building a directional antenna(yagi)

[anerudhan]

i want to build a directional antenna to capture frequencies 144-146 mhz..i have to locate a transmitter transmitting this frequency as a part of my project ...please help me

 

[Gaston]

locating a transmiiter like that is refered to as a "foxhunt" in amature radio. loop antenas are usually used for this. if you google foxhunt im sure you will find all the info you need along with designs for that frequency.

 

[stevez]

A yagi is one kind of directional antenna. A loop antenna offers a fair amount of directivity. As already suggested, amateur radio literature is probably a good source of information. The ARRL website - TIS pages, has a bunch of magazine articles listed. Your school library might be able to assist you in obtaining copies. Access via the website is possible as a member.

 

[Nigel Goodwin]

i want to build a directional antenna to capture frequencies 144-146 mhz..i have to locate a transmitter transmitting this frequency as a part of my project ...please help me

Check the ARRL or RSGB handbooks, or the RSGB VHF/UHF manual, these are essential reading for anyone in amateur radio.

 

[stevez]

A few more things worth noting:

1. The performance of antennas is affected by things, including the user, in close proximity to the antenna. When I teach this part (in local amateur radio classes) I usually demonstrate with an antenna analyzer that anything within a few feet of a 144 mHz antenna can affect it. I would expect that designs used by foxhunters have addressed this issue. Ordinary yagis or other directional antennas are designed to be well away from objects -save for the antenna mast.

2. You may "see" a signal that is actually a reflection (or refraction) - which could throw you off. The way to manage this is less with the antenna and more with the way it is used.

 

[Nigel Goodwin]

I used to do 2m DF hunting on a motor bike (Yamaha DT400), I used a Kenwood 2200 portable, and built a simple yagi that slotted together in seconds using 300 balanced feeder (for the rods), a wooden beam, a few pieces of dowelling, and a handful of rubber bands!

 

[RadioRon]

Here is a book that would be helpful
ARRL's Yagi Antenna Classics

here's where to find it:
https://www.arrl.org/catalog/index.php3?category=Antennas%2C+Transmission+Lines+%26+Propagation

Here is a simple one you can build:
**broken link removed**


Here is one a bit more challenging
https://www.qsl.net/sv1bsx/6EL-VHF/6EL_VHF.html

 

[anerudhan]

thanks for the help ...now the problem is to build a receiver..can i use directly an ammeter to read the current as a measure of intensity of the signal..?

also when i was reading certain circuits i saw preamplification circuits..is it necessary to use them ..?also what is the use of local oscillator in these receiver circuits

 

[RadioRon]

thanks for the help ...now the problem is to build a receiver..can i use directly an ammeter to read the current as a measure of intensity of the signal..?

also when i was reading certain circuits i saw preamplification circuits..is it necessary to use them ..?also what is the use of local oscillator in these receiver circuits

No, you can't use an ammeter to read the current as you suggest. The antenna output impedance is 50 ohms and it will receive signals with power levels around 1 x 10^-14 watts. By ohms law you can calculate that the current will be 0.0000000000000002 amps. You cannot see this on a meter.

The whole point of a receiver circuit is to boost up the signal until a meter can measure it. Inside the receiver there are typically many amplifier circuits to do this boosting, some of which you might call preamplifiers. Some people will also add such preamplifiers outside the receiver for more boost but it is not necessary in your case.

The local oscillator is used to convert the frequency in a "superheterodyne" system design of the receiver. Please study this by referring to google or textbooks.

It is clear from your questions that you have inadequate knowledge to even begin a receiver project. I suggest that you buy one, or buy at least one book and study every page very very carefully. There are many books that can help, but one that covers a lot of ground is the ARRL Handbook.