Hi All,

I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. Am I wrong about
this?).

Thanks!

Nick

On Jun 24, 7:18*am, Nick wrote:
Hi All,

* I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. *My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? *And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. *Am I wrong about
this?).

Thanks!

Nick

I think line isolator is slightly different from balun. Here line
isolator would be necessary for lightening protection etc and hence
better kept at antenna base. Balun is for converting a balanced
structure like dipole antenna to unbalanced structure like coax. Since
monopole antenna is unbalanced structure it does not need balun at its
input.

Amit

wrote in message
...
On Jun 24, 7:18 am, Nick wrote:
Hi All,

I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. Am I wrong about
this?).

Thanks!

Nick

I think line isolator is slightly different from balun. Here line
isolator would be necessary for lightening protection etc and hence
better kept at antenna base. Balun is for converting a balanced
structure like dipole antenna to unbalanced structure like coax. Since
monopole antenna is unbalanced structure it does not need balun at its
input.

A "line isolator" (a choke/current balun) will provide no lightening
protection.

What is being talked about is a choke to prevent currents from flowing on
the outer of the coax.
It is not a balun in the purest sense, but it does stop unbalanced currents
from flowing on the coax.

The problem with an HF vertical is that the ground plane will never be
anywhere near prefect, so it will
not be possible to feed with coax without some current flowing on the outer.

I guess the only problem with putting the choke at the antenna base will be
pickup onto the outer as is
crosses the groundpalne. so I can see the idea of putting the choke at the
edge of the GP might be better.
I guess it either suck it and see, or try to simulate it with NEC.

73
Jeff

On Mon, 23 Jun 2008 19:18:18 -0700 (PDT), Nick
wrote:

Hi All,

I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. Am I wrong about
this?).

---
No.

http://www.electronics-tutorials.com...nna-basics.htm

JF

On 24 Jun, 03:18, Nick wrote:
Hi All,

* I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. *My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? *And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. *Am I wrong about
this?).

Thanks!

Nick

A balun isn't required.

Leon

In message , Jeff
writes

wrote in message
...
On Jun 24, 7:18 am, Nick wrote:
Hi All,

I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. Am I wrong about
this?).

Thanks!

Nick

I think line isolator is slightly different from balun. Here line
isolator would be necessary for lightening protection etc and hence
better kept at antenna base. Balun is for converting a balanced
structure like dipole antenna to unbalanced structure like coax. Since
monopole antenna is unbalanced structure it does not need balun at its
input.

A "line isolator" (a choke/current balun) will provide no lightening
protection.

What is being talked about is a choke to prevent currents from flowing on
the outer of the coax.
It is not a balun in the purest sense, but it does stop unbalanced currents
from flowing on the coax.

It's not a balun in ANY sense of the word. Some of the manufacturers of
the 9:1 isolation/semi-matching transformers (essentially for listening
only) persist in calling these 'baluns'. But baluns they ain't.

Unfortunately, 'balun' has become a generic word for many types of RF
transformer (especially those wound on ferrite). It's a bit like calling
all vacuum cleaners 'Hoovers' - which is what we do, at least in the UK.


--
Ian

In message
,
Leon writes
On 24 Jun, 03:18, Nick wrote:
Hi All,

* I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. *My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? *And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. *Am I wrong about
this?).

Thanks!

Nick

A balun isn't required.

Leon

With this type of antenna, it certainly is not common practice to
choke/isolate the coax, especially the coax braid can be grounded at the
antenna feedpoint and in the shack, and possibly at some intermediate
points along its length. It wouldn't do any harm to use one, but it
probably wouldn't do very much.
--
Ian

On Jun 24, 8:09*am, Ian Jackson
wrote:
In message
,
Leon writes





On 24 Jun, 03:18, Nick wrote:
Hi All,

* I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. *My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? *And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. *Am I wrong about
this?).

Thanks!

Nick

A balun isn't required.

Leon

With this type of antenna, it certainly is not common practice to
choke/isolate the coax, especially the coax braid can be grounded at the
antenna feedpoint and in the shack, and possibly at some intermediate
points along its length. It wouldn't do any harm to use one, but it
probably wouldn't do very much.
--
Ian- Hide quoted text -

- Show quoted text -

Thanks for all the great responses guys! That is exactly what I
had thought, that since the outer coax braid is grounded at the
antenna, and at the shack, and (in my case) at the center with a
lightening protector, then I just couldn't figure out why some antenna
companies, such as reliable DX engineering with their DXE-VFCC H05-A
Vertical Feedline Current Choke, strongly recommend such a line
isolator for verticals. A direct quote from DX Engineering's Website:

"2/5 kW Vertical Feedline Current Choke
If your antenna SWR is already low and you wish to reduce feedline
radiation and improve reception, a Feedline Current Choke is
recommended. Adding a DX Engineering Feedline Current Choke at the
base of a vertical antenna will substantially reduce unwanted feedline
radiation (RFI), reducing the need for improved station grounding.

When quarter-wave antennas are constructed over a good radial system,
they have a feedpoint impedance of about 36 ohms. When they are
constructed over less than a good radial-system there is a loss
introduced into the feed system that adds to the 36-ohm figure. This
improves the SWR but there is a loss in the efficiency of the antenna,
signals transmitted and received have a higher take-off angle and
often there is current introduced on to the feedline.

With a ground-mounted quarter-wave vertical, regardless of the radial
situation, but especially with poor radial systems, the feedline can
become part of the radial system, causing RFI and poor antenna
performance. By using a VFCC at the feedpoint, the feedline is
effectively de-coupled from the antenna system, preventing interaction
with the radial system, improving antenna efficiency. You may notice
improved bandwidth as well.

The Advantages of Using a VFCC:
Prevents unwanted RFI by eliminating feedline current and radiation
All power goes to the antenna, improving efficiency
Reduces noise or unwanted signals picked-up by the feedline
Overcome a less than optimal ground system
Bracket isolates the VFCC case from ground for best de-coupling"


-Nick

On Tue, 24 Jun 2008 09:10:31 -0700 (PDT), Nick
wrote:

Hi Nick,

In response to the ad-copy, I would make some points:

When quarter-wave antennas are constructed over a good radial system,
they have a feedpoint impedance of about 36 ohms. When they are
constructed over less than a good radial-system there is a loss
introduced into the feed system that adds to the 36-ohm figure. This
improves the SWR but there is a loss in the efficiency of the antenna,
This is good, as far as it gets to this point, but...

signals transmitted and received have a higher take-off angle and
often there is current introduced on to the feedline.

Take-off angle is dictated to the quality of ground 5 to 10
wavelengths AWAY (and further) from the antenna.

You could, if you so choose, build 120 radials out 20 wavelengths to
achieve their implied claim of radials contributing to lower take-off
angles.

With a ground-mounted quarter-wave vertical, regardless of the radial
situation, but especially with poor radial systems, the feedline can
become part of the radial system, causing RFI and poor antenna
performance.

For a transmitter, it is unlikely that the listener could tell the
difference between operating into that "poor radial system" and adding
the a vfcc.

By using a VFCC at the feedpoint, the feedline is
effectively de-coupled from the antenna system, preventing interaction
with the radial system, improving antenna efficiency. You may notice
improved bandwidth as well.

This contains a contradiction when it gets to the "improved
bandwidth." Presuming that means wider, it then means more loss. More
loss would come from NOT using the vfcc (or why buy one?).

The Advantages of Using a VFCC:
Prevents unwanted RFI by eliminating feedline current and radiation
All power goes to the antenna, improving efficiency

Uh-huh. Again, will the listener be able to tell? A radiating
feedline might boost their gain - who knows?

Reduces noise or unwanted signals picked-up by the feedline
Overcome a less than optimal ground system
Bracket isolates the VFCC case from ground for best de-coupling"

The noise pickup is the more likely benefit (along with the remote
ground - tied into the service ground as required by code). House
noise can travel down the outer portion of the shield to then develop
across the load of the antenna, and then wrap around to travel back
inside the coax to the receiver. A choke will snub this path and the
remote ground will lower the resistance. The divider action
suppresses local noise.

The same benefit can be obtained from a $25 purchase of bulk ferrite
beads (about 50 of them) which are fitted over a one foot length of
RG-58 (or the distal end of your feedline).

73's
Richard Clark, KB7QHC

Jeff wrote:
I guess the only problem with putting the choke at the antenna base will be
pickup onto the outer as is
crosses the groundpalne. so I can see the idea of putting the choke at the
edge of the GP might be better.

Get the best of both worlds. Put one choke at the antenna
base and one more 1/4WL down the feedline which will be
close to your "edge of the GP".
--
73, Cecil http://www.w5dxp.com