A recent online discussion tried to advise on the need of a common mode choke on a UHF Yagi.
Above is the subject M2 440-6SS, 420-450 MHz Yagi deployed in vertical polarisation (using the other pair of holes for the U bolt).
Now I am a little intrigued as the pic would suggest that the DE is not electrically bonded to the boom, but passes through insulating bushes on each side without direct connection from the centre of the DE to the feed point shield terminal. If so, it is not what is commonly known as a Gamma Match, but a variant.
The assembled experts were divided on the need for a common mode choke, and several online experts decried gamma matches as the cause of woes, one poster citing a presentation by Ian White which does some hand waving about gamma match.
The first point I would make is that if the coax shield is well bonded to the boom at the feed point connector, and zip tied to the boom and masting so as to preserve physical symmetry what might be seen as common mode current on the coax is more generally current flowing on the combined coax shield and conductive boom / masting. Let’s look at a NEC model of a 144MHz gamma matched 4 element Yagi mounted vertically
This model does not include the coax, but we can see than currents flow not only on the Yagi elements, but also on the boom and masting.
Let’s get rid of the ‘nasty’ gamma match.
Well, that is not a whole lot different. The problem is not the gamma match!
We have significant current flowing on the support structure, and if we tied a coax to the boom / masting, much the same total current would flow on both conductors combined. Installing a common mode choke at the DE feed point will not get rid of the significant current flowing on boom and masting.
What is the problem?
We could model this without the conductive mast and feed line, and find that there is negligible current flowing on the boom, the Yagi itself (elements and boom) are quite symmetric.
The masting fairly wrecks system symmetry, and since the mast and boom are not subject to asymmetric influences from the Yagi element current, current is in turn induced on them. The problem is the mounting arrangement.
Aha, what if we used a non-conductive mast?
If you zip tied the coax along the boom and down the non-conductive mast, the results will be much the same as with the conductive mast, the outer surface of the coax shield provides the current path.
It may be possible in this case to install one or more ferrite toroids or sleeves on the vertical coax to discourage current, it would be impractical to try this on larger conductive masts.
- Though some might gratuitously blame the gamma match, it isn’t the problem as demonstrated by the model without the gamma match.
- Exploring a set of models with less and less components shows that it goes wrong when the vertical mast is connected.
- Undesired currents on structure or feedline in common mode are not fixed by simply throwing ferrite at the problem.