I'm putting a vertical back up for transmitting and will
probably receive on my horizontal dipole. The vertical
will be mounted on the top of a 30 foot Rohn 25G tower
with the base grounded in cement. The question arose in
my mind whether to connet the drooping elevated radials
to the tower or not. To my surprise, EZNEC says that RF
current flows in the tower whether the radials are
connected to the tower or not. I'm using mininec ground
for the simulation. The radial fields are obviously
inductively coupling to the tower. Does anyone else
worry about such things?
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

The question arose in
my mind whether to connet the drooping elevated radials
to the tower or not. To my surprise, EZNEC says that RF
current flows in the tower whether the radials are
connected to the tower or not.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There's no reason for the tower to be part of the antenna, so I would
suggest not connecting them. Some current will flow in the tower due to
close proximity induction, but don't make it worse than necessary.
Hopefully the tower is not resonant on any band to avoid additional
losses.

Bill, W6WRT

Bill Turner wrote:
There's no reason for the tower to be part of the antenna, so I would
suggest not connecting them. Some current will flow in the tower due to
close proximity induction, but don't make it worse than necessary.
Hopefully the tower is not resonant on any band to avoid additional
losses.

Unfortunately, the tower is close to resonance on 40m, my favorite
band. Even with elevated radials at 30 feet, it is 0.8 dB down
from a ground mounted vertical, according to EZNEC. Guess I'll go
back to my 2x4 support system. Darn!
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote
I'm putting a vertical back up for transmitting and will
probably receive on my horizontal dipole. The vertical
will be mounted on the top of a 30 foot Rohn 25G tower
with the base grounded in cement. The question arose in
my mind whether to connet the drooping elevated radials
to the tower or not. To my surprise, EZNEC says that RF
current flows in the tower whether the radials are
connected to the tower or not. I'm using mininec ground
for the simulation. The radial fields are obviously
inductively coupling to the tower. Does anyone else
worry about such things?
============================================

Cec, I'm curious but not surprised. What are the approximate
dimensions?

Height of tower.
Length of vertical radiator.
Number and length of radials.
Any loading coils?

Your discovery of current in the tower should not be surprising. It is
just another radial.

It seems to me there should be negligible loss in the tower. There may
be a very small loss in the ground. The tower is just another
radiator. There will be some effect on the radiation pattern in the
vertical plane. It will affect the so-called take-off angle. It may be
advantageous.

Imagine what might happen if the radials were removed.
----
Reg, G4FGQ

Cecil Moore wrote:

Guess I'll go
back to my 2x4 support system. Darn!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2x4s have served well for years. :-)

Bill, W6WRT

I'm putting a vertical back up for transmitting and will
probably receive on my horizontal dipole.

Why? Doesn't make sense. If the vertical is best for
transmit, it should be the best for receive 98% of the time.
And visa versa.. When I ran both a dipole, and a GP,
I almost always transmitted on the antenna that I received
the best signal to whatever path I wanted to work.
If you run a good vertical for DX, and receive on the dipole
you will lose half the advantage of running the vertical
the way I see it.

The vertical
will be mounted on the top of a 30 foot Rohn 25G tower
with the base grounded in cement. The question arose in
my mind whether to connet the drooping elevated radials
to the tower or not. Does anyone else
worry about such things?

I don't think it matters too much as far as the operation
of the antenna. Either will work. But almost all my GP's
had the radials attached to the mounting plate, which in
turn was mounted to the mast. So all mine had the radials
connected to the top of the mast via the mounting.
Same as most any commercial ground plane...
MK

Cecil Moore wrote:
Bill Turner wrote:
There's no reason for the tower to be part of the antenna, so I would
suggest not connecting them. Some current will flow in the tower due to
close proximity induction, but don't make it worse than necessary.
Hopefully the tower is not resonant on any band to avoid additional
losses.

Unfortunately, the tower is close to resonance on 40m, my favorite
band. Even with elevated radials at 30 feet, it is 0.8 dB down
from a ground mounted vertical, according to EZNEC. Guess I'll go
back to my 2x4 support system. Darn!

To fix a problem on a single band, you could de-tune the tower with a
loop trap - a wire running alongside the tower for part of its height,
with a capacitor to bring the whole thing to parallel resonance.

It's described in the antenna books by ON4UN and G6XN (and possibly also
in the ARRL Antenna Handbook).



73 from
Ian GM3SEK


--


73 from
Ian GM3SEK

wrote:


Why? Doesn't make sense. If the vertical is best for
transmit, it should be the best for receive 98% of the time.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Nope. Verticals are notorious for picking up local man-made noise.
Man-made noise starts out with random polarization but the horizontal
component is quickly "shorted out" by the earth's conductivity, leaving
only the vertical polarized component. This is why AM broadcast
stations universally use vertical polarization; better groundwave
coverage. They are not concerned with skywave or local noise.

For skywave, either one works fine, but a horizontal antenna is quieter.

Bill, W6WRT

Reg Edwards wrote:
Height of tower.

Approximately 30 feet.

Length of vertical radiator.

22 feet.

Number and length of radials.

4 x 22 feet.

Any loading coils?

No, an SGC-230 at the base.

Imagine what might happen if the radials were removed.

I imagined that and got very high angle radiation on
some bands. The tower becomes part of the radiator.
--
73, Cecil http://www.qsl.net/w5dxp

wrote:

I'm putting a vertical back up for transmitting and will
probably receive on my horizontal dipole.

Why? Doesn't make sense. If the vertical is best for
transmit, it should be the best for receive 98% of the time.

That's certainly not true at my QTH. The vertical has two
extra S-units of noise on receive compared to my dipole.
I'm assuming the two extra S-units of noise on receive
won't affect my transmitted signal. When I had my 40m
vertical up, I never heard any signal that was better
received on the vertical. +12 dB of noise is virtually
impossible to overcome in actual practice.

But almost all my GP's
had the radials attached to the mounting plate, which in
turn was mounted to the mast.

Conductive mast?
--
73, Cecil http://www.qsl.net/w5dxp