On Wed, Aug 1, 2018 at 10:24 AM, iz oos wrote:
I would use ferrites with permeability close to 800, not the powdered iron toroids like the red and the yellow you have selected (permeability is less than 10).
For a parallel-resonant transformer, we don't want an excessively high permeability; what we want is the core material which will provide the highest Q at the frequency(ies) of interest.? The high-side winding impedance should present a reactance which is near the
geometric
mean (
gm) between the 50 Ohm input and the? Antenna's feed impedance at its end (1500~4500 Ohms).? A 49:1 impedance transformation (7:1 turns ratio) presents a nominal impedance at the antenna terminals of 2450 Ohms as has been previously stated, and for this case the
gm is ~350 ohms.? The reactance of the coil in the parallel circuit should be equal to the
gm on the lower/lowest band of intended use -
I.E. a T80-2 (red) toroid with 36 turns #24 = 7.13uH.? The reactance at 7.0Mhz is ~315 ohms... close enough for our purpose.? On 40 meters a capacitance of between 65pF and 75pF will resonate the matching unit across the band, assuming the antenna wire is resonant (presents zero reactance) at ~7.100Mhz.
On 30 Meters, the same coil will present an inductive reactance of ~450 ohms, and require 32~38pF of capacitance to resonate the system, assuming the wire is resonant (again, zero reactance point) within the 10,100 ~ 10,150Mhz band.
So, a single -Resonant- EFHW tuner can be used (with separate, resonant antenna wires) on two adjacent amateur bands, I.E. 40 & 30 Meters.
On 20 Meters, the reactance of the coil is on the order of 630 ohms and required capacitance on the order of 18pF, and both of these figures present difficulties in actual implementation.? In the first issue, a
gm of 630 Ohms suggests an antenna impedance of well over 6000 Ohms, which is un-achievable in nearly all terrestrial environments; the second issue is that
the 18pF figure is inclusive of stray capacitance, which may be 10pF (or more) depending on materials and physical construction...? this will make tuning on 20M very sharp, and offer only a very limited tuning range because the minimum capacitance of the variable capacitor may well be equal to the capacitance necessary to resonate the antenna system.
Having two separate [switched] coils makes it possible to cover 4 or 5 bands with a single matching unit - 40/30 Meters on Coil #1, and 20/17/15 Meters on Coil #2.? Two wires will? cover four bands: A) 40/20 Meters, and B) 30/15 Meters...? a third wire is required for 17 Meters, if desired.
The
gm for 40 & 30 Meters is 8.33Mhz, so the T80-2 coil should have a reactance of 350 ohms at this frequency; which is 35 turns on a T80-2 toroid.? The total capacitance necessary to resonate the system at 7.0Mhz will be in the range of 70~80pF, and at 10.12Mhz will require 30~40pF to resonate the system, minus whatever stray capacitance is present.
The
gm for 20~15 Meters is ~17.52Mhz, so the T80-6 coil should have a reactance of 350 ohms at this frequency; which is 27 turns on a T80-6 toroid.? The total capacitance necessary to resonate the system at 14.0Mhz will be in the range of 36~44pF, at 18.13Mhz in the range of 20~28pF, and 15~20pF at 21Mhz, minus whatever stray capacitance is present.? The 15pF figure at 21Mhz is very low, indicating that tuning on 15 Meters will be rather sharp, that tuning range may be limited, and that efficiency will be a bit lower on 15 Meters (depending on both stray capacitance and stray inductance in the actual construction).
