Double Wavelength Antenna

[tsunami_australia]

I'm going to be putting VHF back in the car shortly it seems and I'm a stickler for matching antennas where possible (currently two big black sticks on the boot - phone and UHF). The phone will be shifted to the front guard to allow the VHF in which means a nice stainless elevated feed at around 128cm and opposite a big black stick with another on the same side but opposite end.

The full wavelength of CB is around 68cm last i looked (maybe a few off but not much) which only makes 1/2 the VHF length. I will be using a ground independent canister on the bottom for mounting. If I were to go 2x wavelengths of the band in question, how would that affect the radiation pattern and gain? Evey one usually cuts down not up so everyone I've asked has no ideas how it would react. I'm hoping the folks here have had more of a chance than I at testing this.

[Bill_G]

Expect some squirrelly matches / mismatches as you move off center of resonant point. Very steep curves. Apparent gain will drop, but that's more a function of inefficient coupling. Because the antenna is so close to earth, it will still act like a dipole with a null created by the vehicle.

[tsunami_australia]

Transmit will only be across a couple of mhz so bandwidth is not a huge issue. I don't see why gain would drop compared to a regular 6db choked stainless ontop of the can. How would something like that compare with a 1/4 wave or 6.5db on a groundfed can?

[Bill_G]

When you use full wavelength or longer antennas, the gain approaches unity close to earth. Put it in the sky a distance and you get some strange patterns depending on the orientation to earth that could be called directional gain. On a car, I would expect it to act more like a dipole with a distinct, almost critical, resonance curve.

QRZ discussion on fullwave antennas

[tsunami_australia]

Thanks, looks like that thought is on the outer unfortunately. I wanted a length match but looks like it isn't going to happen.