I want to put an eight wire vertical cage around a one hundred twenty
foot steel support for 80 meters. I can center feed the cage with
open wire line, which I assume will be more efficient than feeding the
steel at the bottom. The cage will not be in contact with the ground
or the tower. My thought is that the wires will make a better
radiator than the steel, but I have no idea what the steel "core" will
do to the efficiency of the antenna. I have had no success in
modelling this antenna.

So my question is will the center fed, ungrounded cage dipole provide
any real advantage over feeding the tower at the bottom?

Thanks.

Dave

There is going to be a LOT of coupling from the two halves of the cage
dipole to the tower. It sounds like a recipe for disappointment to me.


As far as a radiator, the steel will work OK because it's got a lot of
surface area. You could always cage it with wires connected at the top
and bottom if you're worried about the conductivity.

I'd feed it at the bottom if I was lucky enough to have a 120 foot
vertical pipe :-)

73,
Dan

You will find hanging a dipole next to a conductor will couple
heavily... I suspect your cage dipole around the tower will cause you
grief...
There has been lots of discussion on feeding galvanized steel towers
with RF... The consensus is that the surface area of the tower makes up
for it's lower conductivity... There appears to be little/nothing to
be gained by straping a few, small diameter wires to the surface of the
tower...
I have run 160 meter receiving tests where I switched between a steel
quarter wave tower and a sloping vertical wire hung off another tower
240 feet away... Both monopole antennas a resonant quarter wave on
160... Relays used to detune the unused monopole..
There is no difference in the received signal strengths that I can
find... At least for 80 and 160 I have no qualms about feeding the
tower... I used to be a believer in adding wires up the tower face,
but I have found the rapture...
If your tower is grounded and it is more than a quarter wave in height,
you can make it into a folded unipole to allow feeding at the bottom...
This will raise the feedpoint impedence, so you will have to use a
parallel resonant tank circuit to match it... I find this better (in
my twisted mind) than taping a few feet up the tower for a feed
point... ymmv...


cheers ... denny - k8do

On Mon, 31 Jul 2006 08:12:41 -0400, David George Johnson
wrote:

I want to put an eight wire vertical cage around a one hundred twenty
foot steel support for 80 meters. I can center feed the cage with
open wire line, which I assume will be more efficient than feeding the
steel at the bottom.

Hi David,

Certainly more complex as you would need to draw the feed off at right
angles for some distance (a quarterwave or more). Do you have a
second 60 foot tower at least that far away to string it out?

As Denny's experience shows, nothing really to be gained (except the
unstated bandwidth increase); and it could be done easier at ground
level feeding the skirt wires that are all connected to the steel mast
at the top. This cage is significantly larger than the mast diameter,
isn't it? It should be, or you are wasting your time.

The cage will not be in contact with the ground

That's fine.

or the tower.

It should be, at the top, as stated above.

My thought is that the wires will make a better
radiator than the steel, but I have no idea what the steel "core" will
do to the efficiency of the antenna. I have had no success in
modelling this antenna.

That is a fairly trivial task: select a sufficient "wire" diameter,
and choose the loss of steel. In the end, for a fat wire, it won't
matter much at the lengths and wavelength you are describing.

So my question is will the center fed, ungrounded cage dipole provide
any real advantage over feeding the tower at the bottom?

Nope.

73's
Richard Clark, KB7QHC

David George Johnson wrote in
:

I want to put an eight wire vertical cage around a one hundred twenty
foot steel support for 80 meters. I can center feed the cage with
open wire line, which I assume will be more efficient than feeding the
steel at the bottom. The cage will not be in contact with the ground
or the tower. My thought is that the wires will make a better
radiator than the steel, but I have no idea what the steel "core" will
do to the efficiency of the antenna. I have had no success in
modelling this antenna.

So my question is will the center fed, ungrounded cage dipole provide
any real advantage over feeding the tower at the bottom?

You know what I think would work better? Four slopers running off at
different angles, 45 degrees from the top of that 120 foot mast. You could
even tune the unused slopers for a better pattern.

Even three slopers would do it, I think.


--
Dave Oldridge+
ICQ 1800667

On Tue, 01 Aug 2006 17:26:02 GMT, Dave Oldridge
wrote:

You know what I think would work better? Four slopers running off at
different angles, 45 degrees from the top of that 120 foot mast. You could
even tune the unused slopers for a better pattern.

Even three slopers would do it, I think.

Hi Dave,

Great advice that maximizes what he has.

73's
Richard Clark, KB7QHC