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Some Pix of the Completed Feedline Project

03 Aug

I wanted to add a few pictures of my feedline project. I will describe it with words then with some photos. I am not sure if I can retain all of these pictures because of space limitations. However, I will wing it initially and see how it goes.

It should be noted that this is a joint project of two Hams and, with deference to privacy, I acknowledge his contributions and would like to say publicly that I would never have arrived at this place without his instruction and patience.

Since this antenna is intended to be used on all bands from 40 meters to 10 meters at legal power, certain performance criteria need to be maintained. Physical and electrical integrity is paramount due to high voltages present along its route. This balanced feedline was constructed to accomplish two goals. One goal was to fabricate a balanced feedline system that would allow legal limit use with no failures due to high swr induced voltages. The second goal was to marry the well constructed physical line with a set of balanced chokes capable of handling legal limit power levels while maintaining electrical integrity. After 4 weeks of service I can safely say both design goals have been met.

Balanced common mode choking in a feedline such as mine is used to reduce common mode RFI from propagating in both directions along the feedline. Balanced common mode choking accomplishes this while having little to no effects on transmission line signaling. Common mode chokes suppress near field RFI from being conducted back along its length into the shack and it also suppresses ac switching and locally generated household noise from propagating up the feedline in common mode to the antenna and then back into the transceiver in transmission line mode. In doing this exercise I have minimized the RFI into my home in the form of TVI and RFI and reduced my noise floor in my receiver to a very low level. The feedline is not doing all of the noise mitigation, but it goes a long way toward dropping my received noise levels. Each choke box contains one choke with a common mode impedance of 2700 ohms at 14 MHz. The impedance curves are presented elsewhere on this Blog. The feedline is constructed of one eighth inch phosphor bronze wire rope. The insulators are fiberglass rod stock cut and drilled and then epoxied to the PB wire rope. The sources were Surplus Sales of Nebraska and U.S. Plastics Corp. The Boxes contain a single choke constructed with Mix 43 Fair Rite toroids and the center conductor of RG-393 coax which is made from teflon. I removed the shields.

The common mode chokes are impedance potholes. No regard is payed to the impedance bumps that they present to the feedline. Choking impedance is 2700 ohms at 14 MHz. Through impedance is about 110 ohms at almost all frequencies between 1.8 and 30 MHz. There are 4 chokes along the feedline. Two at the feedpoint of the antenna separated by 6 feet and two at the exit to my shack, also separated by 6 feet with the first choke located at the tuner output.

Here then are a few pictures. All hardware is stainless steel save the crimp on lugs and swage’s which are standard metal hardware.

Feedline Curing Epoxy

Prepackaged Common Mode Choke.

Choke fitted into its outdoor box

Swaging completed choke onto completed feedline.

Completed feedline with chokes swaged in place

Antenna Mast with Antenna, Feedline and Choke Boxes in place

An interesting aside. While I was redoing my antenna feedlines, I changed the dipole wire from AWG 22 stranded to AWG 10 (105 strand) which modified the antenna efficiency as modeled in NEC 4 from 91% to 99.X %. Wire size alone bumped efficiency 8 %.

I hope this describes in sufficient detail the whole of my feedline project. Should anyone wish to duplicate this project or ask questions I can be reached at the email address noted at QRZ.Com.

Thanks for reading. Best, Chas W7MAP/5

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Posted by on August 3, 2008 in Radio, RFI, Stuff

 

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