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