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|20th August 2008, 12:18 AM||#2|
Join Date: Feb 2004
Location: Recife - Brasil Northeast
Produce a dipole... two aluminium tubes insulated into the middle
each aluminium tube will be 1 pole...so, two wires will go to your receiver...use 75 ohms coaxial cable...with inner conductor and shield... the aluminium length can be half wave length each one, or even 1/4 wavelength each one.
To obtain the correct dimension divide 300 (ligth speed) by your prefered frequency in Megahertz.... for instance 100 megahertz as your prefered FM station.
To this frequency your wavelength is 300/100 = 3 meters.
So... you can build a dipole (two poles insulated into the middle) with 1.5 meters each half side tube (full wave dipole aerial)...or you can use 75 centimeters to each tube (half wave)..... distance between then must be around 5 milimeters to avoid capacitances between electrodes.
You can try your dipole into Vertical position or Horizontal position, a good idea is to be in your roof, above your ceiling, into your home top...or attached into a post or a tree.. the coaxial cable will bring signal for your receiver.... try vertical and horizontal and search for best signal.... if vertical nothing to do...but if you decide by horizontal you have to turn you antenna trying to increase the signal strength measured into your receiver S Meter (units of reception strength.... units "S" ).
The better reception (and transmition too) happens into the perpendicular.... 90 degrées created from the aerial line....so.... the vertical one has all 360 degrees of reception, because the 90 degrees point is all the way around... this does not happens with the horizontal that has maximum and null point (the direction when you think your aerial as a gun barrel.... this is minimum reception).
You can use Television antena booster... a 10 db or 20 db radio frequency amplifier is you are distant from the transmitter aerial... those frequencies are hardly attenuated if have to enter and cross land.... works better when you have visual contact from antenna to antena...so.... not far away from the Horizon....BUT...if you increase the high...installing it high from the ground...your horizon distance is increased.... the radio amplifier, the antenna booster can help a lot... increase noises but this may not appear into the Frequency Modulated signal.... to AM will ruine the signal because of amplitude static noise captured.
Also you can use Yagi style aerial... this one alike a fish spine... used to television.... the dimensions not good to FM radio but will give you some gain that will rid you from the Radio Frequency Amplifier, the antenna booster... the Yagi antenna provides you a lot of gain... can give you 15 decibels easy...but have to point it and 2 or 3 degrees of error will kill your reception.
Also you can call a Radio Amateur to adjust your reception trimmers into your receiver..to give you best match with your antena and increase the sensitivity to the maximum possible to your receiver (2 to 5 microvolts normally).
Use your cable as short as possible...if you decide to use a paralell wire alike a tape... alike insulating tape of adhesive tape format... then cut your wire into multiple of your wavelength (1.5 meters) because this kind of cable (not insulated by external coaxial shield) will receive signal also... the cable will be a part of the antena too.... electromagnetic waves will cross this wire copper and will produce usefull signal to you.
If one antenna cannot see the other (imagine the antenna has eyes)... so...no visual contact... the signal will be attenuated. because will have to cross solid things as buildings, hills and rocks... this goes reducing the signal strengh..but the signal will not be "entirelly killed" because of that..will continue to exist but veryu small in level, you can try to capture it using external antenna.
I think you may be moved to somewere in our forum, as Radio Frequency reception is not really Solid State...but here my contribution to you my dear friend.... remember to be happy...enjoy your life.
By the way... the dipole impedance is 75 ohms... and will be happy and matched with 75 ohms coaxial cable...but will face a 300 ohms input into your receiver (if old)... so... in this case will be mismatched but will work with losses..but gonna work.
If you decide the paralell tape as cable (fita geminada).. this one is 300 ohms impedance...will not match the junction point from the antenna to the cable (mismatch there) but will match the receiver input...once again losses because of mismatch...maybe a transformer...those Balun you can have together your television antenna...the vertical wipe portable antena that comes together your television receiver can be used to...as matches 300 ohms to some 75 ohms input...BUT...you can invert.... so... the 75 ohms antena can enter the 75 ohms side and the 300 ohms side can go to the receiver.
Sadly, Brazil now a days is a time bomb....do not come to the world cup..you may face yourself in the middle of a revolution
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