Snap-on ferrites as common mode choke on ground mounted vertical

[K7JQ]

I'm installing a ground-mounted vertical, with sixty radials stapled to the ground in a symmetrical pattern. The LMR-400 coax will also be running on the ground (or buried slightly under) in between two of the radials. I want to put a series of snap-on ferrites on the coax to choke off possible common mode currents. Is it best to put them right at the feedpoint, or just outside the radius (radiation field) of the radials? Thanks for any input.

73,   Bob, K7JQ

[K1ZJH]

I wonder how much common mode you will have with sixty radials?  I'd run the feedline below the existing radial field.

Pete

[K3VAT]

I'm installing a ground-mounted vertical, with sixty radials stapled to the ground in a symmetrical pattern. The LMR-400 coax will also be running on the ground (or buried slightly under) in between two of the radials. I want to put a series of snap-on ferrites on the coax to choke off possible common mode currents. Is it best to put them right at the feedpoint, or just outside the radius (radiation field) of the radials? Thanks for any input.

73,   Bob, K7JQ

Installation of common mode chokes are best done right at the feedpoint.  After all, what are you choking and why?  As a double-safeguard, you can also install them on your coax adjacent to the lightning protection circuitry.

73, Rich, K3VAT

[K2DC]

It's not clear to me that snap-ons are the right ferrite mix to be very effective with common mode RF.  I have a whole bag full of them and on the few occaisions when I've had antenna issues resulting in RF in the shack, they've never been very effective for me.

73,

Don, K2DC

[W5DXP]

It's not clear to me that snap-ons are the right ferrite mix to be very effective with common mode RF.

In addition to the potential mix problem, snap-on ferrites are not very efficient because they are one-turn devices. One can multiply the choking impedance by the square of the turns by putting multiple turns on a toroid. For instance, it may take 100 snap-on devices to equal the choking impedance of one ten-turn toroid.

[W2ANZ]

I found this question so interesting that I joined eHam just to continue the discussion

Why not take advantage of the feed line as another radial?  I'm thinking of placing my choke  just outside my radial field.  I also wonder if this might be more effective at keeping RF out of my shack than putting the choke at the base of the vertical. With the choke at the base RF currents might couple back onto the feeder at close in distances where the fields are high.  By putting the choke outside the radial field where the fields are lower there will be less opportunity to couple back to the feed line.

With regard to snap-on chokes..  The inherent problem with them is that any gap between the two halves will reduce their effectiveness.  It is better to use solid cores and thread them onto the coax before the connector is installed.  Fair-Rite makes both snap-on and solids in all the materials.... 43, 61, etc.  DigiKey used to carry them - not sure if they still do.  The generic EMI suppression snap-ons that some suppliers carry may not be a suitable blend.  I'm not sure how many cores would be required to make an effective choke - perhaps a dozen or even more.  W2DU published some work on baluns with these cores years ago.

Edit:  found information on W2DU's choke    
http://vk1od.net/balun/W2DU/index.htm
He is recommending 50 cores using type 73 material for HF.
  

[W0BTU]

Go to K9YC's excellent source of ferrite and choke info (http://audiosystemsgroup.com/publish.htm) and download the PDF http://audiosystemsgroup.com/RFI-Ham.pdf
There's nothing like it anywhere on the web. He shows you exactly what ferrite mix you need for your application and how to implement it.

[N8CMQ]

I also have a ground mounted vertical but I have 125 radials under it.
I have my coax inside flexible steel conduit (rodent protection) buried
six inches under the sod. I would not think there are any currents flowing
on the outside of the coax. I do not measure any currents on the outside
of the coax where it enters the house.
 
 The only way you would have current flowing on your coax is if it
is on the surface. Even then, any type of choke isn't going to do any good.
Get it underground and not worry about it.