Toroid matching for LF/MF

Since my antenna was born as monoband for 630 meters, until today I’ve used an autotransformer built around the variometer.
In these days I’ve tried to tune the antenna on 136 kHz, beginning to put some mH over mH to reach my goal. Arrived to 136 kHz I’ve sadly found that the autotrafo was inadequate to reach the correct impedance. So I tought to build a toroid coupler as seen on the web. A short look inside my “oblivion boxes” just to find a big blue toroid, this is a N30 material, usually used for interferences from several-KW triphase motor inverters.

It has a quite hi initial permeability and the work frequency is exactly for our game, from 10 Khz to about 400 Khz. Somebody could turn up one’s nose but since turns on toroid are only ten or so and mine is not a KW station I’ve decided to try to build my new matching unit.

For first I’ve winded 10 turns for primary and then 14 for secondary, here is what I’ve gotten:


I’ve soldered 10 wires for as many as different impedence taps and painted the welds with insulating paint.

The next step was to put the “spider” inside a box to be protected from rain and dust (and cats…).


The matching unit under the front cover

Every screw is provided of an OR rubber.20160721_230748The wingnut give me the possibility to fast retune the antenna 20160721_231302The first test on 136 kHz was an immediate success, SWR was 1.14 at first “wingnut touch”! The same coming back to 474! WOW!
It was time to give POWER!!

Just started the trasmission on 136 I’ve seen power going down “rhythmically”: there was something was arching! Testing on 630 the music was about the same. A brief look on my external ammeter was clarifier. My old autotrasformer was not insulated from coaxial cable, common mode current probably was running over the braid. This new solution now shows me a 3 times bigger current on ammeter. The voltage is of course also higher and the antenna wall insulators cannot resist to this new hi voltage.

So at the moment antenna is perfectly tuned but I can use only about few Watts.

Next step is to increase the insulation to use more than few Watts.

Stay toroidal tuned!

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