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

[Tomas Magyla]

Tom

appreciate sharing your thoughts. I see there are different ways.
Considering the TX Antenna will most likely be half wave feed away from the
radio, the most appealing to me at the moment seems to be opening the feed
line in the shack, on the PA input/ TRX output cable, as PA in RX mode will
be by-passed. Opening the line on the 100w side of cable does not seem to
represent significant challenge for the relays (I can use the garden
variety I have in the junk box).

I am in a process of doing a small switching box with relays that will be
'manually' controlled by a foot switch as the PA is rather old and does not
have fast relays to enable TRX controlled QSK, so I will anyway need a foot
switch to operate with PA. The PA has already been shipped to the island
earlier. Inside the box I have already installed a switch and variable cap
which in RX mode will allow one of the following: Open/ Short/ Shunted with
a switchable inductance L / Shunted with variable Capacitor/ Shunted with
LC. This will hopefully allow to find a suitable combination empirically in
event of the feed line cable being slightly longer than a half wave.

Failing that I could relocate the box outside to where the 1/4 or 3/4 feed
line point is, extend the control cable and short it in RX mode -
considering the relay contacts would need to stay open in TX mode, assume
most of garden variety relays with space between the contacts of 3mm or so
should provide the required isolation.

For a variable Cap I am using 3 x 445 pf broadcast radio cap, no particular
voltage rating as it will be switched in RX only. The TX ant coax cables
have already been shipped to the island so I will only be able to do some
approximate trials in my back yard with a similar length coax.

73 Tomas, VK2CCC


On Fri, Aug 16, 2013 at 8:17 AM, Tom W8JI <w8ji at w8ji.com> wrote:

> **
> <<<
> 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
>