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Thread: More FM02 learning

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  1. #1
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    Default More FM02 learning

    First ("Let me make this perfectly clear" [RMN]) - thanks to ukewarrior for all the work in hosting the group buys for the FM02 and mini-test-antenna and Shure power regulators. Nothing in it for him but the pleasure in helping others.

    I've been struggling to get my FM transmission working as I wanted, and have been following the many threads about antenna design, antenna placement, power supply hum prevention, pin 19 220pF capacitor pilot frequency tuning, etc. My experiences are related, but different, so I'm starting this thread (sharing my learning and perhaps learning more from others) so as not to detract from the other ongoing discussions.

    Antenna design and placement: I'm using the simple separated coax dipole design many others are using. I'm not getting the distance or sound quality I expected (expected=2-to-4 blocks; actual=1/2 block). The antenna is so simple I don't know how it could be bad. There is no dead short between sides as measured by my DVM (~10MOhm). I know the speed of light in a vacuum (and even adjusted for speed of light in atmosphere...) and at 92.9 MHz half-wavelength is about 65.5". I have my dipole in PVC pipe suspended horizontally inside the back side of the house above a window. From others' comments ("I have it leaning against the wall in my basement", "I have it tacked to the rafters in my shed", "I taped the wire to my window"...) I didn't think I needed to go through the trouble of finding a way to get the antenna outside the house. Is that foolish? Does putting it on the outside of the house make that much of a difference? Is it better if it's closer to the natural ground plane (the earth), stuck on a pole in the yard? Do I really need to put it on the roof? I've also intentionally kept the lead coax length to a minimum, about 5'. I thought that would be the next most valuable thing I could do to get all the 50mW of the FM02 out to the antenna. I don't have a SWR meter, and although I see that RS has them for about $50, I'm getting nickel-and-dimed to the poorhouse with this project. If the SWR reading was bad, that would tell me what the problem is, but not how to fix it. How to I fix, or simplify two wires heading in opposite directions?

    Pilot frequency capacitor: From the schematic and board layout ukewarrior posted it is clear that pin 19 is not connected to anything. It is clear that the "near pad" for the unused SMD capacitor location near that pin is connected to some other trace, not clear what. However, the "far pad" for that location looks to be connected to ground. I'll be trying out adding the 220pF cap to between pin 19 (direct solder) and using the far pad as a ground point. I'll post results later. I am currently experiencing what I think others have called the "hiss" or "sssshhhhhh" symptom.

    Audio connection: Well, this is the real surprise to me. I have spent at least 50 hours battling my audio connection. I originally had a wireless connection planned (more below) but had to give in and use a wired connection. What I found was that whenever I plugged the FM02 into my PC headphone jack (front panel) my PC sound system would go crazy. Looking at the PC audio control panel, I'd see that it was disabling/removing every audio recording source and then restoring it about every second. Not only did that render the audio system useless, but you can guess the impact on the rest of the machine. I tried lots of testing and found that it was specific to plugging the FM02 into that jack - nothing else caused that problem. I reloade drivers, finally forced myself to go through the dreaded OS upgrade from the much-maligned Windows Vista to Windows7(64), tried different jacks, connectors, cables... nothing mattered. The FM02 caused the problem no matter what. Finally, after reading one of the other threads (tried to find it again for reference but can't quite get the one post) I got an idea. Frankly, I thought all the concern about "RF traveling upstream" was a problem of small likelihood or impact. Nonetheless, when all else failed, I tried what someone else had recommended (what an amazing concept) and put a snape choke on the audio input line just before entering the FM02 enclosure. It's just a big magnet, and the audio line is shielded, and how could any RF (or other) energy get there anyway? Bottom line: the sound system disruptions stopped. I couldn't believe it, so took the choke off, and the problems returned. I tried reducing the number of turns of wire in the choke: one is not enough, two is marginal, three resulted in one lock-up, four have been running for about 5 hours with no problem. So that leads me back to my antenna design and placement. The dipole is so simple, I don't know how it could be bad. I understand that there is buffering/filtering missing from the FM02 design that might prevent this (but it doesn't have it) but why this issue? My sound system might be more sensitive than others, I guess. My antenna is about 3 feet from the FM02, and the audio cable is 3 feel long. Should I take the additional loss of a longer feed coax length and get the antenna away from the FM02?

    Using bluetooth with synchronized audio sources: Finally, my "great idea" that I had to abandon was a design for untethered (wireless) connection to the FM02 FM transmitter. I don't want to have any cable from my show computer (in my home office) to any other room in the house, or the attic, or worst of all, out an open window to the outside. I found a Belkin Bluetooth audio receiver (meant for iPod/auto use), and a Bluetooth Class-1 USB dongle transmitter. The plan was to have the show computer send the audio out the USB Bluetooth dongle, to the Belkin Bluetooth audio receiver, and the output of that (audio) driving the audio input of the FM02. I know Bluetooth is limited range, somewhat line-of-sight, but thought it would work through one wall or 10' or so. I thought it was an elegant design to get the audio signal to the attic or perhaps just outside the window. As it turns out, the problem wasn't with distance. I have over 100 hours of effort fussing with settings, drivers, configuration, etc, and finally found a way to direct audio to the Bluetooth USB dongle AND to an internal line that my show software (home developed, not Vixen) could also listen to. After all that, I found that there is an apparent latency in the Bluetooth stack so that my video/light control was about 0.5. second ahead of the sound through the Bluetooth channel. That was very distracting and would not be acceptable. I imagine there is some audio stream buffering happening but can't find a way to reduce it. So for ErnieHorning (or Idunbenhad, or dirknerkle, I can't precisely remember) that used a similar setup to get Christmas music out to the yard during setup, it would work fine. However, with the sync issue, it's no good.

    Finally ("That's all I have to say about that." [Forrest Gump]) that's what I have to share. If anyone takes the time to read through this all ("I would have written a shorter, but didn't have the time." [Blaise Pascal]) I hope there's value here for you. If anyone can educate me on the suspected, and dreaded "RF energy traveling upstream on the audio input" issue I'd be grateful. Do I need to get the antenna further from the FM02? Do I really need a SWR meter? Also, do I really need to string coax out the window and put my dipole in the yard to get signal out for a few blocks?

    Thanks.
    Last edited by ags0000; 12-04-2011 at 01:39 PM.

  2. #2
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    Default Re: More FM02 learning

    Great post!

    As lessons are posted, I try to load them into the FM02 wiki.

    Can you please post what 'choke' you bought for your audio solution? A picture?

  3. #3
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    Default Re: More FM02 learning

    RF traveling into the audio lines is caused by one of two things:

    1. High VSWR on the coax sending return RF on both conductors, result is RF on the ground paths. Since the audio ground on the FM02 is the same ground as the RF output connector...there is an easy path for that RF to inductively pass onto audio + lines from their own shield, in turn feeding right back into the audio input of the TX and the cycle repeats.

    2. Antenna indoors too close to equipment, house AC wiring, other wires. Some homes have a mesh of chicken fence wire in the walls, and will absorb and reflect RF energy.

    The chokes I mentioned to use from Radio Shack are typical ferrite cores inside plastic casings with a snap lock. Simply wrap a couple turns or so of your audio or power wire through it, close the choke till it snap locks and bada bing...instant choke inductor to choke off that RF.

    Although this resolves RF getting into the audio + wires and everything, it does not solve the cause of the problem. There is still something causing that RF to get into wires we do not want. The only place that RF should be at is at the antenna.

    Not to sound like a broken record or parrot, but here is where a VSWR meter comes in handy!!!!

    Happy Holidays!!

    RFB

  4. #4
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    Default Re: More FM02 learning

    I can add only a small bit. I just measured my setup, i have my fmo2 laying on the desk as it came, 2 pieces of circuit board, raw, i have the rpsma connector (6 inches) attached to 18 inches of straight coax, which is then looped twice in a 2" circle, then a 3" lead to the antenna. Total feed coax is about 30 inches, but the circuit board is sitting exactly 16 inches from the split in the coax (dipole start). After i moved the router and enet cable, I have no more static or hum and am back on the ac/dc converter , boat battery back outside, I get about 900 feet (need 500 just to road). I had more but some kid a few miles away got a powerful transmitter 10 days ago and is looping the same 6 songs for miles around.


    Yes i found ferrites and attached them to incoming signal and power, I do not think this changed anything.


    DSCN2936.jpg

  5. #5
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    Default Re: More FM02 learning

    Here is a simple low pass filter schematic that will help reduce 2nd and 3rd harmonic down by about 40db.



    L1 and L2 are simple 4 turn close wound air coils 1/4 inch diameter. A 1/4 drill bit works great for a winding form.

    L3 is a 520 uH molded choke. This is to shunt any electrostatic charges collected by the dipole antennas to ground, protecting the transmitter.

    C1 and C3 are 47 picofarad capacitors. Ceramics will work fine.

    C2 is a 75 picofarad capacitor..again ceramic will work fine.

    Build the filter in a metal enclosure.

    This filter will give great performance as it is. It can be peaked by spreading the coils apart a little to maximize the attenuation of the harmonics while peaking the strength of the intended frequency we want to transmit. The filter will cover the entire 20 Mhz FM band.

    Happy Holidays!!

    RFB
    Last edited by RFBurns; 12-04-2011 at 03:56 PM.

  6. #6
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    Default Re: More FM02 learning

    Thanks for the replies. I'm now thinking that there may be something wrong with the FM02. I was considering that the short lead (5') from FM02 to split in the coax to form my dipole antenna was the problem - but after seeing the picture in post #4 and reading how short that lead is, that explanation seems unlikely. I'm afraid I may have a defective unit. I just bit the bullet and tried the antenna up in the attic (just 5/8" or 3/4" sheathing (not sure of code here) and asphalt shingles (one roof only)) between the antenna and the great outdoors. I tried horizontal and vertical orientation. Walked in the front yard with a small iPod-like unit with FM radio, and heard static less than 100' away from the roofline. I then took the antenna out of the attic and propped it up (shield side down) in a vertical orientation, and also tried setting it horizontally in a small tree (3' above the ground). I was 75' away, with my eyes on the antenna, and found static. I'm not talking about the overdriving, hissing, "ssshhhh" sounds, it's static, which varies based on my position in the yard. It's what I'd expect if I were on the fringe of reception area - if I'm in just the right spot (which is close to the antenna) the static is not present (or discernable over the audio content).

    Any other ideas (other than getting an SWR meter)? As noted before, even if I had an SWR meter and it read 2:1 or more, I'd have something to follow, but I just don't see how I could have made a mistake with the antenna - unless my coax is bad. It is Carol RG59/U, manufactured in the USA, purchased at Home Depot a month ago. I measured >10MOhm resistance between shield and inner conductor on the feed to the antenna.

    Anyone in the Bay Area with a known-good FM02 installation willing to try my FM02 in that system and see if it works (or try my antenna and see if it doesn't work)

    Oh, and I visually checked the SMA-BNC connector and it looks OK (not sure what I'd see otherwise, but I read some had defective ones) as well as the SMA connector on the board, and the solder joints on the board to the SMA connector.

    Did I do some bonehead mathematics when I arrived at 65.5" for the half-length dipole length? Maybe I should trim it a bit to get closer to 0.47 lambda to achieve the magical infinite impedance mentioned in one of the links provided by RFBurns?

    Anything at this point, I'm out of ideas, and hate being stuck like this. Thanks.
    Last edited by ags0000; 12-04-2011 at 07:17 PM.

  7. #7
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    Default Re: More FM02 learning

    Quote Originally Posted by ags0000 View Post
    Did I do some bonehead mathematics when I arrived at 65.5" for the half-length dipole length? Maybe I should trim it a bit to get closer to 0.47 lambda to achieve the magical infinite impedance mentioned in one of the links provided by RFBurns?

    Anything at this point, I'm out of ideas, and hate being stuck like this. Thanks.
    Obviously you got more problems than an antenna issue. At 75 feet, even with a 6 inch piece of wire sticking out of the center conductor of the SMA connector at the TX, you should get that signal.

    Now as to what caused this incredible reduction in range is anyone's guess..including me..since there is no way at the moment to check the output of that TX and the antenna system.

    Don't worry I wont say the terrible 4 letters and word. And it's not a curse word either!!

    We do not need nTh decimal place precision here to attain a signal @ 75 feet, not even 200 feet or 2 blocks. But just to toss my 2 centavos out here after spending some time reading all the complaints about loss of range and hum noises, and all the steps taken by many to resolve these problems, the problems seem more severe than most are considering, or there is something common to all these setups that are resulting in the same issues spanning hundreds of miles apart from one system to another.

    Now you gotta ask yourself why would so many living in different parts of the nation, and world, all have such common problems repeating again and again?

    The history record is not a good one for the FM02 in these applications (light shows). Perhaps with all the information available to date, something is bound to break this common ice burg everyone is running into.

    Happy Holidays!!

    RFB

  8. #8
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    Default Re: More FM02 learning

    For 92.9 I get 30 1/8" for each half. I'm not sure thats enough to put your SWR high.

  9. #9
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    Default Re: More FM02 learning

    I'm back to basics. I just measured the real resistance (impedance at DC) between shield & inner conductor to be >10MOhm. I measured the continuity of the SMA-BNC connector - good. I disassembled the antenna from the PVC conduit and measured <1Ohm resistance between each half and the respective connection to the BNC connector on the coax. I measured the voltage in to the FM02 and it's 11.78 vDC under load. (I have the Shure regulator all the way up, I guess I can use a stronger wall wart as a supply once I get this figured out. I think my final transistor is fried. RFBurns (or anyone else), is it replaceable on the FM02?

    I have no music for my show...

  10. #10
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    Default Re: More FM02 learning

    You are all way ahead of me...but here's my humble input.

    After reading up on the related posts, I mounted my FM-02 in an enclosure with the Sure regulator and two 2200uF caps for filtering. I plumbed in a jack for the audio and added a clamp-on ferrite bead for good measure, one loop.
    I built a dipole antenna from RG-59 I got from Ace Hardware. Used the calcs from Idunbenhad's thread. Found the nylon liner had a foil wrap under the braid that was tedious to remove, but got it off.
    Bought an SMA to F male adapter at RS to connect the antenna with the factory connector.. Have about 6' of cable before the split. Used two thumb tacks to mount the leads vertical in my garage about 3' off the concrete floor on the sheetrock. Wood frame house, wood siding. Metal folding garage door.
    Powered up the regulator to adjust the voltage, powered down and connected power to the FM-02 and connected the antenna and my computer line out in the back.
    I get about 1/8 mile indirect and more line of sight, crystal clear.

    Sounds like our systems are similar...

    I hope this helps in some way. Sorry to hear yours is giving you issues.
    Alex

    X3 Ren48LSD, X4 Ren16SS, X2 Ren24SS, X1 SRen32, FM-02
    X36 Super Strips, X9 Dumb RGB Mini Trees, ~25K lights (mostly LED)
    ~250 channels programmed with Vixen+ and run by Vixen 2.1

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