Does a HF vertical antenna need a balun?

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

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.

A balun isn't required.

Leon

In message , Jeff writes

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.

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.

st

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

Hi Nick,

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

This is good, as far as it gets to this point, but...

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.

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.

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?).

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

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).

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".

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

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).

When I was into 2-meters 40+ years ago, I preferred gamma-matched verticals.

...Jim Thompson

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).

-Nick

Hi Nick,

Well, no argument perceived. 1000 feet would come with its own loss. This is published in dB attenuation/100 feet for almost any type and grade of coax in the market (and those that are no longer made). This is truly Differential Mode loss.

Winding too much into a coil would not necessarily achieve the results expected due to self-resonance (it could act like a shorted choke, and not very useful). You can achieve astonishingly high Zs with a coiled transmission line with very little effort (roughly 8-12 turns on a liter pop bottle - empty of course - for HF). It will peak at some frequency and provide adequate isolation in nearby bands; however, the ferrite chokes can be made to cover a decade of frequency - or much more (3-30 MHz), if you care to really go whole hog (3MHz-3GHz) with

One proviso should be observed: Whatever problems find their way onto the shield of the coax (Common Mode circuit) can arrive over the air. If they do, they can induce themselves into the below the choke point as easily as at the antenna. The solution here is to repeat the choke construction 1/4 wavelength from the feedpoint (a second choke, that is). If you are choking a wide band, you might try distributing the ferrite beads over a greater length of coax than the 1 foot it would normally inhabit. I have a 20 foot ferrite choking section that I can insert, inline, into my feeds. It has one bead every 3 or 4 inches. I wouldn't advise this as replacing the feedpoint choke, but rather the second choke some short distance away.

Note that I made a distinction about interference coming in "over the air." This is not the only way as most home based interference comes into the receiver by conduction - sharing a poor ground. Most folks who swear that chokes don't work (or at least theirs don't so none do), are suffering from ground loops. A choke can help, but not nearly as well as combining it with conventional grounding solutions. Poor advice abounds in this topic too.

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

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.

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 -

t?

ng?

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.

-Nick