the U-bolt and the element 
to boom plate. This is the largest air gap and prevents such an occurance. 
Having the 40 mtr resonated at 7.230 and operating through a tuner down on 
RTTY (7.080 or so) is causing a higher than typical voltage condition at the 
center of the element. The normal 2:1 VSWR bandwidth of the 37' element is 
130 kHz, so the lower 2:1 point is at about 7.165. The tuner is doing quite 
a lot of "work"! If the PVC slot is not oriented at the air gap, the higher 
voltage at the center of the dipole can an eventually breakdown across the 
PVC to the U-bolt, which is tight against the PVC and attached to the 
element to boom mounting. The mount is at a much different potential than 
the element, essentially ground.

The PVC is right within reach. It might be possible to clean it and rotate 
the slot to the gap. If necessary, new insulators can be added without 
removing the element (or antenna).

If you would like new insulators, please respond to this e-mail address and 
it should rach me!



Have a good day and 73,
			Tom, N6BT
			Force 12 Antennas and Systems
			(Home Page   http://www.QTH.com/force12 )







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