on behalf of bri9hu (firstname.lastname@example.org
Sent: Thu 7/01/10 4:54 PM
Excellent practical info about baluns on: http://home.comcast.net/~markwa1ion/exaol2/z_transformers.pdf http://www.dxing.info/equipment/impedance_transformer_bryant2.doc http://www.dxing.info/equipment/impedance_matching_bryant.pdf
on behalf of n0sya (email@example.com
Sent: Mon 6/28/10 9:30 AM
Hi Rowland, the multipliers realting to baluns are the values that one would select to multiply the impedance of the antenna or transmissions line, wether coax, single wire, or twin lead. These values are fixed by the turns ratio of the wires on the balun as well as the core material. Baluns are typically made on certain ferrite or powdered iron cores with specific turns ratios, various mixes of cores and turns windings nets you the different multiplier values, just as in any other transformer of the electrical variety.
Antennas transfer most energy when matched to a transmission line with their native impedance, a dipole cut for any specific frequency usualy exhibits an impedance of 75ohms at that frequency, so the naked dipole is a fair match to 50 ohm coax or better yet, 75ohm coax, but then since one is going from a balanced circuit (the dipole) to an unbalanced circuit (the coax), one may still want a balun, but in this case a 1 to 1 ratio is best as you may have guessed. Now if your antenna is a long wire (wich is also an unbalanced antenna), wich may have an impedance at the frequency of interest of 300 to 2000ohms or so, one would use a balun transformer of the required multiplier value to get best energy transfer from the high impedance antenna to the low impedance transmission line, and vice versa.
It's all about getting the most energy to and from the load.
With a folded dipole (really a loop antenna) like the T2FD antenna around 600ohms impedance nominaly, you would use a 4 or 6 to 1 balun as the matching transformer to get best energy transfer to the coax lead in.
There are two reasons one wants a balun transformer, to match the load (antenna) impedance to the line (coax or twin lead or single wire) impedance so as to get maximum energy transfer, and to go between what is called a balanced line, like twin lead or a dipole antenna where there are always two conductors, to an unbalanced condition such as with coax cable and uhf sockets on the back of radios where you have one conductor surrounded by a "ground".
Most folks employ baluns (meaning balanced to unbalanced transformer) simply to get maximum energy transfer, but they are very usefull for keeping unwanted rf currents from flowing where one does not want them such as on the outside of the coax lead. One can effect suitable balun action of the keep-rf-off-my-coax variety by merely slipping come ferrite or powdered iron cores onto the coax line near the ends, the cores, if of correct material for the frequency range, will do a fine job of reducing rf flowing on the outer sheild of the coax.
Just some thoughts for you.
Antennas in general:http://www.hamuniverse.com/n4jaantennabook.html
> To: UDXF@yahoogroups.com
> From: firstname.lastname@example.org
> Date: Mon, 28 Jun 2010 16:12:21 +0000
> Subject: Re: [UDXF] Re: BALUN
> At 13:28 28/06/10, Chris wrote:
<previous email snipped>
> Whatever the turns ratio/impedance ratio, a long wire balun will seldom match
> properly to a coax feedline. The long wire balun, although a compromise at
> matching impedances, does provide a screened feedline to the receiver which will
> not usually pick up noise, particularly if the screen is earthed/grounded near the
> antenna feedpoint. Thus the antenna can (hopefully) be positioned well away from
> the domestic noise environment, avoiding the rubbish usually induced into a
> single-wire feedline from TVs, monitors, PSUs etc.
> 73 de Jim,