what one length of a simple wire dipole for general scanning

[WARLOCK]

can anyone please sugest what length to cut the wire to make a simple dipole for general scanning,118.000mhz to 400.000 mhz say? mainly to cover the 2 airbands.

[crusty]

No one dipole will cover that range of frequencies efficiently, the nearest you'll get is an off-center fed dipole as it will work on its even harmonic as well as it's fundamental: Mounted vertically the bottom leg (attached to coax shield) would be 40cm and the top leg (attached to coax center) would be 80cm.

A better option would be to create a vertical nest of center-fed dipoles joined at the same feedpoint as follows.
Dipole 1: 60cm each leg
Dipole 2: 28cm each leg
Dipole 3: 20.5cm each leg

Experimentation will show which suits your needs.

[W-U-B]

400 Mhz is almost a harmonic of 118 Mhz

So a full wave at 118mhz will be something like 2.5m long, which will also be pretty much a 1/4 wave at 400 Mhz

(quick and dirty figures cos I haven't got any charts handy)

[crusty]

The feedpoint impedance of a full-wave is very high WUB (thousands of ohms), it will also not cover the bands Warlock is most interested in (118-138 & 225-400MHz).

[W-U-B]

It's a scanner, RX only

[crusty]

A few hundred ohms mismatch you can get away with for receive only. A few thousand will result in a significant reduction in signal-voltage capture, weak signals may be outside the detection capabilities of the scanner. Your proposed design also places the fundamental resonance point at 114.5 MHz which is below the 118-138MHz airband. Additionally, at 400MHz it would be almost three and a half wavelengths long, not 1/4 wavelength as stated. Antenna design characteristics are reciprocal, be they for receiving or transmitting (though not as critical for receive purposes). This is why commercial airband antennas are not a full wavelength long.