Friday, June 3, 2011

Field Test -- KD8JHJ Home Brew 40m Antenna




The data from the MFJ-259B:  7.050 MHz  =  10.5 : 1 SWR

5.914 MHz  =  2.1 : 1 SWR

5.818 MHz  R = 37    X = 37

The good news is as I swept the band with the analyzer I detected a well defined dip in the SWR readings.  Unfortunately the bottom of the SWR curve did not coincide with my required frequency range.

At this point I believe it necessary to discuss some technical details concerning the construction of this antenna.  The greatest length aluminum tubing I could receive via UPS was a 6 foot section.  Great! I thought.  I could make an antenna mast 12 feet tall -the bigger the better.  While I was at it and had extra material I made the top mast 3 feet long instead of the prescribed 2' 6".  The coil winding data is intended for use with a 10' steel TV mast pipe.  I did not expect a match right away because of my deviation from the plans.  The interesting question now is what to do? start messing with the loading coil or maybe cut the aluminum tubing down to match the plans?        





3 comments:

  1. That's awesome. Where will you start? I suppose fiddling with the coil might be best, that way if you may be able to remedy the problem without sacrificing any of your tubing length. Looks like a pain in the butt to set up (stake down)! Two person job I imagine?

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  2. Hey phattire,

    I did get out on Sunday for some more testing. I took 4 turns off the loading coil and set it all back up. This brought the SWR at 7.050 MHz down to 7.5 to 1 and also moved up the low SWR dip to 6.118 from 5.914 with the original 20 turns on the coil. That is the outcome I expected so thats cool except it's looking like removing turns from the coil will not make the antenna workable as I still need to shift the dip nearly 1000 KHz more. Removing 4 turns only netted about 200 KHz shift.

    Set up and tear down is pretty easy and a one man job. The antenna will stand on it's own without guys but it would not take much breeze to topple it over so the guys are essential. If it was a breezy day I would need someone at the base to keep a hand on it until the upwind guys are secured at least.

    The pain in the butt is the fact that my back yard is too small for me set this antenna up so I have to load it up in car and take it to the park.

    It does look nice set up if I do say so myself. Been a long time coming.

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  3. Absolutely, it looks really good set up. Impressive! BTW, did you get my email I sent this weekend?

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