Basic help about Yagi please

[Willen]

I want to transmit FM RF with high
gain antenna like Yagi, help me in
basic way plz:
1. I should have to feed rf in two ends
of dipole rediator only, isn't it?
2. I am confusing in Main Rod (which
connects reflector, dipole rediator and
directors) component. Does it should
have to be conductor? Are they all
small rods and main rod connected
electrically via Main Rod?
3. It is too much directional antenna
with high gain. May I know, in which
direction it can transmitt? And how to
stand this Yagi in the sky to transmit
horizontally?
4. Impedance matching Balun (75
ohm - 300 ohm) is necessary for Yagi
too, isn't it?

 

[davenn]

I want to transmit FM RF with high
gain antenna like Yagi, help me in
basic way plz:
1. I should have to feed rf in two ends
of dipole rediator only, isn't it?

First off, you have not told us what frequency you want the Yagi for ???

Most (not all) yagi's use a loop dipole feed

here's one for 433MHz that isnt and is pretty easy to construct it could be scaled to use on other frequencies....

2. I am confusing in Main Rod (which
connects reflector, dipole rediator and
directors) component. Does it should
have to be conductor? Are they all
small rods and main rod connected
electrically via Main Rod?

the boom can be metal or other material like wood or plastic
if it is metal as in the example pic above, then consideration needs to be given on how to mount the driven element

3. It is too much directional antenna
with high gain. May I know, in which
direction it can transmitt? And how to
stand this Yagi in the sky to transmit
horizontally?

the higher the gain the sharper the beamwidth (ie. the more directional it is)
it transmits in the direction you point it

you need to mount it on a pole, using U bolts anyone selling TV antennas will supply U bolts or a hardware store

4. Impedance matching Balun (75
ohm - 300 ohm) is necessary for Yagi
too, isn't it?

That depends on the impedance of the coax.... do you plan to use 75 Ohm or 50 Ohm coax ?

whether you need a 4 to 1 BALUN depends on the type of driven element you use
in the example above it is a 1 to 1 BALUN
for a folded dipole driven element in a Yagi, the impedance is usually around 200 Ohms
this makes a 4 to 1 BALUN and 50 Ohm coax ideal

cheers
Dave

 

[Willen]

First off, you have not told us what frequency you want the Yagi for ???

Most (not all) yagi's use a loop dipole feed

here's one for 433MHz that isnt and is pretty easy to construct it could be scaled to use on other frequencies....






the boom can be metal or other material like wood or plastic
if it is metal as in the example pic above, then consideration needs to be given on how to mount the driven element



the higher the gain the sharper the beamwidth (ie. the more directional it is)
it transmits in the direction you point it

you need to mount it on a pole, using U bolts anyone selling TV antennas will supply U bolts or a hardware store



That depends on the impedance of the coax.... do you plan to use 75 Ohm or 50 Ohm coax ?

whether you need a 4 to 1 BALUN depends on the type of driven element you use
in the example above it is a 1 to 1 BALUN
for a folded dipole driven element in a Yagi, the impedance is usually around 200 Ohms
this makes a 4 to 1 BALUN and 50 Ohm coax ideal

cheers
Dave

-Does it work in transmitter as well as receiver?
-So then...if I make a normal dipole using normal metal tube for my 50 ohm, 1 watt FM transmitter, there is no need to use Balun, isn't it? (because I think coax has same impedance of around 50 ohm and normal dipole like mine has also same impedance around 50 ohm) am I right? Please make clear!

 

[davenn]

a standard straight dipole is 75 Ohms, it will work ok with a 50 Ohm feed with only a small amount of loss

A folded dipole has a 300 Ohm feedpoint and would require a 4 to 1 BALUN to bring it down to 75 Ohm and
again making it ok to use with 50 Ohm coax

Dave

 

[Willen]

a standard straight dipole is 75 Ohms, it will work ok with a 50 Ohm feed with only a small amount of loss

A folded dipole has a 300 Ohm feedpoint and would require a 4 to 1 BALUN to bring it down to 75 Ohm and
again making it ok to use with 50 Ohm coax

Dave

-Almost '0' shape dipole is folded dipole, isn't it?

-What is the impedance of rabbit ear antenna (with feeder wire/ribbon wire? Is it general ''straight dipole, 75 ohm'' too? If it is straight dipole of 75 ohm, why TV uses ''75/300 ohm balun'' on this rabbit ear antenna? Balun used between TV and ribbon but not between rabbit ear and ribbon wire? Why?

 

[davenn]

if they are straight telescopic rods that do have some up down movement, then the antenna should be 75 Ohms impedance and could be fed with coax directly to the TV's antenna socket.
You just have to remember that rabbit ear antennas are really a cheap and nasty solution and the manufacturers dont necessarily follow good design practice. Hence you are likely to find all sorts of weird and wonderful variations, some of which will work better than others.
They are also really designed for strong signal areas and as a result those poor manufacturing practices dont make too much difference to reception.

Dave