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Thread: For the FM transmitter techs. Attenuators.

  1. #1
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    Default For the FM transmitter techs. Attenuators.

    Right now I have an FM Transmitter that is a little too strong. Don't have the time or money right now to spend on and EDM or the like. I do have a lot of electronic parts lying around so I was wondering if I could make my own attenuator to knock down the signal to something a little more acceptable. My transmitter is set to low power which is 100mW. I can get my signal about 1/2 mile away. I'm looking to get just a few hundred feet. The antennae on the transmitter has TNC connectors. From what research I have done, all I need to do is get a male-female TNC cord and cut it and build a Pi-network attenuator with 3 resistors and place it in between the transmitter and the antennae. According to my calculations, to be close to "legal" FCC part 15, I would need 30dB of attenuation. So, that mean a 830ohm resistor in series with 2x 51ohm resistors in parallel.

    First, is my thinking correct to accomplish what I'm trying to do? Second, would this ruin the transmitter if I tried?

  2. #2
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    Default Re: For the FM transmitter techs. Attenuators.

    Im not sure about your math, but your approach seems sound to me.
    There is some loss in all cabling and connectors so your best bet is to experiment.
    You can also detune by simply using a less efficient antenna.

    No, you won't hurt your transmitter. The point is that all RF is just energy and it either radiates out your antenna as RF or is absorbed and turned into heat by the wires and in your case, the added resistors.

    The key about being compliant is to never ever step on a commercial signal.

    It is nice to see someone that is concerned about their signal strength. Good for you.

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    Default Re: For the FM transmitter techs. Attenuators.

    Have not checked your calculations for the resistor network, but 30 db attenuation would take you down by a factor of 1000, taking you to 0.1mw. So if your at 1/2 mile now, that might be a bit too much attenuation. I would suggest 10db attenuation, taking you to 10mw and giving you about 250'. Or you could try 6db attenuation taking it down to 25mw or approximatly 600ft. It is rf, and depending on your topography, antenna, your results will vary.
    40yr telecom tech.
    Last edited by aknflyer; 10-19-2019 at 02:47 AM.

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    Default Re: For the FM transmitter techs. Attenuators.

    Thank you guys. I will do some testing with various values, 10db 20db 30db. It's only a matter of resistors. May take a bit of time but, do some testing and post back with my results.

  5. #5
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    Default Re: For the FM transmitter techs. Attenuators.

    Quote Originally Posted by ukewarrior View Post
    It is nice to see someone that is concerned about their signal strength. Good for you.
    I too commend you for your efforts to stay within limits. I am currently working on mine, I am currious as to your results.

  6. #6
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    Default Re: For the FM transmitter techs. Attenuators.

    The parts are in and the testing has begun. I'm posting preliminary information to give others a chance to chime in. So, this all began with the realization that my FM Transmitter was probably a little too powerful. I've had it for years, way before I got into the lights hobby. I only used it once or twice a year for outdoor movies and for that, it was only on for about 2hrs. Now, with the lights, it going to be on for hours everyday for a couple of months. With a signal as strong as it is I started to think about attenuation. After doing some research, I figured I would try to make my own attenuator vs. just buying a pre-made one, which by the way, seem really expensive for what they are.
    My plan was to try different attenuators and just walk/drive around getting basic range measurements. Nothing fancy, I can't afford the test equipment. Here is a page to help with the calculations (https://daycounter.com/Calculators/). Under the "RF" section is the PI and T-pad Attenuator Calculator. Putting in various dB values and using 50 ohms for characteristic impedance (that's my antennae), I got a bunch of resistor values to use. Limited by my supply and what the local electronics store had, I can make 5, 10 and 25 dB attenuators.

    Parts:
    20" TNC Male to TNC Female nut bulkhead RG58 cable (ebay)
    HAMMOND 1411B ENCLOSURE, UTILITY, ALUMINIUM, GRAY (local store) (found on amazon, but pricey)
    1 Watt resistors (my transmitter is .5W max, so doubled for safety)



    1/2" Hole for bulkhead connector
    5/32" Hole for antennae cable


    Finished design with antennae attached.
    Yes there is a cover, but then you wouldn't be able to see that awesome electronic work inside.


    Final (semi-final) design
    Pi Attenutaor design, currently set for 25dB
    Ground braid is attached to bare metal of box to prevent stray signals
    Not fancy, but it does actually work
    Still need to attach zip-ties for cable strain relief


    What I've found in my design is that I cut the cord on the bulkhead side a little too short. Unfortunately, when inserting the antennae, it pushes the female connector into the cable and doesn't make a good connection. There isn't enough friction to keep the inner conductor in place, oops. Careful pliers work allowed me to push the conductor and connector back out. Certainly, not built for constant dis/connect use.

    Results so far:
    My transmitter, on low setting, is set for .1W and that gets me about 1/2 mile range on a good day. Not the greatest signal at that range, but the radio will still pickup spots and reflections. First, I tried a 5dB setup but that didn't seem to have any effect. I believe weather and atmospheric conditions actually made the signal go further, approx 2000ft+. Upto about 2500ft+ the signal was spotty static but still, barely, present. Next, I tried the other end of the spectrum and went straight to 25dB. Great results as far as knocking the signal down. I went from 2000ft+ to about 300ft. At about 200 ft, I start to get static and at 400+ I lose the signal. This is actually what I was looking for but, the downfall is that the signal might be just a little too weak. Driving up and down my street, things like my mail box or the tree in front of my house causes static. That's only about 100ft away. Un-obstructed signals are actually quite clear. I have parts to make a 10dB but, I think that's not enough, so I'm going to order some resistors and try to find a "happy" medium. I'm thinking the 15dB - 20dB range will work for me. In this experiment, I'm not going for full on regulatory standards, but I'm not looking to draw unwanted attention either by having my own broadcast radio station. My philosophy is "out of sight, out of mind." Don't think that I'm some sort of electrical engineer, believe me, I'm not. I just like to experiment, play, break things and try to learn something along the way. So anything you do is your own liability to you and your equipment. I just hope that this will help contribute to the Christmas lights hobby, keep everybody safe and legal (to a certain extent) and allow this hobby to thrive for many years.
    Attached Images Attached Images

  7. #7
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    Default Re: For the FM transmitter techs. Attenuators.

    So your using the pi network. 2ea 56 ohm resistors, and one 470 ohm resitor. I do have the test equipment to verify the levels. I will set it up and give some real measurements to confim with.

    Btw, I find it easier to solder a small wire to the shield, heat shrink it, and use the small wire to make the other connections. Cleaner.
    Will get back with an update.
    Last edited by aknflyer; 10-25-2019 at 02:52 AM.

  8. #8
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    Default Re: For the FM transmitter techs. Attenuators.

    According to the calculator, the optimal value should be 460 ohm, but that's not the most common value. I had a 470 ohm. But like all electronics, the values aren't perfect. Even 2% tolerance gives +-9.4 ohms, so that's why I'm using 470. I would like to test with 15dB using 130ohm, 2x 75ohm and 20dB using 240ohm, 2x 62ohm. Since I would have to order those, I can get those actual values.
    Yeah, it's not the prettiest. It did look better on the first run w/ 5dB, but that's when I discovered the antennae pin pushing the connector into the cable. I had to cut the cable shorter to be able to push it back out. I figure, once I get the values that work for me, I'll neaten it up and make sure none of the solder joints will touch the box.
    Aknflyer, thank you for your help and insight.
    Last edited by Cmndr Brain; 10-25-2019 at 09:53 AM.

  9. #9
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    Default Re: For the FM transmitter techs. Attenuators.

    Yeah, you will not find a 460 ohm resister, I would ballpark it, use the most common resistors. It will not hurt the transmitter, the 1/10ths of a difference will not matter. Sounds like a cheap connector if the pin is not fixed. My issue has been the small space in the case of the dsp pll transmitter I use, makes it hard to pull out the ground lead. I may just switch to the Vast I have, since it seems they may have fixed the rds. At least it has an actual connector output.
    Will let you know the results.

  10. #10
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    Default Re: For the FM transmitter techs. Attenuators.

    So, after another trip to the electronics store and more testing, I think I've finalized my attenuator. My final resistor values are 120 ohm for the series resistor and 83 ohms for the parallel resistors. This give me about a 14dB decrease. This gets me about 500ft before the static and about 700ft - 800ft before I lose the signal. Between those, depending on obstructions it "fights" with 2 stations that don't come in clear anyways. A religious station that's 69mi away and a country station that's 88mi away. But, small obstacles, like my mailbox, doesn't seem to interfere with the signal. I think I'm going to run it this way this year and see how it goes. I hope my attempt to keep within legal limits doesn't attract any unwanted attention and this will help others looking to do the same.

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