Doublet antenna as vertical T

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Currently as a transmitting antenna I use a 2x15m - about 100 foot doublet antenna fed by a ladder line of about 600 Ohm. This length of the doublet's arms is due to the location of the highest points on the property, where I could hang the antenna and the place where the antenna box stands (remote matchbox). I would have had the opportunity to mount a full-size dipole for 80m, but the center of the dipole fell in an unfavorable place for me (the ends of the antenna almost in the branches of trees or over a metal roof).

The antenna box is about 20 m from the house (in a straight line). I could run a symmetrical line (ladder line) and have the antenna box in the shack, which would probably be very convenient. But then I would lose the possibility of using my antenna as a vertical. So it was necessary to run coaxial cable to the antenna box. I've had opportunity to buy cheaply 40m of used LMR-600 cable, which is perfect for such applications. On the bands where it will be used the losses will be negligible.

Additional benefit of placing the antenna box at a distance is that it avoids interference. During tests with the box in a shack with a symmetrical line, I was able to reset the 5Ghz ubiquiti radio on the roof to factory settings. Additionally, my shack is on the second floor of an old building - about 6 m above ground level. In the case of higher bands there is a problem with grounding, because such a 6 m wire going to even the best radial system - starts to act as a part of the antenna system (acts as a counterweight). We want to avoid that.

A military variometer with a large Q used here as Fuchs match. The left green crocodile clip is connected to the ladder wires tied together, the right green crocodile clip is connected to the ground. The white crocodiles are the variable capacitor connectors - the capacitor rotor "hangs" in the air - it is not connected anywhere. The capacitor stators are disconnected, which increases the voltage resistance. The gap between the capacitor plates is about 4 mm, so the total gap is 8 mm, which is at least 8 kV. This should be sufficient for QRO operation. The red crocodile connectors are the output to the TRX.

The picture was taken in full sunlight, so unfortunately not much can be seen. For now, everything is set for testing at the minimum SWR around 3.73 MHz - SWR 1.07. You can set the whole band to be tuned to almost 1.0 in the center and 2.0 at the edges - in case you can't afford to control the box remotely (you can then "tweak" the box on the radio). I would recommend however, if there is such a possibility - at least remote capacity switching, which will allow you to tune the antenna with one "snap" to the lower and upper part of the band.
A prototype current choke wound with 3m of teflon cable on a ~100 mm outer diameter core used to power three phase motors from an inverter. At low power it works great, unfortunately I don't know the type of material it was made of, or how it will work at QRO.
The capacitor from the radio station. Nominally it probably had about 800 pf. In this circuit the stators were disconnected, the rotor is not connected anywhere. The crocodiles are connected to the stators, its capacitance decreased by about 4x, but the resistance to high voltage breakdown increased by 2x, losses on the connectors were eliminated. This particular capacitor from the picture was also used in a symmetrical antenna box, where it was needed to have high capacitance (PI filter). On the left side you can see automotive relays that can be used to connect additional capacitors in parallel (silver mica capacitors for high voltages). When choosing a capacitor/inductance it is worth to choose these parameters in such a way as to achieve resonance in the middle of the capacitor's tuning range. This will give us a certain range for tuning the system in case of drought or heavy rainfall. In case of choosing the capacitor so that its tuning range is on the edge (min or max) we can sometimes have problems with tuning - there will be too little or too much capacitance.

Unfortunately, there is no free lunch. This type of arrangement requires a fair amount of switching. In this post I showed how it looks for a vertical on 80 m. You can similarly tune such a doublet on 40 m. Unfortunately, this requires changing the taps on the coil and changing the capacitance of the capacitor. If you want to work on contest on one band - there will be no problem - you can tune once and work. If you need to work on multiple bands - without a neat way of remote tuning - the whole thing becomes quite complicated.

Currently, it seems to me that the optimum solution will be remote switching on 160/80 m with a beefy components and for higher bands using a K3NG box with "tweaked" components for QRO using this antenna as a doublet.

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