Hello Barry,
Last things first - OK on the antenna. It is a large beast and uses a bit of real estate. Btw antenna builders think that antennas are wonderful (and they are!) but there is a section of the community (wives, councils and non-believers) who think otherwise.
Re:Baluns. The word Balun is made from the words BALanced to UNbalanced.
A balun is actually a BALanced to UNbalanced transformer.
What is balanced and unbalanced?
The best way to explain it is in thinking about our normal 240V power. One wire, the neutral, is connected to ground at the power station and the active reach us, via sundry transformers,directly. This is an example of an UNBALANCED system - one wire is at earth potential.
Now, you can get a transformer that converts the 240V mains to 12V
some of these transformers also have a centre tap (12V with 3 wires labelled 6-0-6) If the centre tap is connected to ground then you have 2 wires (each is an "active") and the potential between each "active" and ground is 6V.
- this is a balanced system.
If you connected one of the wires(other than the centre tap) to ground then you have 2 voltages (groung to centre ... 6V and ground to other terminal ... 12V) - this is an unbalanced system.
What about the Dipole antenna? One of the points with di-pole antennas is that they are two pieces of wire in the air (I'm stating the obvious to make a point) neither of which is connected to ground. Notice
that it is balanced with respect to ground - just like our transformer - 2 "actives" !! This dipole will present a balanced load to a transmitter.
So to feed a dipole you must present it with a balanced feed.
Yes, people do connect a co-axial cable directly to the di-pole but this is not the best. Notice that the co-axial cable is an unbalanced cable (the outer is connected to ground). Yes this works, but anything works when you receive powerful signals (Rabbit ear antennas work well in the city but badly if at all in the country).
The next thing is matching:
The antenna picks-up some of the transmitted power and feeds it to the receiver - here is the important part: The receiver input is designed to look like a resistor of 52 ohms (or 75 ohms if you like!). For the receiver to get the max power from the antenna the antenna must also have a resistance of 52 ohms (or 75 ohms) this is called matching the antenna to the receiver. If there is a mismatch then the amount of received power available to the rx will be reduced and there is no way that it can be recovered!!
Now a dipole antenna will "look like" 72 ohms (and balanced!) but that is under some strict conditions. A lot of the time it will not be 72 ohms.
When it is close to the ground or near other "antennas" the resistance will drop.
We now get back to baluns - the balun is a transformer OK! If you put a resistor on the input the output also looks like a resistor. Depending on the design of the balun the input and output resistance will "look" the same (52 ohms in - 52 ohms out) except that the input balanced and the output will be balanced - this is a 1:1 balun. You can have a 4:1 balun in which case 300 Ohms on one end gives 75 Ohms on the other end - notice 75 Ohms matches
a standard co-axial cable (unbalanced) and 300 Ohms matches a standard 300 Ohm TV ribbon (balanced). Most Tv antennas have these 4:1 (75 to 300 Ohm) baluns - have a look at yours. The length of 300 Ohm ribbon on the balun is quite short (100 to 150mm) - enough to connect
it to the antenna elements after that its all coax. to your TV.
Hope that answers:
" ... Whats a balun? Why 300ohm lead? ... "
And finally, A dipole (if split in the centre) will have a range of resistances from say 40 ohms in the centre to many thousands of ohms at the tip. So using the matching method Iv'e chosen you can adjust the antenna for a perfect match. A single piece(unbroken) of wire fed the way I've drawn is also a dipole.
Why do we need a good match? To get the max signal to the receiver - the signal's are low enough as it stands.
Antenna diagram? Next post - need a cuppa after this!
Cheers and beers,
Jerry
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