FM transmitter need help

Discussion in 'Wireless & RF Design' started by imaan, Jul 24, 2014.

  1. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    Hi friends
    I'm living in a country where broadcasting FM is free and legal unless you don't interfere with others.
    So I got a 15W FM transmitter plus a half wave dipole antenna, installed on a 5th floor's roof.
    Our city is surrounded by ocean so there is no serious barrier.
    But the signals are not going very far as I was expecting.
    Now I need advice how can I make my system more powerful since I'm not professional in this field.

    Thanks
     
    Stacy Xie likes this.
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    add an amplifier, it dosnt need to be linear, linear is for am and ssb.
     
    imaan likes this.
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    "broadcasting FM is free and legal unless you don't interfere with others" Let me see... So you are saying that you must interfere with others in order for it to be legal." How many negatives are you allowed in a sentence?

    How much of the 15W transmitter power is actually getting to the antenna?
    What frequency are you operating the transmitter on?
    What are you using for feed line? How long is it? How much loss does it have? What is the standing wave ratio?
    Is the transmission line impedance correct for the transmitter output impedance and the antenna input impedance? Do you have any impedance matching devices connected which are properly adjusted to achieve maximum power transfer?

    Do you have any friends who ARE professional in this field?
     
    imaan likes this.
  4. Synaps3

    Member

    Jun 5, 2013
    67
    2
    First off, I would suggest you get an SWR meter to find out how much power is actually getting to the antenna. Also, you should know that FM broadcast band is very line of sight, so if there are many objects blocking your antenna, the height won't make that much of a difference - otherwise it makes all the difference.

    Secondly, you say 15W. I'm making the assumption it is one of those chinese CZH units. If it is, I'd watch out, no matter what country you're from. Those things have terrible spectral purity and harmonics. You could always construct your own LPF (low pass filter) for the unit which could improve things.

    Thirdly, you could also add an amp like suggested, but do not do so if it is one of those chinese units. They are not clean enough for amplification even with an LPF. You can verify the quality of your transmitter by searching the PCB for a chip called BH1415F (it may be located under a cage). If you're transmitter has this chip, it's crap. If you'd like a high quality unit, I'd recommend PCS electronics.
     
    imaan likes this.
  5. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    Make sure your dipole is vertical.
     
    imaan likes this.
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    They're going a long way. Whether or not they're detectable at long range will depend on the sensitivity of the receiver. What do you define as 'not very far'? Is the receiver a normal domestic FM radio?
     
    imaan likes this.
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Please show photos of the "15W FM transmitter".
     
    imaan likes this.
  8. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    Thanks a lot everybody for your kind replies.
    I'm going to order a Micronta SWR/power tester to test the swr for my transmitter.wondering if it's a good brand.

    @ wmodavis: yeah I meant it's legal unless you interfere other signals.
    -I've ordered a SWR meter from USA, hope it can help me finding out how much of the power is going to the antenna.
    - I'm running on 89.00 FM which was empty and not interfering with others.
    - The 50 ohm cable came from the company which is 15 meters.
    -the cable matches with the transmitter.
    - and no, have no one helping me here !

    @ alfacliff
    What kind of amplifier? how powerful?
    I'm using a behringer mixer instead, donno if that helps!

    @Synaps3
    Yeah unfortunately it's Chinese.. but so far it's not so awkward however I don't believe in Chinese products!
    about amplifier somebody told me using amplifier can burn your transmitter ! now I'm confused...

    @THE_RB
    it's a brand new of CZH-A15 :(

    @Alec_t
    can you tell me how long is the max range of this system?


    Thanks everybody again..
     
  9. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    I'm sure you made sure the SWR meter is appropriate for the frequency you are using.
    Actually they are for measuring standing wave ratio not power but often have an approximate calibration for power which can be highly frequency dependent.
    How accurately do you need to know the power?
    SWR measurement is mainly for determining if the antenna is properly impedance matched to the transmission line.
     
    imaan likes this.
  10. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    It was a long time I was looking for a suitable and good swr meter on eBay, finally found this one.
    Can you tell me if this is the right product I need?
    It says its frequency range is 3 to 30 MHz operation frequency range..
    Is it related to my FM frequency which is set on 89.00 ?
    Thanks again
    [​IMG]

    And this is my transmitter :
    [​IMG]

    And antenna:
    [​IMG]
     
    Last edited: Aug 6, 2014
  11. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Specifications are needed to know if it is right for your application. Picture is pretty though. Maybe someone else has access to specs. I personally would not buy it until you are sure.
     
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  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
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    That's a bit like asking 'how long is a piece of string' :). Theoretically the range is infinite. In practice, assuming a normal domestic FM radio is the receiver, and bearing in mind all the surrounding tall buildings which will block/reflect signals, it could be anywhere from hundreds of metres to tens of kilometres. I don't think there's a definitive answer.
     
    imaan likes this.
  13. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    yeah right.. the max range I got recently was 12 kilometers that I could hear clearly. it might go beyond 15 KM.

    still confused about the swr meter
     
  14. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    My Fm transmitter's specs:
    Freq range :87.5MHz~108MHz

    Power :15w

    Ripple or harmonic waves: <= -60dB

    Tuning Step : 100khz

    Stability of Frequency: ±5ppm Less than 10ppm ( better system)

    Freq. Response: -55dB(100~5000Hz); -45dB(5000 ~ 15000Hz )

    Audio Input Connector: 3.5mm headphone connector
     
  15. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Was referring to the specs of the SWR meter.y
     
  16. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    OK this is it:
    Frequency Range: ..........................................3 MHz - 30 MHz

    Input RF Power: ......................................1 Watt - 2000 Watts

    Impedance: .......................................................50 Ohms

    Minimum Input Power for Calibration: ..............................1 Watt

    Power Meter Accuracy at 50 Ohms Load Impedance
    5W: ..................................+- 0.5W
    50W: ..................................+- 5W
    500W: ..................................+- 50W

    SWR at 50 Ohms Load Impedance: .......................................1.1

    SWR at 25 Ohms Load Impedance: .......................................2.0

    SWR at 100 Ohms Load Impedance: ......................................2.0

    Dimensions: .........................1 7/16 x 5 9/16 x 1 1/2 inches (HWD)
    (60 x 165 x 90 mm)
     
  17. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Glad you included the dimensions. Nice to know how big it is. But that tells me that the max frequency is 30MHz. Will your transmitter go down that low? I don't think so.
     
  18. imaan

    Thread Starter New Member

    Jul 24, 2014
    17
    1
    No my FM frequency can't come down that low.
    So I must get a swr meter which supports my range?
     
  19. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    "So I must get a swr meter which supports my range? "

    2 + 2 = 4

    In other words YUP!
     
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  20. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    I have found that those SWR units are good to ~ 250 MHZ with anything over QRP power. It should be ok. Replace dipole with 5/8 wave ground plane with at least 4 radials. Tune antenna to your frequency. Don't settle for low swr......adjust antenna for flat....1 to 1 swr. Measure swr at transmitter only. Place antenna as high and clear as possible. Use shortest feed line possible. I don't know how long your feedline is....but a high loss, long run of coax is nothing but a dummy load....nothing gets to the antenna. Use the highest low-loss cable that you can afford. On flat terrain you should get dependable 20 to 30 mile range. If there is no high powered station close to your frequency, you should be ok. If you are on a hill, mountain, peak or tall building and not too long of a feedline......the range will be much greater. Remember....your listeners judge you by what you say. Good Luck.
     
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