There used to be an expensive T2FD by RF Systems that could be
remotely switched to T configuration for use below 3 MHz.

If one wants to cover 0.1-0.54 and 3-20 MHz or so, a diplexer may
allow using the T2FD as TWO specialized antennas:
- a LF "Tee" with both upper and lower wires fed in phase
- a T2FD fed across as usual
with no switching, and rejection of strong signals in the AM broadcast
band - which are a problem for some.


This is what I though up:

For LF, use the T2FD as a short T, with all four semi-arms fed in
phase. Connect each side of the classical T2FD feed point, and each
side of the resistor, to 4 separate unbalanced Low Pass filters (Fc =
..55 MHz), all outputs in parallel, feeding in-phase a stepdown RF
transformer, unbalanced-unbalanced, LF-optimized, 50 ohm out.

For HF, connect only the T2FD feedpoints to a balanced High Pass
filter (fc = 2 MHz), feeding a step-down balun, HF-optimized, 50 ohm
out.

If a single coax is desired, a low-Z, unbalanced, LF-HF combining
diplexer could be used. As an alternative, two coax lines - perhaps
RG9 for two way HF, RG58 for LF receiving.

Result: one antenna for both LF and low MF (Tee) and HF (T2FD), no
remote switching needed, AM BC rejection - particularly strong if a
diplexer is used to output on a single coax.

I still have to work out if the out-of-band load of the front ends of
the BPFs is small enough not to ruin performance too much. I am
thinking of multiple T networks.

LPFs: four unbalanced, Zi=Zo=~3000ohm @ .55MHz, feeding in parallel a
4:1 or so unbalanced transformer to 50 ohm coax. Zi at HF intuits to
be horrendously high - good! Are numbers in the ballpark?

HPF: one balanced, Zi=Zo=300 ohm @ 2MHz, feeding a 4:1 balun into a 75
ohm coax.
How do I get a steep rise in load viewed from MF / LF? Are T networks
steep enough?

Comments? If you can convince me this is dumb, I'll save myself a lot
of crunching, but it would be a pity.

TIA

Filippo