Monday, February 28, 2011

The T2SFM Antenna

The T2FD (aka TTFD, Terminated Tilted Folded Dipole) may be know to many of us. No, I am not going to discuss the T2FD here. It is just mentioned since it forms the basis for the following design considerations.
The only thing we should keep in mind, the span of a T2FD is 30% of the lowest wavelength, not 50% as a regular folded dipole would be. The distance of the fold is 1% of the wavelength.

In a country as densely populated as Holland, one needs to think of room saving antenna solutions regularly. The goal of the game is hence to reduce the span of antenna setups.

The aerial I am thinking of needs to the at least the popular QRSS bands 80m - 30m, possibly 160m and 20m.

First design step, the a quarter-wave radiator for 30m and tilt it. The length of such radiator would be 7.6m.

Second step, a trap terminating the 30m radiator. This trap also acts as loading inductor for the lower frequencies. As a fist assumption, I was randomly picking 10µH, the resulting capacitance is 25pF.

Third, the folding distance is chosen to be 50cm, reflecting an operation wavelength of 50m, which is somewhere between 40m and 80m obviously.

The last and remaining bit would be the second "T" of the name, i.e. the termination. I figured 600Ohms could be a good value to start from. When putting this into reality, one needs to remember that the termination resistor should not be induction and should be rated at least 30% of the power applied to the aerial.

There the MMANA script a hacked in:

80m - 20m T2SFM
0.0,    0.0,    0.0,    -7.0,   0.0,    3.0,    8.000e-04,    -1
-7.0,   0.0,    3.0,    -7.0,   0.0,    3.5,    8.000e-04,    -1
-7.0,   0.0,    3.5,    0.0,    0.0,    0.5,    8.000e-04,    -1
0.0,    0.0,    0.0,    0.0,    0.0,    0.5,    8.000e-04,    -1
1,    1
w1b,    0.0,    1.0
2,    1
w2c,    0,    2.17,    114.0,    0.0
w4c,    1,    600.0,    0.0
800,    80,    2.0,    1
2,    0.0,    1,    600.0,    120,    60,    0.0
Mod by Joachim, PA1GSJ 2/28/2011 9:55:09 PM
Created by Joachim, PA1GSJ 2/28/2011 9:08:01 PM

The first simulations look promising, still a lot room for optimizations. Would be time to also start putting stuff together and try out if this contraption is any good.

Had another thought about the 30m trap. The easiest way to make a trap would be a coax trap. With a diameter of 4cm, a length of 4cm a coax trap would have a self-inductance of 2.17µH and a capacitance of 114pF. Purely mechanical, 117cm of RG58 are required in order to form 8 turns on a 4cm diameter.

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