Hello,

Have just started reading about, and studying antennas.

Simple question regarding the classic dipole for receiving (although I
imagine it's exactly the same for transmitting):

Hope I can explain my question clearly:

I understand that If the dipole is fed with coax, one leg is tied to the
outer coax shield.

Regarding the end at the radio -

Is the coax shield "always" tied directly to the radio's chassis ground,
(which is also earth ground for my set) ?

Or, are the outer and inner coax conductors sometimes fed into an
isolation transformer, and therefore the coax shield would not connect
directly to the radio chassis ?

Thanks,
B.

I understand that If the dipole is fed with coax, one leg is tied to the
outer coax shield.

Regarding the end at the radio -

Is the coax shield "always" tied directly to the radio's chassis ground,
(which is also earth ground for my set) ?

This is almost always the case. The coax shield may also be grounded
at one or more locations between the antenna and the radio chassis
(e.g. where it enters the building), for both RF-grounding and
lightning-safety-grounding reasons.

Or, are the outer and inner coax conductors sometimes fed into an
isolation transformer, and therefore the coax shield would not connect
directly to the radio chassis ?

This can be done (with an isolated "unun" transformer) but it is
rarely done.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Wed, 14 Dec 2005 18:52:00 -0500, "Robert11"
wrote:

Hello,

Have just started reading about, and studying antennas.

Simple question regarding the classic dipole for receiving (although I
imagine it's exactly the same for transmitting):

Hope I can explain my question clearly:

I understand that If the dipole is fed with coax, one leg is tied to the
outer coax shield.

The coax shield does not *have* to be tied to one leg of the dipole, a
balun could be used to connect the unbalanced transmission line to the
approximately balanced load. A balun is a device which accomodates the
transition from balanced to unbalanced or vice versa, without
significantly disturbing either environment.

If the coax shield is directly connected to one leg of the dipole (and
that is often done), the coax doesn't just provide a means for
transferring energy from the feed point to the receiver (using the
outside of the inner conductor and the inside of the outer conductor),
but the outside of the outer conductor is now connected at one end to
the dipole leg, and it and everything connected to it forms part of
the receiving antenna itself.

The ins and outs of it!

Owen
--

B. wrote:
"Is the coax shield "always" tied directly to the radio chassis ground,
(which is also earth ground for my set)?"

"Always" may embrace too much. Usually, the coax shield is almost
directly connected to the radio chassis ground.

B already implied that his dipole has a potential problem as its
connection to the grounded inner surface of the coax shield is shared at
this point by connection to the outer surface of the same coax shield.
This inherently unbalnces the load on the inner surfaces of the coax.
The center conductor has only one load, its half of the dipole. The
inner surface of the coax shield has two loads, its half of the dipole
and the outer surface of the coax.. This is fixed by by a balun between
the balanced dipole and the unbalanced coax.

On the other hand, you may have no objection to distortion of the
dipole`s pattern by radiation from the coax exterior. In some cases it
won`t hurt. In other cases it may be beneficial. It is somewhat
unpredictable, but many dipoles are directly fed by coax and are
satisfactory.

Best regards, Richard Harrison, KB5WZI

On Wed, 14 Dec 2005 23:58:47 -0000, (Dave Platt)
wrote:

and the radio chassis
(e.g. where it enters the building), for both RF-grounding and
lightning-safety-grounding reasons.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As has been explained here many times before, you can not "RF ground"
your coax in the sense of connecting it to earth ground. The
wavelength of the ground wire prevents a true ground from happening.

You do need a safety ground (for the AC mains) and a "ground" to your
radio chassis, but that's all. I put "ground" in quotes because it
isn't really connected to earth. The word "common" would be more
accurate, but most hams think of their chassis as "ground". So be it,
as long as the true condition is understood.

73, Bill W6WRT

Bill Turner wrote:

As has been explained here many times before, you can not "RF ground"
your coax in the sense of connecting it to earth ground. The
wavelength of the ground wire prevents a true ground from happening.

You do need a safety ground (for the AC mains) and a "ground" to your
radio chassis, but that's all. I put "ground" in quotes because it
isn't really connected to earth. The word "common" would be more
accurate, but most hams think of their chassis as "ground". So be it,
as long as the true condition is understood.


"Ground" is one of the most misued terms among amateurs that I
encounter. Somehow, labeling something "ground" imparts magic properties
-- "grounded" things don't radiate, you can't get a shock or rf burn
from "grounded" items, "grounded" items are free from the same rules
that all other conductors must follow. None are true.

If your rig is mains-powered, you need a mains safety ground, as Bill
said. This effectively connects the equipment to the Earth at mains
frequency. If lightning is a threat, you also need a lightning ground,
which effectively connects your antenna to the Earth when lighning
strikes the antenna or nearby. Making an effective safety ground is
simple; making a lightning ground is an art and science in itself.

If you have a balanced load for your rig (for example, a coax or
twinlead feedline with equal and opposite currents on the two
conductors), you don't need a "radio ground", that is, a low impedance
connection between your radio and the Earth at radio frequencies. The
only time you need one is if you've managed to make your rig a part of
the antenna by allowing currents on the outside of a coax feedline, or
unbalanced currents on a twinlead feedline. And getting this low
impedance connection can be difficult -- or impossible, if multiple
frequency bands are involved.

Roy Lewallen, W7EL