G4YDM described his balun at Ham Radio – What Is a Balun and How to Make One Cheaply.
With a title like that it is sure to have wide appeal, but it isn’t anything too novel, it is simply an air solenoid of 50Ω coax cable as a common mode choke, commonly known as an Ugly Balun.
He gives some instructions for one of several constructions:
When wrapping your coax around the pipe don’t use too much force as it may damage the inner braid and space the turns away from each other by a millimetre or two. R-G-2-1-3 coax around 21 feet used with 5 inch pipe will handle 400 watts pf power.
Above is a pic of the third construction which appears to be 21′ of RG213 on a 5″ PVC former:
He gives some performance measurements adjacent to the pic above:
Using a dummy load connected to the choke and transmitting 100 watts from my transmitter indicated an S.W.R. readings of around 1.5 to 1 at 3.5 Megahertz when testing 28 Megahertz the S.W.R. reading came down to 1.1 to 1 which is an excellent match. …
The test describe above seems to simply be a dummy load connected to one end of 21′ of RG213 and the transmitter with VSWR meter feeding the other end. To be meaningful we need to know the impedance of the dummy load, indeed to be meaningful it needs to be 50Ω, so lets assume that is the case.
This is a simple transmission line problem, and we would expect that the input VSWR of a length of approximately 50Ω line to be approximately 1.0, but it is substantially different at 1.5, why?
There could be many factors contributing, but if we assume that he used a validated good VSWR meter and dummy load for his published measurements (and they could be checked easily), a possible explanation is that the RG213 is not 50Ω. Could that be the case?
He has specified a radius of curvature of the coax on the form of 63mm, half the minimum specified by Belden for their 8276 RG213 coax, so it is quite possible, likely even, that the tight wrap of RG213 has deformed the shape of the coax profile and altered its characteristic impedance. (This deformation and conductor migration is an even greater issue for foam dielectric cables.)
If we assume that velocity factor remains approximately 0.67 by virtue of the solid polythene dielectric, we can calculate the electrical length of 21′ at 3.5MHz to be 40.8°, and find the value of Zo that would account for an input VSWR(50)=1.5.
Above is a Smith chart solution to the problem. If the sole reason for input VSWR(50)=1.5 is deformation of the coax altering its Zo, then the Zo was approximately 37Ω.
Migration of inner conductor is a problem with small bend radius, and the greater distance subject to small radius bends, the greater to contribution to impedance transformation.
Sure it is cheap, but is it a good implementation?
Properly executed “Ugly Baluns” baluns or common mode chokes have three notable features:
- very low insertion VSWR;
- a very narrow, very high impedance peak that gives excellent common mode reduction in that very narrow band; and
- low cost.
They are fairly ineffective as a wide band common mode choke, and this one on the author’s reported measurements has poor Insersion VSWR. It probably does have very high common mode impedance over a narrow frequency range, but no measurements were given to show that band coincided with any ham band.