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Thread: FM transmitter antenna

  1. #16
    Stay-Puft! rcktpwrd's Avatar
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    Default Re: FM transmitter antenna

    I have a question about the connectors/adapters needed if building an antenna like @Tripplett posted. I see cables with the pl259 end that connects to the so239 socket.
    What do you use to adapt/connect the pl259 to the F connector on the back of the Ramsey fm25b? I am confused when trying to look up what is needed...
    Joel


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    Super Moderator EricR's Avatar
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    Default Re: FM transmitter antenna

    Quote Originally Posted by rcktpwrd View Post
    I have a question about the connectors/adapters needed if building an antenna like @Tripplett posted. I see cables with the pl259 end that connects to the so239 socket.
    What do you use to adapt/connect the pl259 to the F connector on the back of the Ramsey fm25b? I am confused when trying to look up what is needed...
    I made the same antenna. I just ordered this to connect to fm25b. It came in the mail a couple days ago and looks correct but I haven't verified that. I had bad luck finding the right thing on Amazon and radio shack didn't have it. This was $11 after shipping.
    Type F-Male to SO-239 / UHF-Female Coaxial Adapter Connector ARS-G248


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    Super Moderator EricR's Avatar
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    Default Re: FM transmitter antenna

    Just checked and it definitely fits.


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    Stay-Puft! rcktpwrd's Avatar
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    Cool Re: FM transmitter antenna

    Thanks @EricR!
    I think I found the same type adapter on Amazon to go along with the other prats needed to build the antenna.
    Are you using a 50 ohm cable to connect from the transmitter to the antenna?
    Joel


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    Super Moderator EricR's Avatar
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    Default Re: FM transmitter antenna

    Quote Originally Posted by rcktpwrd View Post
    Thanks @EricR!
    I think I found the same type adapter on Amazon to go along with the other prats needed to build the antenna.
    Are you using a 50 ohm cable to connect from the transmitter to the antenna?
    Yes 50 ohm is recommended. I know nothing about this stuff except what I have read. This is what I got. They sell shorter if needed and beware this cable is cream not black as pictured. Cream is actually better for me. My antenna is going to be permanent in my garage attic with this cable coming down to the work bench and FM transmitter.

    https://www.amazon.com/gp/aw/d/B00D6...236_SY340_QL65


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    Stay-Puft! rcktpwrd's Avatar
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    Default Re: FM transmitter antenna

    Excellent! I had that exact cable in my shopping cart! Looks like I have been on the right track with what I have been reading and figuring out.
    Not sure that I need anywhere near 50 feet of cable but it will give us plenty of options for placing the antenna...
    Joel


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    Super Moderator EricR's Avatar
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    Default Re: FM transmitter antenna

    Quote Originally Posted by rcktpwrd View Post
    Excellent! I had that exact cable in my shopping cart! Looks like I have been on the right track with what I have been reading and figuring out.
    Not sure that I need anywhere near 50 feet of cable but it will give us plenty of options for placing the antenna...
    Yep. I only need 25 but this way I can put this setup anywhere in the garage. Nothing worse than being a couple feet short. If you ever figure out how to test the signal strength let me know. I never could determine where you place the ground to test it.


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    Super Moderator EricR's Avatar
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    Default Re: FM transmitter antenna

    Quote Originally Posted by EricR View Post
    Yep. I only need 25 but this way I can put this setup anywhere in the garage. Nothing worse than being a couple feet short. If you ever figure out how to test the signal strength let me know. I never could determine where you place the ground to test it.


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    In case you care. I tested it and everything worked this weekend. I got about 1/3 mile of clean reception with default settings that it came with from eBay.
    Do people mount these somehow or just throw them in a box?


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    Stay-Puft! rcktpwrd's Avatar
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    Default Re: FM transmitter antenna

    Cool deal Eric! Glad to hear everything works properly!

    I'm still waiting on parts to show up so I can build the antenna. I have yet to plug in the transmitter...
    Joel


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    Default Re: FM transmitter antenna

    All of my parts have arrived!
    Now I need to find an open fm frequency and build the antenna.

    How critical is the length of the radials? Do they need to be exactly the calculated length of a specific frequency or will it work OK if it's close?
    Joel


  11. #26
    Havin fun ! kidcole's Avatar
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    Default Re: FM transmitter antenna

    Quote Originally Posted by rcktpwrd View Post
    All of my parts have arrived!
    Now I need to find an open fm frequency and build the antenna.

    How critical is the length of the radials? Do they need to be exactly the calculated length of a specific frequency or will it work OK if it's close?
    I don't know which antenna you got, and the Amazon link that Eric posted will not work for me. It actually looks like Amazon is down! Very strange. In general, the rods angling down should already be the correct size. The antenna will perform best if the angle of each of the rods is the same. If there is a top rod that is adjustable, then your manual should show you the exact size you need at the top (for the frequency you are broadcasting). Also make sure that if there is a little plastic piece on the top rod, remove it before you do your measurements. I didn't do that when I first got my antenna, and when it didn't go very far I finally figured out to check the measurement without that plastic protector. Turns out I was off by 1/8th inch .. It made a huge difference when I got the measurement right. Then I put the guard back on. Keeps you from sticking the thing in your eye !

    another very important thing about broadcasting distance, is to get the input impedance and input signal strength right. To do this, make sure you have a small mixer between your computer and your FM transmitter. Use the fixed output of the mixer for the FM transmitter's input. Then use the variable output of the mixer for your outdoor speakers. For most mixers, you will set the audio input channel to about 60 percent and never touch it. That should give you an amplE signal into your transmitter. Then use the mixer's master volume to set how loud the outdoor speakers are. When you have it all correct, you should be able to turn your master to zero (no sound outside) and still get your FM broadcast.

    The last thing you can do it play around with the channel setting which drives the transmitter input. Drive out to see how far you are broadcasting in 2 or 3 directions. Then come back and push the channel volume up to about 80 percent. Drive out again to see if you are going further, or if you are going shorter. If you lost distance, then you are over-driving the transmitter. This overdrive causes the broadcast to be shorter. Usually with a laptop driving the fm transmitter (without the mixer), this happens more easily. You will also hear a very "crusty" or fuzzy sound through the FM radio. This all gets cleaned up when you set everything correctly.

    With my setup in Cary (when I had my antenna on my roof) I could hit downtown Cary when everything was set right. That was close to 2 miles as the crow flies. But if I turned up the FM input, the sound was crusty and barely went past the shipping centers at Kildare Farms and Cary parkway. So I took the antenna off the roof, mounted it on a mic stand on my porch, and dropped the settings back down on the mixer. The result was a nice clean signal for about two tenths of a mile. But there was one angle (towards Dariusz' house) where I was still going about a half mile. So keep in mind that the broadcast distances are not the same for 360 degrees. My front porch made it more directional to the front, particularly since there was a hill behind my house.

    I think someone posted a really nice lityle Behringer mixer that sells for about $50 a while back.
    Thanks,

    Denny Cole
    https://www.facebook.com/groups/Cole...ristmasLights/

    Back to Work <unretired> so I went Static in 2017. Planning xLights when I retire <again>. Maybe 2019 ?

    Falcon - 3 F16V3 & 1 PiCap, Sandevices - 2 E681 & 4 E6804, 288 Channels Lynx Express, 108 Channels DC DMX,
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    Stay-Puft! rcktpwrd's Avatar
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    Default Re: FM transmitter antenna

    Thanks for the info Denny. No mixer this year, may add one next season, just going to run from the laptop into the transmitter.

    I am building a 1/4 wave antenna, using the link/guide that tripplet posted.
    Joel


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    Havin fun ! kidcole's Avatar
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    Default Re: FM transmitter antenna

    Quote Originally Posted by rcktpwrd View Post
    Thanks for the info Denny. No mixer this year, may add one next season, just going to run from the laptop into the transmitter.

    I am building a 1/4 wave antenna, using the link/guide that tripplet posted.
    Let us know how it works. If you get distortion, it's likely the impedance mismatch. Try the laptop set fairly low volume, like 40% at first. Then go out and see how far you are broadcasting. The other thing to check is to see if your FM volume from your car (on your station) is similar to other stations. Slightly lower is typical but not a huge difference. Then bring up your laptop volume a small amount and check distance and signal again. There will be a point where turning up your laptop volume will reduce the broadcast distance. But usually the distortion will happen first when using laptop direct to FM input. This is because the transmitter design is counting on a much higher impedance input (such as an audio signal) to yield the transmission power. The lower impedance of the laptop is designed to push headphones which are pretty much mini speakers. It's been a long time since I've worked on laptop design but I think they still use 50 ohms or less. Whereas typical line audio signals are 600 ohms. If I get some time and coming into Cary one day in December, it might be fun to bring my mixer to you, and experiment with your setup to see what difference we can determine.
    Last edited by kidcole; 10-13-2016 at 01:47 PM.
    Thanks,

    Denny Cole
    https://www.facebook.com/groups/Cole...ristmasLights/

    Back to Work <unretired> so I went Static in 2017. Planning xLights when I retire <again>. Maybe 2019 ?

    Falcon - 3 F16V3 & 1 PiCap, Sandevices - 2 E681 & 4 E6804, 288 Channels Lynx Express, 108 Channels DC DMX,
    10' Pixel MegaTree, CoroFlakes w/Pixel Modules, Pixel RBLs, 2 Pixel Matrix 16x25, 10" RGB Ornaments, 7x230 Pixel Icicle Matrix,
    Classic 20' AC Megatree, TIR Destiny RGB Spots, RGB Blowmolds, Wireframes, and Inflatables with External Light Control

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    Default Re: FM transmitter antenna

    For those reading this post discussion who may not have seen what kind of mixers you need, here's a nice deal on one of the popular mini mixers among the Christmas guys. This one allows the fixed line signal to the FM and still lets your master volume bring outside speakers up and down (without affecting the FM signal). So you fade down your outside speakers (or fade to zero) and still have a strong clear FM signal.

    Behringer XENYX 502 Mixer Works Great Needs Power Supply | eBay

    It's missing the power supply but check on the Behringer website for the specs. I bet it's a standard 9v or 12v wall wart. I have some extras if you don't have one. Check first if you decide to buy. Or there are a bunch of new ones for $39 .. Seems they are down by $10 this year.

    Brand New Behringer XENYX 502 3 Channel 5 Input Compact Audio Mixer | eBay

    If or when you do decide to buy one, get in touch with me and I'll help you get the right audio plugs for it. Most low cost mixers are 1/4" jacks. It's ideal to find mixers with RCA jacks but those are generally more expensive. It's really easy to find the adapter plugs you need for 1/4" on eBay ...

    For those reading this post, here's what I use. It has no tone controls but it's all RCA jacks in and out. Low noise mixer. Has all the features you need for FM broadcast along with outside speakers. These have long been discontinued so they sell pretty cheap now.

    Audio Technica Model Am 150 Stereo Mixer Tested Working Great | eBay
    Thanks,

    Denny Cole
    https://www.facebook.com/groups/Cole...ristmasLights/

    Back to Work <unretired> so I went Static in 2017. Planning xLights when I retire <again>. Maybe 2019 ?

    Falcon - 3 F16V3 & 1 PiCap, Sandevices - 2 E681 & 4 E6804, 288 Channels Lynx Express, 108 Channels DC DMX,
    10' Pixel MegaTree, CoroFlakes w/Pixel Modules, Pixel RBLs, 2 Pixel Matrix 16x25, 10" RGB Ornaments, 7x230 Pixel Icicle Matrix,
    Classic 20' AC Megatree, TIR Destiny RGB Spots, RGB Blowmolds, Wireframes, and Inflatables with External Light Control

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    Registered User Tripplett's Avatar
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    Default Re: FM transmitter antenna

    I use the Behringer 502 you mentioned and it can be powered from a USB adapter so 5v. I love it. It is popular enough that there is even built in features in FPP to have audio go from the Pi to the 502 through USB digital then out to your speakers.

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