On 21 Sep, 12:14, Helmut Wabnig .... .-- .- -... -. .. --.
@ .- --- -. dot .- - wrote:
On Fri, 21 Sep 2007 14:28:47 GMT, Navtex-Fan wrote:
Hi group,

I'm a SWL and am specially interested in Navtex DX-reception.My problem
is (was) the heavy QRM from about every TV-set, monitor, PC in the
neighbourhood.

I'm currently using a T-aerial: horizontal wire 20 meters long, ca. 10
meter above ground, vertical part ca. 10 meter down from the middle, =
1:9 "magnetic" balun = 25 meter RG58 to receiver. Receiver is located
on the first floor, and is grounded via a thick wire to the earth pin
of the house. Typical reception sounds like this:

http://users.pandora.be/dirk.claessens2/div/not_grounded.wav

...humming, hissing, hard to decode, nightmare.

In despair - and against all advise in antenna textbooks (ground
loops!) -

I have never seen "ground loops" mentioned in an antenna book.
Ground loops are a main problem in audio frequency equipment,
not in HF.

In high frequency equipment GROUND EVERY POINT YOU CAN
to create a quasi-reference potential area as large as possible.

The usual way to overcome stray noise from nearby sources
a

1.) Electrostatic shielding of the antenna.
Use coax for the horizontal dipoles, and ground the coax shield
at the ends but do not create short circuited loops.
(do not confuse with "ground loops")
2. Use a noise eliminator which feeds a phase shifted signal
from a small local antenna into a combining network.
3. Improve grounding as you did.

To me it is not quite clear what happened in your case,
but antennas are and have always been much try-and-error.
w.



I decided to make an additional earth point at the balun. I
drove two 1 meter 1/2 inch copperpipes into the ground, and connected
these to the shield of the RG58 at the balun. Grounding at the receiver
was unchanged. The result was stunning:

http://users.pandora.be/dirk.claessens2/div/grounded.wav

QRM totally gone!
Can anyone explain this?

Thanks for any insight - Dirk- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Hi, let me clarify your problem. The most efficient radiator is one
wave length long where it is considered to be in equilibrium with a
parallel electrical cuircuit
instead of the usual series circuit. For most efficient radiation both
the capacitance and the inductance
must act as a energy storage such that when the terminals are shorted
the energy is released in a burst such that radiation can begin,
the electrical circuit by the way is often called a TANK cuircuit. In
the case of a fractional wave length radiator the pendulum type
radiation is not available for radiation and thus it travels up the
outside of the coax where it can radiate or become part of the "real
resistance" of the radiator in a similar way a vertical 1/2 wave
antenna would do when half of the applied energy is absorbed by the
ground.
Per the replies you will see what these currents are named when one
has an antenna that wants to work as a full wave length long even tho
a 1/2 wave length of the radiator is missing for max efficiency.
Hope that helps
Regards
Art KB9MZ