[TenTec] balun noise?

[Bob McGraw - K4TAX]

Here's the "balun" solution.  These work great, cost little and can handle more power than you can generate.  Do note, RG-213 is the coax to use.  Don't use the foam stuff.  Thanks to Rich Measures, AG6K for the write-up.

73
Bob, K4TAX

Building a no-grief 1.8MHz to 30MHz 50ohm-balun is easy. No costly ferrite-cores are needed, just a short length of 3 to 5 inch size plastic pipe, about 25 feet of 50ohm coax plus some nylon cable ties. Solid-dielectric coax is best for this application because foam-dielectric has a tendency to allow a change in the conductor to conductor spacing over a period of time if it is bent into a tight circle. This can eventually result in voltage breakdown of the internal insulation. The required length of the plastic pipe depends on the diameter and length of the coax used and the diameter of the pipe. For RG-213/U coax, about one foot of 5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For 3.5MHz to 30MHz coverage, about 18 to 20 feet of coax is needed. This length of coax is also adequate for most applications on 1.8MHz. The number of turns is not critical because the inductance depends more on the length of the wire (coax) than on the number of turns, which will vary depending on the diameter of the plastic pipe that is used. The coax is single-layer close-wound on the plastic pipe. The first and last turns of the coax are secured to the plastic pipe with nylon cable ties passed through small holes drilled in the plastic pipe. The coil winding must not be placed against a conductor. The name of this simple but effective device is a choke-balun.

Some people build choke-baluns, without a plastic coil-form, by scramble-winding the coax into a coil and taping it together. The problem with scramble-winding is that the first and last turns of the coax may touch each other. This creates two complications. The distributed-capacitance of the balun is increased and the RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage. The single-layer winding on the plastic coil-form construction method solves these problems since it divides the RF-voltage and capacitance evenly across each turn of the balun.

A more compact, less ugly, 1 to 1 impedance-ratio, 50ohm trifilar-wound (with wire) ferrite-core balun could also be used but there would be some tradeoffs. Ferrite cores are not cheap. Also, the air-core of the coax-balun can't saturate like the ferrite-core and, unlike ferrite-core wire-wound baluns, single-layer wound coax-baluns almost never have an insulation breakdown problem. Also, a trifilar-wound balun does not like to work into anything but a perfectly balanced load. With an imperfectly balanced load, the coax-balun will not, as does the trifilar balun, generate a differential, third RF-current on the outside of the coax that brings the RF to the input of the tuner. The choke-balun is not fussy. It will work as well into a less than perfectly balanced load as it will into a perfectly balanced load, and do so without the possibility of creating a differential RF-current on the station ground and fricasseeing the operator's fingers.



----- Original Message ----- 
From: "Jim Brown K9YC" <k9yc at audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Friday, November 23, 2007 2:44 PM
Subject: Re: [TenTec] balun noise?


> On Fri, 23 Nov 2007 09:46:03 -1000, Ken Brown wrote:
> 
>>Usually a balun is used to connect an unbalanced feed line, such as 
>>coax, to a balanced antenna such as a dipole. The balun should improve 
>>the current distribution on the dipole and reduce the current on the 
>>outside of the coax. 
> 
> Most so-called "baluns" of the type Ken describes are really common mode 
> chokes, and they do exactly what he says. 
> 
> See http://audiosystemsgroup.com/RFI-Ham.pdf  for a tutorial on many things, 
> including these chokes ("baluns". 
> 
> 73,
> 
> Jim K9YC
> 
> 
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