Which Choke to use ?

This is a loaded question and regardless of what I write,

I'm sure the answer will blow up in my face!

(But I'll try anyway - hi)

(Suicide is Painless)

 I will begin by showing again where all we should use a choke:

ANT+Choke ¦ -Coax-Coax-Coax*- ¦ Choke ¦ House ¦ Coax ¦ Choke ¦ Station

*As stated on the previous page, longer runs of coax (more than about 50 ft.) should be broken up with RF-Chokes, similar to how we break up the steel guy lines of towers with insulators.

Please keep in mind that here in Germany we are limited to just 750w and I have always run legal power.  The information I present here is, to the best of my knowledge, valid for that power level, but I can only speculate on what would happen with more power.

The RF-Choke (or Choke-Balun) at the feedpoint (ANT ¦ Choke) will vary significantly, dependaning on the type of antenna, its CMC characteristics, power level, and local environmental factors.

I can only describe solutions that I have used successfully for antennas which I have built.  I cannot define what to use for every possible antenna one might want to build.

Never-the-less, this will cover several types of antenna, albeit not all.

APPLICATION-DEPENDANT  (It's Horses for Courses)

How do we define the applications?

Ian White, GM3SEK divides Common-Mode-Current problems into two categories:

• Soft CMC Issues

• Hard CMC Issues

Luckily, most of the time, most of us are only dealing with Soft CMC Issues.

• For Soft CMC Issues, usually a Maxwell Choke is all that is required.  

• HOWEVER, it is important to understand that there are different 'flavors' of Maxwell Chokes, each with slightly different characteristics.

SOFT CMC PROBLEMS:

Old-School textbooks would have us believe that a choke needs only 10x the impedance of the transmission line, so for 50 Ohm coax, it would need a CMI of 500 Ohms.  AND that's what many RF-Choke vendors are offering for sale. 

DON'T  WAST  YOUR  MONEY!

IMO a good Maxwell Choke must have at least 1000 Ohms of CMI.  The better Maxwell Chokes have even more CMI.

(See RF-Choke Copmarison Chart in the middle of 'Things to Watch Out For')

If a Vendor does not state the CMI or dB of his product in terms for frequency (ham band), then I would not buy his choke.  There are serious vendors who do state the specs of their products. 

A good choke costs about $30.  (So does a bad choke!)

IN GENERAL:

Most of the time, most hams must only deal with Soft CMC Issues.

The Choke* used 'in-line' with the coax, both inside and outside of the house is much easier to describe, so I will begin here.

I have had good experience using 'good-quality' Maxwell chokes here. 

So far they have always sufficed in the presence of 'soft' CMC.

So just what is a 'good quality' Maxwell?

In this case I will define 'good' to mean a CMI that is >1000 Ohms.  

Not every Maxwell choke available commercially has this much CMI.  Many are more in the range of just 500 Ohms.   Remember, if a Vendor states the Specs of his Choke, make sure it shows the frequency range.  Many state the CMI for the frequency it performs best.  On other bands it might be significantly worse.  Chokes with a CMI of just 500 Ohms may still suffice but more is better.   Play it safe.

A very good Maxwell choke will have 1500 Ohms of CMI.  That's about the limit with this technology, unless we start cascading chokes - or choose chokes with a higher CMI but only for a limited frequency spectrum.

So remember, 1000 Ohms is the minimum goal for a good Maxwell.

It is rare that general purpose chokes need more CMI and when they do, it is usually not across the entire HF spectrum but rather just on one or two bands.  In this case we could also use a simple Guanella choke which has much more CMI but is reactive and has a smaller usable bandwidth.  Info on Guanella Chokes will follow on a subsequent page.

HARD CMC PROBLEMS:

These are the ones that really bite your fingers or lips when you transmit.

Certain types of antennas are far more prone to hard CMC problems than others.

• Antennas LESS prone to having hard CMC problems:
  • Symmetrical antennas, such as dipoles or loops.
• Antennas MORE prone to having hard CMC problems:
  • Asymmetrical antennas, such as Off-Center-Fed Dipoles (OCFD)
  • Symmetrical antennas with metal objects under one side (like a big school bus or a garage with a metal roof)
  • Vertical dipoles with the bottom of the antenna close to the ground

In order to make the right choice of choke or choke-balun, it is important to understand the characteristics of Common Mode Current of the antenna across the frequency range it is to be used.

In cases of hard CMC problems, a MAXWELL Choke is NOT good enough!

You will need to use a 1:1 Guanella choke or a "Modern Ugly Choke".

(Reminder:  "Modern Ugly Choke" = Coax loops through hunks of Ferrite)

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SIDEBAR ON GUANELLA BALUNS:

When considering a choke-balun for antennas such as the Off-Center-Fed Dipole, keep this in mind:

• A 1:1 Guanella Balun on some frequencies can have up to 8 k-Ohms of CMI - albeit not over the entire HF spectrum.  

• A 4:1 Guanella Balun is simply two 1:1 Guanella Baluns connected with their inputs in parallel and their outputs in series.  Although this quadruples the output impedance, it also halves the CMI since their coils are in parallel.  For this reason, the 4:1 balun is not nearly as good as a 1:1 Guanella balun in impeding the flow of CMC.

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If the application calls for a 4:1 matching device, a 4:1 Guanella balun is NOT ALWAYS GOOD ENOUGH!  In that case you will need a 1:1 Guanella choke AND a 4:1 matching device.  IMO a combination of 1:1 Guanella and 4:1 Ruthroff, called a "Hybrid Balun" is what works best here.

CAUTION:  if you google "Hybrid Balun", you will find many different designs. As always, I can only report on my experience with the baluns I have built and used, and I refuse to speculate on how others might work.  

I prefer to place the 1:1 Guanella on the coax side and the 4:1 Ruthroff on the antenna side.  Others do it the other way around, with the 1:1 Guanella on the antenna side.

CAUTION 2: a reputable company in Texas is selling a 4:1 Hybrid Balun for OCFD antennas.  IMO it will work OK for 40m and above but has insufficient CMI for an 80m OCFD unless you can hang it 100 ft. in the air!  My opinion is based on hundreds of hours of field testing and analysis, and I have documented measurements to back it up.