The T2FD antenna
Posted: Mon Oct 27, 2025 2:59 pm
the T2FD "all band" antenna was a military secret till around 1950 when informations about this aperiodic "all band" antenna started to spread
https://en.wikipedia.org/wiki/T2FD_antenna
yet, the data about calculation behind this antenna arem't clear, so after some ferreting on the intertubes, I collected enough informations to be able to put together some NEC model and determine some baseline, and.. here we go
First of all we calculate the Wavelenght for the LOWEST frequency we want to use the antenna on, at this frequency the antenna has "no gain" and to see some significative gain we'll need to DOUBLE the base frequency this means that, if our lower frequency is 7 MHz, we'll start seeing gain at about 14 MHz; as for the termination resistor and the impedance transformer, the combo offering best result a 1 KOhm non inductive resistor (RATED AT 40% of max TX power) and a 16:1 transformer such a combo gives a good match over the whole antenna working range, from lowest freq up to 5x that freq, so for example from 7 to 45 MHz
if someone is interested, the code below is the 4NEC2 model for a T2FD, all sizes are in meters
as you can see, the span is 1/3 wave at lowest desired frequency, while the spacing is 1/100 wave; the range goes from the lowest frequency (Fl) up to about 5x Fl as I already wrote
by the way, the antenna can be used for RX and TX, and its main advantage is offering frequency agility since it allows to quickly jump from a frequency to another w/o the need to adjust an ATU
https://en.wikipedia.org/wiki/T2FD_antenna
yet, the data about calculation behind this antenna arem't clear, so after some ferreting on the intertubes, I collected enough informations to be able to put together some NEC model and determine some baseline, and.. here we go
First of all we calculate the Wavelenght for the LOWEST frequency we want to use the antenna on, at this frequency the antenna has "no gain" and to see some significative gain we'll need to DOUBLE the base frequency this means that, if our lower frequency is 7 MHz, we'll start seeing gain at about 14 MHz; as for the termination resistor and the impedance transformer, the combo offering best result a 1 KOhm non inductive resistor (RATED AT 40% of max TX power) and a 16:1 transformer such a combo gives a good match over the whole antenna working range, from lowest freq up to 5x that freq, so for example from 7 to 45 MHz
if someone is interested, the code below is the 4NEC2 model for a T2FD, all sizes are in meters
as you can see, the span is 1/3 wave at lowest desired frequency, while the spacing is 1/100 wave; the range goes from the lowest frequency (Fl) up to about 5x Fl as I already wrote
Code: Select all
CM ------------------------------------------------------------------
CM T2FD antenna :: feedpoint 800 Ohm (use a 16:1 BalUn)
CM ------------------------------------------------------------------
CE
' frequency and lambda
SY freq=7.000
SY fwrk=freq*2 ' start of gain
SY wave=(300/freq)
' wire radius
SY wire=0.00125
' span and width
SY lfac=0.333
SY sfac=0.01
SY leng=wave*lfac
SY spac=wave*sfac
' arms and sides
SY arms=leng/2
SY side=spac
' termination resistor
SY resi=1000 ' Ohm
' heights
SY hght=10
SY hlow=hght
SY hupp=hlow+side
' conductors segmentation
SY segl=51
SY segm=11
SY segs=1
' feedpoint and termination placement
SY feed=segl/2
SY pres=feed
' arms
GW 1 segl 0 -arms hlow 0 arms hlow wire
GW 2 segl 0 -arms hupp 0 arms hupp wire
' junctions
GW 10 segs 0 -arms hght 0 -arms hupp wire
GW 11 segs 0 arms hght 0 arms hupp wire
' ground
GE 1
GN 2 0 0 0 13 0.005
' conductors
LD 7 0 0 0 2.1 wire ' teflon
LD 5 0 0 0 58000000 ' copper
' termination resistor
LD 1 2 pres pres resi 0
' enable extended kernel
EK
' feedpoint
EX 0 1 feed 0 1.0 0.0
' test freq
FR 0 1 0 0 fwrk 1
' end of model
EN
