High Pass T Circuit

High Pass “T” Coupler Circuit from http://www.w0qe.com/Papers/Antenna_Tuners.pdf

Trying to get 4 bands 160 – 30M (and maybe even 20M as a bonus) is tricky with a 2 component L circuit, so I will follow Larry, W0QE’s lead and start with a High Pass “T” circuit – which gives great range at the sacrifice of a little loss. I plan on trying to find one position on the roller inductor and tune using the KPOD just the two variable capacitors:

I’ll move the two capacitors outwards which will give enough room to put the roller inductor in the middle. 

I will first try to use a fixed inductor in between the caps using this slight variation on the circuit:

The fixed inductor might work out instead of a roller inductor . . .

When I tested the use of three components, I did so as a balanced pi circuit. I have a very good feeling that by changing the wiring and making a High Pass T for the Inverted U that I will be able to match all bands 160 – 30M, and using one fixed inductor

I should have the control wire run and the top wire replaced and the u.RAT tested by hand later today after work. I’ll test various components on the u.RAT board. I will go with one of several possibilities – and use clip leads to test a variant of a 2 component L – C circuit and then the 3 component High Pass T circuit – if I have trouble matching 160 – 30M (higher bands being a bonus).

Speaking of higher bands, are there any higher bands that would work using the Inverted U?

Well, sort of – 20 and 17M look great on the elevation angle, but their azimuthal patterns are pretty weird, not even worth thinking too much about.

Here’s my test plan:

  1. Try just one variable capacitor series feed (maybe inductor as well, but I fully expect a capacitor in series will match some bands – but not all)
  2. Try variations on the 2 component L – C
  3. Try the T with a fixed inductor

The u.RAT board will either end up driving one L and C or two C’s in the circuit. If I find that the fixed coil needs several taps to make this work, then I could use the one last spare wire in the control cable to add in a high powered relay and switch in maybe 2 different taps. I would change the KPOD code so that one of the function buttons changes the tap by applying power to the relay. It will have a default setting and a push of the button toggles 12v to that relay – it would mean one GPIO pin – so its really easy to do. I am very confident that one of these methods will work and work well. I can also look into the many relay driver boards that would allow a more exotic set of switchable taps – but I might exceed the number of wires needed in the control cable. I’m hoping I don’t have to go this route at all actually. Just having two steppers doing all the work would be what I really want – so I can get this thing on the air ASAP.


2 Comments on “High Pass T Circuit

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