[TowerTalk] L or shunt? what to do?

Carl Braun Carl.Braun at lairdtech.com
Sun Jan 5 09:29:22 EST 2014


Im working on installing an 80/160 inverted L that will be approximately 165' in total length with the vertical portion being 88' tall and the rest of the wire (77') running out to a 80' tall palm tree.  This will give me approximatley 5/8w on 80 and 5/16w on 160 and will allow me to use a single antenna wire into my matching networks at the base.

Ive thought about shunt feeding the tower and have researched the archives here to get some ideas but I have two concerns:

1)       Im using a Tri Ex TM 490 (90') Skyneedle and am concerned about good grounding between the tubular sections if I was to shunt feed the thing. Im not crazy about running a "continuity wire" down the length of the needle.  Thus my interest with an inverted L off of the tower.  I've constructed a 1 ½" copper pipe around the base of the needle and the control panel that measures 6' x 4' that will act as my radial "plate".  I plan on having 50 1/8 wl radials and 10 ¼ wl ground radials when finished
2)      I was unable to get a real feel for where the tower resonates.  I have a 5 element Telrex monster on top of the Needle that uses grounded elements and Im fairly sure the antenna resonates below the 160M band.  Im assuming this as I disconnected the coax where the Telrex feeds into my control panel at the base of the Needle and placed an MFJ analyxer between the braid of the yagi coax and GND.  The MFJ was beginning to dip when I ran out of real estate on the meter...the MFJ bottoms out at @1750KHz.  Im assuming the tower and antenna resonate at @1700KHz
3)      I have concerns that I could not resonate the tower on 80m if I shunted it

Considering these items I wanted to keep things tidy so Im installing the inverted L off of the tower (36") and starting from there.

Here are my questions:

1)       Will I have to de-tune the tower in order to get the L to resonate on 80 and 160?
2)       Am I better off continuing to try and find a resonant sweet spot on the tower and shunt instead of an inverted L?
3)       I would expect 10-20 ohms at 160 and approx 40-60 ohms on 80 with the full sized 165' vertical.  Am I close?

Comments from the gurus?

Tnx de AG6X

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