SYMMETRICAL vs. ASYMMETRICAL
We tend to clasify antennas as being eithr "Symmetrical" or "Asymmetrical".
(some people call them ballanced or unbalanced)
Examples of Symmetrical Antennas:
- Delta Loop
Examples of Asymmetrical Antennas:
- Mobile Whip
- Off Center Fed Dipole
Like Antennas, Matchboxes come in two flavors:
- Asymmetrical (unbalanced)
- Symmetrical (balanced)
MOST MATCHBOXES AVAILABLE TODAY ARE ASYMMETRICAL.
BUYER BEWARE !
Their manufacturers usually market them for matching both asymmetrical and symmetrical antennas.
They simply include a 4:1 balun and claim the matchbox will match symmetrical antennas. NOT!
(at least not very well)
The question is not "if" they can do this but rather "how well" can they do this?
The truth is "not as well as their marketing would lead you to believe."
WHAT'S THE DIFFERENCE?
- The technical difference is easy to understand:
- In a Symmetrical Matchbox, everything inside the matchbox is built symmetrically to the two wires feeding the antenna.
- In an Asymmetrical Matchbox, one line from the antenna goes through the matching circuitry, the other line is connected to ground and passes straight through to the ground side of the coax leading to the transmitter.
- The practical difference is not quite so easy to understand:
If you are running 100w or less, you probably won't see a big difference. The problem emerges when running high power.
The problems are caused by Common Mode Current (CMC) on the antenna feedline. With an asymmetrical matchbox, we use a balun, typically a 4:1 balun to help match the impedance and hopefully reduce CMC. THIS IS PART OF THE PROBLEM. DO NOT USE A 4:1 BALUN UNLESS NOTHING ELSE IS AVAILABLE. INSTEAD, USE A 1:1 CURRENT BALUN.
REASON: SEE: http://www.karinya.net/g3txq/tuner_balun/
Unfortunately if the SWR is too high on the balun, it does not function well and you can get trouble with CMC. This is compounded when running high power. This is more predominant in 4:1 baluns than in 1:1 baluns. The result is "RF in the shack."
You burn you fingers on the key, you burn your lips on the mic, etc.
Worst Case: some of your equipment may randomly go into FAULT mode.