Dual band vertical antenna for 2 and 4 meter band (70 and 145 Mhz )

Because a 2 and 4 meter vertical is hard to find , I decided to built one.
It's a 1/2 wave with trap.
For both frequencies the impedance is high. About 500 ohms for 2m and 800 ohms
for 4 m.

This can be matched with the right dual band matching network. Consisting of two coils and two C's.
I designed it with a smith chart program. Although you have to retune it somewhat in the
' real world' . First I used plain coils, later isolated.

There is a 5.6 pf and 2.2 and 5.6 pf in parallel. Take at least 500 V types.

This is the circuit, designed with smith chart program.
2 meter section is 89 cm; 4m section 67 cm.

Description
The first (2m) part is 89 cm long. The lower part is of 12.5 mm alu rod and 65 cm long. It's attached with 2 pieces of 18 mm cutting board. In this tube comes a 30 cm long 10 mm tube that can be tuned with a hose clamp.
Then follows the trap. It consists of a 10 mm int. diameter PE tube. external 12 mm. The tubes are attached which 3 mm bolds and nuts. The coil consists of 4 windings, int. diameter 12.5 mm. Attached with cable lugs.
The parallel C is of 6 cm RG-58. When you tune it, start with about 8 cm and then cut it untill there is resonance. This coil is the easiest. For tuning connect it with a high impedance 145 signal, and there shouldn' t be any influence.
Above is 67 cm of 10 mm tube for the 4m band. Because the trap extends this part, it seems less then a half wave.

The dual band matching section consists of 2 coils and 2 C's. When they have the right values, it tunes both bands. It took a lot of  'try and error' to find it out. I also used a 1;4 transformer with FT140-61 toroidand transformed both bands to 200 ohm. To transform this to 500-800 ohm is less critical. However you need large coils (500 and 650 nH)

Trap with 200 nH coil and 6 pf RG-58

Here's the 3 pf fine tuning C

Results

It works fine. SWR is below 1.3 on both bands.

Indoor testing. Sorry for the French..

Determining coils and c' s

Testing with FT140-61 coil, 2.5 windings. This gives larger bandwidth and the step up coils have to be larger; 500 nh and 650 nH. But more complicated.

indoor testing..