Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Plz help me convert FM receiver to 2m for a Foxhunt

Status
Not open for further replies.

ramesh2000

Newbie level 4
Joined
Dec 16, 2008
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,339
2m receiver

Hello everyone

I am Ramesh and am planning to take part in a Radio Detection Event (A foxhunt).
For the event I have to build my own receiver(readymade not allowed but can use ICs)
Morse code will be transmitted b/w 144-146 Mhz FM.

Plz help me out:

1) I have never really built anything so complex so I was thinking of modifying an existing FM(88-108 Mhz , schematics are relatively easy to find on the net) circuit to work at 144-146 Mhz.

I figure that I'd need to change a couple of capacitors and I'd be done.Plz tell me if this is true..and if yes , how should I go about it? Can any1 plz suggest a schematic and what modifications shld be done to it?

2)I have searched a lot for 144 Mhz FM receiver but haven't found any good schematics...Plz could some1 tell me ?

3)Has any1 taken part in these competition?Can you tell me your experience?And any other info a first-timer like me would need to know??

Thanks!!!
Ramesh
 

dpechman

Advanced Member level 4
Joined
Mar 12, 2002
Messages
102
Helped
3
Reputation
6
Reaction score
1
Trophy points
1,298
Location
Brazilzil
Activity points
555
convert fm to m

i dont have any expertise in rf, but i growup reading electronic magazines and i have tryied some radio toys many years ago. i remember to cut 2 or 3 turns from the main coil in superregenerative radios to reach 144Mhz. the capacitors you can try some value lower of original.
check this one **broken link removed**
this seems very sensitive, but will give you the idea.
in more elaborated schematics i have no experience.
regards
 

    ramesh2000

    Points: 2
    Helpful Answer Positive Rating

cyberrat

Advanced Member level 3
Joined
Jun 19, 2001
Messages
899
Helped
87
Reputation
174
Reaction score
43
Trophy points
1,308
Location
In the sewers of the U.K.
Activity points
5,859
fox hunting receiver 2 m

You will also need a directional aerial and signal meter otherwise your not going to find them.

Foxhunts are fun, take some sandwiches as it can take a while to find them :D
 

    ramesh2000

    Points: 2
    Helpful Answer Positive Rating

ramesh2000

Newbie level 4
Joined
Dec 16, 2008
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,339
simple receiver 144 mhz

Thanks a lot for yor reply!

I read the link , but wasn't able to understand everything very well.Bt will certainly read up on that

Can any1 tell me how the simplest FM radio using a popular IC (like TDA7000 , etc) works?As in which capacitor/inductor controls what?

Here is a link which attempts do do something similar...Can any1 plz tell me how the original FM radio was made?
www.qslnet.de/member/la3za/dokumenter/CheapVHF_SPRATWinter04.pdf

Thanks

Added after 10 minutes:

@cyberrat

Ya , I know that Directional antenna etc. are imp , but being a first timer I am worried just about building the basic receiver :( I'd be more than happy if my receiver actually detects something.

Plz plz help me out.

PS:If I do make it , i'll certainly take sandwiches along! :)
 

dian

Member level 3
Joined
Apr 16, 2006
Messages
61
Helped
15
Reputation
30
Reaction score
6
Trophy points
1,288
Location
Indonesia
Activity points
1,574

    ramesh2000

    Points: 2
    Helpful Answer Positive Rating

ramesh2000

Newbie level 4
Joined
Dec 16, 2008
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,339
tda7000 144mhz

Thanks for those links!
But unfortunately I am unable to find many of the components in India.Worse , the shop owners have no clue about the replacements(They are merely traders , have very little understanding of electronics).

That is why I'd like if someone could explain me how a basic FM IC driven circuit works.

Like TDA7000 , etc. , so I could simply modify the existing capacitors and be able to boost the range to 144-148 Mhz.

Also could some1 list the most popular/easy to use FM ICs?

Plz help me out .

Thanks
 

dian

Member level 3
Joined
Apr 16, 2006
Messages
61
Helped
15
Reputation
30
Reaction score
6
Trophy points
1,288
Location
Indonesia
Activity points
1,574
simple 2m receiver

TDA7000 designed for FM broadcast 88-108MHz like the data book said. How ever there's some people said TDA7000 family (TDA7088) have ability to operate until 150MHz. You may try to changes the oscillator frequency, (pin 5 & 6 in TDA7000) by changes the smallest value of L and C.

Other method is try to make a down converter for this chips.
*. create fixed freq oscillator (using crystall) (ie. 45MHz)
*. mixed it with input freq (ie. 144MHz to 148MHz)
*. give the mixed signal to TDA7000 (ie 144-45 = 99 to 148-45 = 103)

try this link for your TDA7000 modification
**broken link removed**
 

    ramesh2000

    Points: 2
    Helpful Answer Positive Rating

pranam77

Advanced Member level 5
Joined
Apr 22, 2008
Messages
2,777
Helped
553
Reputation
1,104
Reaction score
369
Trophy points
1,363
Location
INDIA
Activity points
16,406
2 meter receiver

Converting a FM radiop is very difficult. Dedicated kits built with FM reciever IC's but those are fixed frequencies with crystals as refernce. You may go for any regenerative recievers whic are available on the net. But those too will not have a PLL option. You may have to tune manually. Any ways... Best of luck
 

    ramesh2000

    Points: 2
    Helpful Answer Positive Rating

ramesh2000

Newbie level 4
Joined
Dec 16, 2008
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,339
fox hunt circuit

@Dian

Thanks ! I don't understand everything you said bt I am reading abt it.

@Audioguru

Plz suggest some newer and better FM chips

@Pranam
Plz tell me more.I dont care if I have to manually tune it.

In fact I'd be glad even if my receiver picks up something(forge the morse code signal) , cuz that'd give me a big hint as to where the transmitter is.

Is there any simple antenna/ckt that can pick uup "some" signal at 144-146 Mhz(with/withouut decoding the FM)?
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,440
Helped
2,150
Reputation
4,300
Reaction score
2,002
Trophy points
1,393
Location
Toronto area of Canada
Activity points
59,559
superregenerative receiver 144mhz

With radios so cheap lately, nobody makes a good radio IC anymore. Even the Sony radio IC is not made and the newer Philips IC is also not made anymore.
 

pranam77

Advanced Member level 5
Joined
Apr 22, 2008
Messages
2,777
Helped
553
Reputation
1,104
Reaction score
369
Trophy points
1,363
Location
INDIA
Activity points
16,406
144-146mhz circuits

Hai Ramesh, Thanks for pressing the "Helped me" button. Its my pleasure to have helped some one. I have attached here two articles related to regen receivers. The article from QST describes the regen principle and the other is a VHF regen receiver which cover 140 to 150 MHZ. Good luck.
 

    ramesh2000

    Points: 2
    Helpful Answer Positive Rating

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,440
Helped
2,150
Reputation
4,300
Reaction score
2,002
Trophy points
1,393
Location
Toronto area of Canada
Activity points
59,559
modifying fm radio for 2m

I guessed correctly that somebody would post a one-transistor regen "radio" in this thread.
 

pranam77

Advanced Member level 5
Joined
Apr 22, 2008
Messages
2,777
Helped
553
Reputation
1,104
Reaction score
369
Trophy points
1,363
Location
INDIA
Activity points
16,406
simple receiver fm

Audioguru..you guessed right. Ramesh wanted exactly that which is very easy to build with too littele components. Ramesh...Thanks for pressing the "Helped me" button. Its my pleasure to have helped some one. Good luck in the Fox hunting contest.
 

ramesh2000

Newbie level 4
Joined
Dec 16, 2008
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,339
pll 2m 144mhz

Hey every1 , thanks a lot for your help.I am evaluating varios designs and will see whcih1 to build.

Also can you guys help me out with the antenna?
I am planning to use the simplest 3 element Yagi Antenna (1 reflector , 1 driven and 1 director).
The construction seems very easy.

But can some 1 plz explain me the bare basics?As in what orientation wrt hidden transmitter do I get to hear the loudest signal(on my earphones )?

Thanks
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,440
Helped
2,150
Reputation
4,300
Reaction score
2,002
Trophy points
1,393
Location
Toronto area of Canada
Activity points
59,559
fm receiver modifcation

The sound level from a sensitive FM tuner does not change with RF signal strength from the transmitter.
The sound volume from an AM tuner changes with RF signal syrength if its AGC is disconnected.

Is the fox transmitting AM or FM?

Most foxes transmit FM so the receiver uses an RF signal strength indicator circuit to determine the direction that is sensed by a moveable directional antenna.
 

pranam77

Advanced Member level 5
Joined
Apr 22, 2008
Messages
2,777
Helped
553
Reputation
1,104
Reaction score
369
Trophy points
1,363
Location
INDIA
Activity points
16,406
norwegian regen receiver

Hai Ramesh. The answer to this question is a bit difficult. Ok....here it is. Basically to get best reception fron any antenna it should be tuned to minimum SWR and maximum gain using any Antenna analyser or transciever with a SWR meter. For hunting the fox you need a signal strength meter which will display the received signal strength of the transmitter and you need to move towards the direction,in which you get more and more strength. It is possible only if you use a commercial receiver in which you will have a calibrated S-Meter in analogue or LCD bar graph form. You cannot guess the signal strength by heraring the signals. Rest of my answer is given by Audioguru. Good luck
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top