Does a HF vertical antenna need a balun?

[Nick]

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

 

[[email protected]]

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

 

[Jeff]

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

 

[Leon]

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

 

[Ian Jackson]

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 Jackson]

In message

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.

 

[Nick]

In message








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.

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

 

[Cecil Moore]

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".

 

[Cecil Moore]

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.

Nick, think about it. If you ground the feedline at exactly a
common-mode standing wave voltage node, the ground accomplishes
absolutely nothing. You certainly don't want to use "plumber's
delight" techniques in your ground system.

 

[Nick]

Nick, think about it. If you ground the feedline at exactly a
common-mode standing wave voltage node, the ground accomplishes
absolutely nothing. You certainly don't want to use "plumber's
delight" techniques in your ground system.

Thanks Richard and Cecil.
Cecil, I understand what you are saying, but still why not use the
outer shield of the coax as one of the radials, and simply choke off
the coax just before it enters the shack? I also don't want to add
what may be a useless choke at the antenna's feedpoint, since that
would decrease the antenna system's efficiency due to the choke's
resistive losses...

Thanks!

-Nick

 

[Nick]

Hi Nick,

A choke at the feedpoint does NOT add loss.

The Choke (which may be resistive insofar as the bulk material's
property) is in the Common Mode circuit.  The load is in the
Differential Mode circuit.  You want a very, very high Z (or R, it
makes no difference) in the Common Mode circuit.  The choke is
entirely transparent to the Differential Mode circuit.

If you don't understand the difference between the two modes (and it
is not a common topic outside of electronics design) then, no doubt,
others will fill in (or simply go to wikipedia).

73's
Richard Clark, KB7QHC

Thanks for the further info Richard! I guess I don't really fully
understand why the pure resistive losses in the coax choke wouldn't
affect the efficiency of the monopole. If I understand you correctly
(and I may not!), then even if we increase the impedance of the coax
choke to insane levels, such as by making it 1000 feet long and
winding it on a six inch diameter PVC form, then the huge losses
normally inherent in such a long run of coax would not be seen at all
by the vertical because it is wound on the PVC form? (I'm not arguing
with you at all, I just really want to know).

73,

-Nick

 

[Jeff]

I guess I don't really fully
understand why the pure resistive losses in the coax choke wouldn't
affect the efficiency of the monopole.

A simple way to think of it is: Ferrite beads over the outer of a coax has
no effect on the signals flowing 'normally'
up and down 'inside' it. The beads only effect signals that are flowing on
the outer (treating it like a piece of wire rather
than a coax transmission line). I know that this is not quite the full story
but it is a simple explanation of the situation
under discussion.

Jeff

 

[Cecil Moore]

Cecil, I understand what you are saying, but still why not use the
outer shield of the coax as one of the radials, and simply choke off
the coax just before it enters the shack? I also don't want to add
what may be a useless choke at the antenna's feedpoint, since that
would decrease the antenna system's efficiency due to the choke's
resistive losses...

Unless the coax is up and in the clear, common-mode power is
probably wasted whether you choke or not.

If your choke at the shack happens to be placed at a common-
mode voltage-maximum/current-minimum, it will be ineffective.
One reason for placing a choke at the antenna feedpoint is
that is probably the maximum current point where the choke
has a good chance of being most effective.

 

[Michael Coslo]

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,

Wad is dis? Is that something to protect your coax from becoming too light?

Jes kiddin'!


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.

Since they make them, they're happy to sell them to you.

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.

I am really at a loss as to how this isolator improves reception.

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.

Am I a hopeless noob, or are there a lot of things wrong here?
Feedline radiation = RFI. and almost a lure toward poor 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.

Are there tests showing this somewhere?


Anyway, you are probably best served by placing as many radials as you
can handle - but more than 4. I just went till my knees and back said
"no more", then came back the next day and laid more. Have as good a
grounding system as you can get, and good lightning protection. IMO, if
that isolator is helping any, it is just trying to mitigate a problem
that should be taken care of elsewhere.

- 73 de Mike N3LI -

 

[Nick]

Wad is dis? Is that something to protect your coax from becoming too light?

Jes kiddin'!

  then I just couldn't figure out why some antenna


Since they make them, they're happy to sell them to you.

   A direct quote from DX Engineering's Website:


I am really at a loss as to how this isolator improves reception.


  Am I a hopeless noob, or are there a lot of things wrong here?
Feedline radiation = RFI. and almost a lure toward poor station grounding?


Are there tests showing this somewhere?

Anyway, you are probably best served by placing as many radials as you
can handle - but more than 4. I just went till my knees and back said
"no more", then came back the next day and laid more. Have as good a
grounding system as you can get, and good lightning protection. IMO, if
that isolator is helping any, it is just trying to mitigate a problem
that should be taken care of elsewhere.

        - 73 de Mike N3LI -

Thanks again guys. I'll have to study common versus diff mode
currents on coax cable again to gain a better understanding of the
situation! And as was stated by many, a good radial field should
mitigate the situation anyway.

73,

-Nick