Experience anyone with this one ? Imagine a vertical half folded dipole.
One side is grounded. Like this:

o ____________________________
l
______________________________l
l
ground.

o = input

Drawn laying down but of course it stands upwards. All is a 1/4 lambda long.
But now the trick: a coaxial cable can be shorted at the end too. But the lenghts needs
only be 0.66 times ! So 40 metres high becomes only 26 metres.
Don't bother the shielding, that is no obstacle for magnetic fields.

On Dec 7, 5:02*am, "D. Heizinga" wrote:
Experience anyone with this one ? * Imagine a vertical half folded dipole.
One side is grounded. Like this:

*o *____________________________
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * l
______________________________l
l
ground.

o = input

Drawn laying down but of course it stands upwards. All is a 1/4 lambda long.
But now the trick: a coaxial cable can be shorted at the end too. But the lenghts needs
only be 0.66 times ! *So 40 metres high becomes only 26 metres.
Don't bother the shielding, that is no obstacle for magnetic fields.

Can't see much point. Makes more sense to just run a 1/4 wave to me.
Sure, the coax has a slower velocity factor than wire, but that's not
going to make a 26m antenna show the same performance as a
wire 1/4 wave. It will still be a "short" vertical vs the monopole.
I'm not sure how the current distribution would be effected.
Would need to model it.. But I'm too lazy to mess with it at the
moment. :/
BTW, there will be some extra loss in the mix using the coax vs
plain wire if you are feeding the center conductor. Much the same as
you would see using a coax dipole such as the common "bazooka".
Also, there is no advantage to using a folded vertical vs a monopole
if avoiding ground losses are the intent.
That used to be a fairly common belief that has been pretty much shot
out of the water in the last several years.
So... Where's the beef?

D. Heizinga wrote:

Experience anyone with this one ? Imagine a vertical half folded dipole.
One side is grounded. Like this:

o ____________________________
l
______________________________l
l
ground.

o = input

Drawn laying down but of course it stands upwards. All is a 1/4 lambda
long.
But now the trick: a coaxial cable can be shorted at the end too. But
the lenghts needs
only be 0.66 times ! So 40 metres high becomes only 26 metres.
Don't bother the shielding, that is no obstacle for magnetic fields.

The antenna as drawn is a common folded monopole
configuration for VHF antennas.

What you are proposing using coax would simply be a shorted
1/4WL stub with very little common-mode current and therefore
very little radiation. Virtually all of the current would be
contained on the inside of the coax and the feedpoint impedance
of the shorted 1/4WL stub would be extremely high. If one could
succeed in shoving some current into that sky-high impedance, the
major losses would be I^2*R and dielectric with very little
energy "lost" from the system as radiation - bad idea.
--
73, Cecil http://www.w5dxp.com