yagi antena to 50 ohm amplifier

[estenolotienenadie]

Hello please i need to now how can i conect my yagi antena to a 50 ohm preamplifier. I want to build a microstrip balun like this https://www.cst.com/Academia/Examples/Quarter-Wave-Transformer but i dont have idea how to conect it. Please if you can help me i will be very happy.
Im sending some pictures of what i need but i dont have the real dimentions. If you have any example with dimentions please post it here. I need to cover uhf band.
Thank you.

 

[BradtheRad]

At one time the nominal impedance of TV antennas was 300 ohms. These were yagi type. When 75 ohm coax lead became the style, there were matching transformers. Attach it to a 300-ohm twin lead connection, and you had the output through a 75 ohm F connector.

It's possible you could use the same matching transformer. Or, find its schematic to supplement information you have now.

 

[estenolotienenadie]

Thanks for your replay, in fact new antennas arrive with balun on it, this antenna is home made, is not original, also im going to use 50 ohm wire from antenna to my tv.
Thank you.

 

[BradtheRad]

If you are receiving a strong signal, then you might get by with a simple length of wire, plugged into the center of the F receptacle. I've seen it work with modern tv's.

A yagi antenna is easy to make. You can add elements to add sensitivity and directionality. This can help solve problems with multipath from local stations.

I once built my own yagi from wood sticks and copper wires, so I could receive TV channel 13 from 90 miles away. I followed instructions in the ARRL handbook. Some days reception was good, but usually it was snowy.

 

[davenn]

Thanks for your replay, in fact new antennas arrive with balun on it, this antenna is home made, is not original, also im going to use 50 ohm wire from antenna to my tv.
Thank you.

so ... is it a folded dipole or a straight dipole ?
As Brad said, 300 Ohms was standard and that was for a folded dipole ( driven element) yagi
Actually to be more precise, a folded dipole on it's own is 300 Ohms. Once within a Yagi configuration with a reflector and directors
the impedance drops considerably.
Using a 4 : 1 BALUN is still a good way to go as the impedance may be relatively close to 50 Ohms anyway

The other thing to consider is that for receive, matching isn't going to be overly critical

MY question to you is ..... Why are you using a 50 Ohm preamp in a 75 Ohm system ???

you do realise that the input to the TV is 75 Ohms ?


Dave

 

[estenolotienenadie]

so ... is it a folded dipole or a straight dipole ?
As Brad said, 300 Ohms was standard and that was for a folded dipole ( driven element) yagi
Actually to be more precise, a folded dipole on it's own is 300 Ohms. Once within a Yagi configuration with a reflector and directors
the impedance drops considerably.
Using a 4 : 1 BALUN is still a good way to go as the impedance may be relatively close to 50 Ohms anyway

The other thing to consider is that for receive, matching isn't going to be overly critical

MY question to you is ..... Why are you using a 50 Ohm preamp in a 75 Ohm system ???

you do realise that the input to the TV is 75 Ohms ?


Dave

Listen i dont know anymore what to do, my brain is hot, i have one antenna yagi UHF 300 ohm, with 30 meters 75 ohm coaxial so i brought Ecoflex 10 coaxial that is very good becouse 30 meters is too large and too much looses, but Ecoflex 10 is 50 ohm, also i have one minikits MMIC preamplifier that also is 50 ohm, my tv is 75 ohms so i dont know anymore what to do, i want to put this ecoflex becouse is low lost coaxial and i have my antenna on the top, the amplifier is 50 ohm, so how i conect all this??? 300 ohm antenna yagui, 50
ohm the coaxial, 50 ohm the preamplifier and 75 ohm the tv?
I dont understand why is so complicated all this. I hope you can help me.
Regards,
Alberto.

 

[estenolotienenadie]

Hello, thanks for your replay, the problem is that my antena is on the top so i need like 30m coaxial and if i put 30m RG6 is not good , thats why i have now Ecoflex 10 but is 50 ohms, i will put it anyway becuse the preamplifier is 50 ohms, but the problem is that trv is 75 ohm, i dont know anymore what to do, believe me, i am thinking and thinking and my brain want to explote, Why electronic is so complicated? why MMIc are only 50 ohms? Sometimes i think that i am the person that give me myself all this problems, Here in Peru you cant find one good antenna, all antennas are home made, any shop sale good antena, so i have to build my antenna myself, next, i live in the first floor, i have to use 30m coaxial, i got one very expensive in holand but is 50 ohms, if you need a good amplifier for all uhf band you have to buy the one with MMIC, but is also 50 ohm, and tv are 75 ohm, so why like this so complicated.
Thank you.

- - - Updated - - -

At one time the nominal impedance of TV antennas was 300 ohms. These were yagi type. When 50 ohm coax lead became the style, there were matching transformers. Attach it to a 300-ohm twin lead connection, and you had the output through a 50 ohm F connector.

It's possible you could use the same matching transformer. Or, find its schematic to supplement information you have now.

Thanks for your replay, if you say this that antena no have the output 50 ohm, how is posible to conect it to 75ohm tv? Why the best antenna preamplifier are 50 ohms? 50 ohms, 50 ohms, everywhere i reed i see 50 ohms, matching network, loose power....., if you dont put correct impedance and things are not so easy to understand for me at list.
Thank you
Alberto.

 

[BradtheRad]

I'm sorry I got mixed up. 75 ohm coax became the common type in commercial appliances, replacing 300 ohm twin lead.

50 ohm is a figure which was settled on as a compromise for radio equipment handling high power. The article below tells what differences there are between 50 and 75 ohm type.

https://cablesondemandblog.com/word...ence-between-50-ohm-and-75-ohm-coaxial-cable/

- - - Updated - - -

I went back to my post #2, and changed occurrences of 50 ohms to 75 ohms.

Although these ohm values are supposed to be 'standards', they are really a compromise, according to the article. You may get adequate performance even if your matching is imperfect.