Aluminium plank antenna

[VK2FXXX]

Gday guys

You may get a laugh out of this .
I drive an old toyota van around. On the roof racks of this van I have aluminium planks  strapped tight to rubber topped (insulated racks.)
Do you guys think if I loaded a 4m long alumium plank up ,it would be an effective hf antenna?
My gut feeling tells me no,it would be useless,what do you guys think?
Brendan
VK2FXXX

[WB6BYU]

The issue is the spacing between the aluminium and the roof.   The greater the spacing the better
it would work.

Actually a 4m plank is a bit short all by itself.  You could turn it into a loaded dipole for 10m,
but performance will drop off quickly as you go lower in frequency.  You could make a vertical
antenna fed against the van with a loading coil and use the plank as a top hat - again, wider
spacing will help.

[VK2FXXX]

Yes ,It would be more like a transmission line than an antenna I think.
I have at the moment on the roof racks ,a 4m long plank (aprox13 ft),a 3m long plan(10 ft) a 4.5 m long aluminium ladder.It just seems like too much aluminium too go to waste ,not being used as antennas   
The plank only sits about 6 inches off the roof. Its difficult to have a mobile antenna on the roof as I always pulling things on and off the racks and an antenna would be in the way.
Im not too interested in mobile ops at the moment anyway ,as my licence class only permits 10w pep. Gotta get off my a$%#@ and upgrade.
Brendan

[WX7G]

Fed differentially - as a dipole - it can radiate well but will have a low input impedance due to the proximity of the roof.

[W3JKS]

Fed differentially - as a dipole - it can radiate well but will have a low input impedance due to the proximity of the roof.

On the other hand, at night the corona discharge would be a real eyecatcher! 

[VK2FXXX]

Gday again
I see the problem. I just modelled as 4m long 8" diameter aluminium pipe in mmana.
At 3.6 mhz  In free space R0.35 -j1065 ,  6" over perfect ground  R 0.002 -j426.
Similar to small loops ,so I imagine similar problems with loss in component connections ,Inductor loss in atu would be huge.
Hmmm. If I put an al vertical tube at each end with a wire between .....
Ill be back.

Just modeled a loop 4m x 1m rectangle .Bought R up to about  0.558  and J + 241.
This is 6" over perfect ground. (3.6 mhz)
mmana suggests -6dbi for the antenna.
Now Ive got to put the model over a simulated van!
Still tough on an atu.
I might give it a try tomorrow!
Cheers
Brendan

[WX7G]

You could feed the car as an antenna. Perhaps a gamma match of sorts would be the way to do it.

With a conventional vertical mobile antenna the car forms the other half of a vertical dipole so we're used to using the vehicle for the antenna.

[K1CJS]

Just to inject another thought into this discussion--it may not matter much, but directivity may also show up as an issue.  Just think, you're having a nice QSO, and have just started to discuss a very interesting subject---and you turn a corner and lose the other guy!