Topband: How to detune a wire Inverted L? How to 'detune'electrical fence

[Tom W8JI]

<<<
A poor man's version of L worked satisfactory for me in 2010 VK9LL operation, without any tuner at the base, the bottom terminal of the vertical wire directly connected to the center conductor of the coax cable. I had around 90m ??? of feedline going from the shack to TX antenna, to bring the TX antenna to the clear. It was not a perfect match (I had a MiniVNA with me ant taken measurements but dont remember the feedpoint impedance straight of my head). >>>

What you do really depends on what you are comfortable with doing. One case requires a control line and remote relay, the other moves stuff inside or close to the shack.

One possible solution is to add a T connector 1/4 wave electrical from the antenna, and short it there with a relay. The relay has to handle higher voltages but almost no current, provided you time it right.

Another is to go 1/2 wave away, and just open the line for RX. This requires a higher current lower voltage series relay. Provided you time it right.

Timing is a huge issue with some radios, and no problem with others. Many radios can just use the TX line used to key amplifiers. Others will actually require an external sequencing or interlock system (which is no easy task for multiple modes).
 
<<<<What would the methodology be to calculate the load at the shack end to detune it TX ant? Would inserting any significant X move the TX ant resonance point further way do the detuning job?>>>

I would do it emerically, by using the coax you intend to use at a home test setup. Exactly how you do it depends entirely on the test equipment you have and how comfortable you are. It can be done with almost no equipment and a little skill, or less skill and more equipment. 

<<<The location of TX ant for 2013 operation is about 60m away from the shack. Another idea expressed was if the L is fed with a half wave, the open terminal at the shack end during RX would transform into an open contact at the feedpoint making the L float above the ground?>>>

That's correct, although it can be made to work with any cable length if you use an inductor, a capacitor, or an open or short. One of the four will open any length line at the antenna. So would a properly located stub. There are a dozen ways to do it, and the choice depends on what you can do and want to do,  more than what works.

73 Tom