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Facing Balun transformer's problem please!

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Willen

Well-Known Member
I want to use dipole Ant to my 500mW FM transmitter but facing a problem to connect balun transformer. I found a balun on rabbit ear antenna of TV. How may I connect it to my transmitter? (input?, output?) I know nothing about balun, so having trouble....
 
You probably don't want to use the balun. On an antenna like that, the balun tries to match the high-impedance of the antenna to the low-impedance of the reciever circuitry. Little 1/2-watt FM transmitter circuits are generally designed for high-impedance output drive already.
 
Hi Willen,

What is there to figure out here? The balun can only hook up two different ways. You have a primary and a secondary. Call either end primary then call the other end secondary.
Connect the primary to the FM transmitter and the secondary to the antenna. Try the transmitter out by transmitting something.
Then remove the balun, connect the secondary to the FM transmitter then the primary to the antenna. That's the second way to connect it. If it works better with primary to FM transmitter then keep it that way, or if it works better with secondary connected to FM tranmitter then keep it that way instead.

Do you know the impedance of your transmitter and antenna? Those TV baluns are made for 75 to 300 ohm coupling usually. Since that's a 1 to 4 ratio, if you have a 50 ohm antenna and 200 ohm transmitter it might also work for that.
You could look up the impedance of a dipole antenna, the kind you have, on the web.

There's also a chance you can rewind the balun to match your impedances if 1 to 4 doesnt work, but you'd have to know the impedance of your FM transmitter and of the antenna too.
 
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Hi Willen,

What is there to figure out here? The balun can only hook up two different ways. You have a primary and a secondary. Call either end primary then call the other end secondary.
Connect the primary to the FM transmitter and the secondary to the antenna. Try the transmitter out by transmitting something.
Then remove the balun, connect the secondary to the FM transmitter then the primary to the antenna. That's the second way to connect it. If it works better with primary to FM transmitter then keep it that way, or if it works better with secondary connected to FM tranmitter then keep it that way instead.

Do you know the impedance of your transmitter and antenna? Those TV baluns are made for 75 to 300 ohm coupling usually. Since that's a 1 to 4 ratio, if you have a 50 ohm antenna and 200 ohm transmitter it might also work for that.
You could look up the impedance of a dipole antenna, the kind you have, on the web.

There's also a chance you can rewind the balun to match your impedances if 1 to 4 doesnt work, but you'd have to know the impedance of your FM transmitter and of the antenna too.

I think I have 50 ohm impedance transmitter and I have a general dipole almost like rabbit ear antenna (with vertical position). And trying to connect TV's balun, does it match? Which thing determines the impedance of a dipole?
 
Hi,

I think the impedance of a 1/2 wave dipole is around 100 ohms, so you'd have to match to that. The 75 to 300 ohm balun would not do that. You'd probably have to make your own balun. Depending on frequency range this might not be hard to do.

Do you only have to transmit at one frequency or a bunch of frequencies?
 
Why are you using rabbit ears for an antenna? It is directional in two directions and has poor response in two other directions.
Use a whip antenna connected directly to the coax cable and a ground plane. A vertical whip antenna transmits or receives all around.
 
Hi,

I think the impedance of a 1/2 wave dipole is around 100 ohms, so you'd have to match to that. The 75 to 300 ohm balun would not do that. You'd probably have to make your own balun. Depending on frequency range this might not be hard to do.

Do you only have to transmit at one frequency or a bunch of frequencies?

i want to trnsmit single friquency at 100MHz. How to modify to this 75-300 ohm balun of Tv?
 
Why are you using rabbit ears for an antenna? It is directional in two directions and has poor response in two other directions.
Use a whip antenna connected directly to the coax cable and a ground plane. A vertical whip antenna transmits or receives all around.

without balun? Monopole? Show me any figure please
 
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Hi,


One way to try this would be to take apart the balun and remove the turns (might want to remember how it was wound in case you want to restore it later) counting the N turns as you remove them.

Take some solid core wire and wind it around the core. Wind N turns then strip the end of the wire, then wind another 0.4 N turns in the same direction as the first winding, possibly right over top of the first winding. The first winding has two ends, the start end and the finish end. The second winding also has two ends, the start end and the finish end. Take the finish end of the first winding and twist it together with the start end of the second winding. Now the primary is the first winding, and the secondary is the first winding plus the second winding. So the primary is the start end of the first winding and the finish end of the first winding (that also connects to the start end of the second winding) and the secondary is the start end of the first winding and the finish end of the second winding. This is an autotransfomer balun and with those ratios it matches 1:2, so 50 ohms to 100 ohms.

The original balun should have a transformer core that is like a figure 8. You'll be winding around the center leg of that figure 8.
What i hope is that this works for 500mw. That's only 1/2 watt so it should work.
 
a standard TV balun should work at 100Mhz. there's no need to use a balun to match a dipole or monopole antenna, since they are 50-75 ohm antennas to begin with. the balun is for matching to a 300 ohm antenna such as a folded dipole, which is an antenna made from 300 ohm transmission line.
 
Hi,
The original balun should have a transformer core that is like a figure 8. You'll be winding around the center leg of that figure 8.
What i hope is that this works for 500mw. That's only 1/2 watt so it should work.

Where is the figure 8? Oh sorry, I understand. You mean- ferrite core shape is like '8', isn't it? Does it work at 1.5 watt too? Make me clear about number of turns and SWG of wire. Which ends are input rf input+gruond and which ends are rf output and ground?
 
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Hi,

I dont know about 1.5 watts. See the restriction is how many turns you can get on there with the required wire gauge.

To find out the primary turns i told you you have to count the number of turns that are on there already when you take it apart.
The number of secondary turns then equals that times 0.4 or that approximately. The wire gauge has to support your current which i dont know, but you could look that up i guess. My guess is that #22 AWG would work ok, or even #24 at 500mw but again it depends
on the current you need. If it gets hot you'd have to use more wire or larger wire i guess.

The secondary turns would be on top of the primary turns and wired like this:

o---(((((((((()---o---(((()---o

Here the (((((() are the windings. The big one is the primary and the small one is the secondary. That's the wiring, but the secondary should be wound right on top of the primary if possible.
So say you count 10 turns, then the primary should have 10 turns and the secondary 4 turns. If there are less than 10 turns you'll end up using an approximate number of secondary turns that's all.

There's no reason why you shouldnt experiment with this either. Try more or less secondary turns and see if it transmits farther one way or the other.

With a single frequency there are other ways to do this too. A balun is usually required when there are a wider range of frequencies that have to be dealt with.
 
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If I make a dipole and balun (using coax) this:

www.hem.passagen.se/communication/ipod.html
>Does it affective?
>Can I transmit more than 5KM using this balun and dipole by 1watt FM Tx in almost open area?
>Here, the core of coax balun is almost 2 inchs, am I right?
 
WARNING

My anti-virus software does not like the link in post #14 above.
Go there at your own risk.

WARNING

JimB
 
Hi Willen,


Apparently there is a problem with the sites you are using. Perhaps try to upload the photos to this forum instead.

A balun for a single frequency can be made with an ordinary length of coax cable. There's a trick to it im sure you'll find on the web, but if you try different lengths you can probably match it up pretty good setting aside the theory and instead using experiment to find the optimum length. It is nothing more than a length of coax that extends one side of the feed line before it gets to the antenna, so one side of the line sees a longer distance to the antenna than the other side.

This is hard to get to work for multiple frequencies, but since you only need it to work at 100Mhz (with some reasonably small deviation bandwidth) it should work ok. If you need other frequencies you would switch to a different length cable.
 
the standard TV baluns are made with either the figure-8 core or a ferrite bead with 6 holes in it, and both are useable from 50Mhz to 700Mhz, so i don't see what the problem is.... for a regular dipole, and any normal FM transmitter, you don't even need a balun. 300 ohm is for folded dipoles, which are more like an elongated loop antenna, whereas a regular dipole is just two wires cut to 1/4 wavelength. think of a folded dipole as a dipole with an additional piece of wire connected from one end of a regular dipole to the other end
 
Can I transmit more than 5KM using this balun and dipole by 1watt FM Tx in almost open area?
Don't you think the range is also determined by the sensitivity of the FM radio and its antenna? Radios have varying amounts of sensitivity from excellent to very bad. A very bad radio has a range maybe only across a street (30m).

A dipole antenna is directional. It is good in two directions and very poor in two other directions.
 
A dipole antenna is directional. It is good in two directions and very poor in two other directions.

quite easily fixed in the FM band by making the dipole vertical......
the coax feeding the dipole needs to be brought in horizontally to minimize stray coupling between the antenna and the coax. the length of horizontal coax needs to be at least λ/10 long (0.3m at 100Mhz) if not longer. an interesting way to make a dipole, is by stripping back 1/4 wavelength (72cm@100Mhz) of shield braid over the PVC jacket, leaving 1/4 wave of the center conductor as the other half of the dipole. by attaching a plastic hook to the end, you have a vertical dipole that can be put up just about anywhere. if you do this with 75 ohm cable, be sure to remove the metal foil from the insulation around the center conductor.
 
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Hi Willen,


Apparently there is a problem with the sites you are using. Perhaps try to upload the photos to this forum instead.

There, he made a general dipole and balun. He make 10 turn and almost 2 inch diameter inductive coil balun twisting coax near at dipole.
According to him (look his text):-
I will explain how to build a simple but very
good dipole antenna, and it only took 45
minutes to build.
The antenna rod is made of 6mm copper
tube I found in a shop for cars. It is actually
tubes for the breaks, but the tube works
great as antenna rods.
You can use all kinds of tubes or wire. The
benefit of using a tube, is that it is strong
and the wider tube diameter you use, the
wider frequency range (bandwidth) you will
also get. I have noticed that the transmitter
gives highest output power around
104-108 MHz so I set my transmitter to 106
MHz.
The calculation gave the rod length of 67
cm. So I cut off two rods at 67cm each. I also
found plastic tube to hold the rods and to
give it a more stable construction.
I use one plastic tube as boom and a second
to contain the two rods. You can see how I
used black duct tape to hold the two tubes
together.
Inside the vertical tube are the two rods and
I have connected a coax to the two rods.
The coax is twisted 10 turns around the
horizontal tube to form a balun (rf choke) to
prevent reflections. This is a poor mans
balun and lot of improvement can be done
here.
I placed the antenna on my balcony and
connected it to the transmitter and turned
on power supply. I live in a medium city so I
took my car and drove away to test the
performance. The signal was perfect with
crystal clear stereo audio. There are many
concrete building around my transmitter
which affects the transmitting range.
The transmitter worked up to 5 km distance
when the sight was clear (could not obtain
line-in-sight). In city environment it reached
1-2km, due to heavy concrete.
I find this performance very good for a 1W
amplifier
 
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