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

[Tomas Magyla]

Tom

it is a 1/4 wave L, with the vertical section dictated by a 18m Spidermast
telescopic rod.

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).

I would be more tempted to use the same poor man's version as I will be
cutting the handle of my toothbrush to fit all the radio gear into the 14kg
airline baggage allowance.

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?

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?

Thanks heaps

Tomas VK2CCC



On Fri, Aug 16, 2013 at 12:39 AM, Tom W8JI <w8ji at w8ji.com> wrote:

> Q1) Would a simple relay disconnecting the vertical wire at the TX antenna
>> feedpoint be good for detuning TX antenna? (I am using a piece of wire
>> attached to 18m Spidermast for the vertical section) and another piece of
>> horizontal on the top.
>>
>> Q2) Are there any practical solutions for detuning TX antenna by using
>> some
>> circuitry at the shack end instead?
>>
>> Q3) What in practice could be done about the impact of the electrical
>> fence? (other then moving RX antenna as far away as possible/ and
>> considering using a shielded coax loop / or Waller Flag to 'null' the
>> fence
>> out?)
>>
>>
>>
> Assuming it is a 1/4 wave L, anything that electrically opens the base of
> the antenna at the feedpoint with maximum possible impedance detunes the L.
>
> Depending on any networks or transmission lines between the detuning
> switch and the element, the optimum termination at the switch might be
> almost anything. It is quite possible to detune the antenna from the shack,
> although it will generally require a certain value of inductance or
> capacitance to be switched in. I detune my 200 ft tower, fed through an L
> network, about 50 feet from the element and L network with a short.
>
> Tom
>