Tower Resonance Breaker?
 

I have a very small yard and a 70ft tower with a beam on top. I want to put
up transmitting antennas for 80m and 160m. For the space I have, some form
of vertical seems the best. I do not want to shunt feed the tower as a
vertical because the feed lines run a long way to the tower from the house
along a messenger cable that is 10ft above the ground (no buried coax). The
feed lines would become part of the radiating system and would put a lot of
RF into my shack.

I would like to string a wire quarter wave 80m vertical alongside the tower
but I know it will heavily couple to the tower. I would also like to string
a 160m inverted-L alongside the tower. This would likely couple into the
tower too.

I have seen resonance breaking "traps" described in ON4UN's book. He calls
them "resonant linear traps" shown on page 9-27 section 2.2.11. He forms a
rectangular single turn coil in parallel with a variable capacitor using the
tower as one side of the rectangular coil.

Does this work? Do you have to use the entire length of the tower to form
the "trap"? Could I just use a 4ft segment of the tower half way up with the
electrical effect of breaking the tower up into two shorter pieces?

I have also seen what is called a "resonance breaker" described in the ARRL
Radio Frequency Interference book. It is similar to the one described above
except it is a two turn coil made of insulated wire. The coil is twisted
open to form a figure eight and then has the variable capacitor added. The
advantage of this configuration is that no electrical contact is needed
between the conductor you wish to break "resonance" in. The "trap"
magnetically couples with the conductor to impart the high impedance.

Does anyone have experience with these things? Can I use a GDO to set the
resonance of the "trap"?

Jeff - KA9S

KA9S-3_Jeff wrote:
Can I use a GDO to set the resonance of the "trap"?

Just be sure that the trap is in free space and
you are verifying the frequency of the GDO with
a good communications receiver.
--
73, Cecil, W5DXP


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I have a very small yard and a 70ft tower with a beam on top. I want to put
up transmitting antennas for 80m and 160m. For the space I have, some form
of vertical seems the best. I do not want to shunt feed the tower as a
vertical because the feed lines run a long way to the tower from the house
along a messenger cable that is 10ft above the ground (no buried coax). The
feed lines would become part of the radiating system and would put a lot of
RF into my shack.

Shunt feeding the tower is probably the simplest option.
Why not coil the feedlines into chokes when they leave the tower, or
put stacks of ferrite beads on them to keep RF off them?

Torsten
N4OGW

On 10 Mar 2005 13:18:18 -0600, wrote:


I have a very small yard and a 70ft tower with a beam on top. I want to put
up transmitting antennas for 80m and 160m. For the space I have, some form
of vertical seems the best. I do not want to shunt feed the tower as a
vertical because the feed lines run a long way to the tower from the house
along a messenger cable that is 10ft above the ground (no buried coax). The
feed lines would become part of the radiating system and would put a lot of
RF into my shack.

Shunt feeding the tower is probably the simplest option.
Why not coil the feedlines into chokes when they leave the tower, or
put stacks of ferrite beads on them to keep RF off them?

Torsten
N4OGW


Or, use an UNUN.

I am not familiar with the specific antenna you are describing, but in
the ARRL Antenna book, it describes an antenna where the coax is taken
up the tower where the shield is grounded to the tower and the other
half of the 'dipole' is a wire strung out to where ever.

It's just an option.


--
73 for now
Buck
N4PGW

I agree with Torsten. You need make no changes to your present run of
cables (along a messenger cable) other than adding clamp-on chokes to each
wire (including the messenger cable). If the "run" is a significant part of
0.25 WL at a frequency of concern, you may find it desirable to place a
second set of clamp-on-chokes about 0.25 WL from the first set or near the
house entrance (whichever comes first).

You will need radials in order to have reasonable efficiency. An
impedance bridge will help in finding the connection spot on the tower where
the real part of Z is 50 (or 70) ohms.

Let us know what you do.
73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
wrote in message
...

snip

Shunt feeding the tower is probably the simplest option.
Why not coil the feedlines into chokes when they leave the tower, or
put stacks of ferrite beads on them to keep RF off them?

Torsten
N4OGW