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miscellaneous resonances that can be found with such combinations as half an element to the boom, up the boom to the opposite side of a different element. A change of that nature between the Cushcraft 402CD and the XM240, the latter a "beefed up" "electrically identical" upgrade of the former, is widely believed to explain the latter's propensity to detune the 15 meter section of close tribanders. This was a characteristic not found on the 402CD, probably the best mini 40 meter beam going. (Yes, I know this is a Force 12 reflector, and that Natan is going to choke on his milk and cookies whenever he reads this. The 240N is not a "mini" beam, but 85% of full size.) What the 402CD didn't have going for it was BEEF. There were a number of strength modifications available for it, various sources. Cushcraft's answer to the need for beef was the XM240, which, it turns out, really needed F12's larger element mounts, PVC wraps for the element, and larger U-bolts, all to insulate the elements. On the local scene, receiving wise, an F12 antenna with the insulated elements will do better at resisting picking up off-pattern local noise, due to the lack of the aforementioned miscellaneous resonances. On the other side of the coin, if one is using a shunt-fed tower with a top F12 antenna as a 160 meter transmitting antenna, at 1.5 kW, the highest ground connected point will carry a large peak RF voltage. These RF voltages will see the insulated elements as another place to go, and can jump the gap and create carbon paths. I will probably connect the center of my C31's twenty meter director and reflector via a 1.8 MHz series circuit to the boom. This will keep the RF from building up to a peak value to jump across the PVC. At all the other points, the RF voltage will be 15 feet or more from the peak RF voltage which now resides out at the tip of the front and rear 20 meter elements, which reduces it significantly. - - . . . . . . - - . . . . - - . . - . . 73, Guy k2av@contesting.com Apex, NC, USA ----- Original Message ----- From: Danny <ON7NQ@pandora.be> To: <Force12Talk@qth.com> Sent: Monday, August 28, 2000 7:42 AM Subject: [Force 12 Talk] Isolated elements > Hello > > Is there any specific reason why Force 12 isolates the elements > from the boom ? > > > 73 Danny - ON7NQ - > > ON7NQ@pandora.be > http://users.pandora.be/ON7NQ (best viewed with java enabled ) > http://www.qsl.net/on7nq > > DXC : ON0DXA > WWDXC :EA7URC-5 , PI5EHV-8 > #CQDX chatbox > > > > > -------------------------------------------- > Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. > Force 12 Web site: http://www.qth.com/force12 > > Submissions: send to Force12Talk@qth.com > To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com > Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk > For problems with the list, contact n4zr@qth.com > > > -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact n4zr@qth.com |
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