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Thread: kind of off topic?

  1. #1

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    Hi all. I'm looking to build a yagi antenna for a very specific use. I have a wireless video sending unit that has PCB antennas and some small non shielded wires terminated on PCBs, which I believe to be antennas. I THINK I've isolated the exact antenna I need, and want to do the following

    1) add a bulkhead, like SMA or TNC
    2) add a yagi antenna
    3) build a yagi antenna

    I've come to the conclusion that I really need to make my own as those on ebay are garbage and not tuned for the 5ghz range. With that said, I have these questions

    1) If I add a bulkhead, should I use shielded wires, and does it matter?
    2) is there somewhere I can go on the internet, or if someone can tell me the formula or be REALLY nice and give me the dimensions to build my own?
    3) Ive heard that you don't want your elements to be connected to a conductor, so using PVC or wood down the center is better - Is this true?
    4) does someone have a better idea for what I'm trying to achieve?

    I'm using a Brite View BV 2322 wireless HDMI and it goes 150ft with the external antennas I've added, but I need more distance, like 200+ feet - I'll answer this now - Why? Because we are a video production company and we need to send a signal to a monitoring station BUT we don't have the budget to buy all the real stuff, you know, $15k per unit. I'm sure I've asked some questions I could find with the search button, but I just want someone who can dial in what I'm looking for to know what I'm asking without having to wonder. I don't know if I read this correctly or not, but I heard something about horizontal and vertical waves - does this mean that i need to use two antennas 90 degrees relative to each other? I use two antennas now and I keep them 90 degrees relative, and honestly, it seems to make a difference by 50 feet. I don't know If I'm self implementing coloration and causation but again, it's all I THINK at this point.

    Thank you in advance to anyone posting. ANY material you throw at me I'll read. I want to understand everything I can. Math is not my strongest point but I'll do my damnedest. I have mathematician friends so I can use them if needed.

    Jacob

  2. #2
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    I'd forget all you propose and just mount the PCB antenna at the focal point of an old satellite dish, pcb's working at that frequency are "fiddly" to put it mildly.

    Aligning the dishes/polarization at either end of the link may be a job too, but it should work once you have them within a few degrees...

  3. #3

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    understood, but I ask what type of dish we talking here? Massive 80's keeping up with the jonses pizza pie dish or directtv whiskey tango dish? HAHA. Also, the pcb antennas arent really satellite boards, they're on the daughter board that is 168 pin connected to the mainboard. While fashion is NOT my primary concern and mounting the whole thing in the center of the dish is doable. Ill post some pics of the unit to give a better idea. I'm experiencing a massive migraine right now but i'm trying to make the best of my day... staring at a white screen and feeling pain.

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    Quote Originally Posted by dinosquirrel View Post
    what type of dish
    I reckon a pair of 80cm ones will do, provided the kit at the focal point doesn't shadow the face of the dish too much (they don't look that big).

    Bigger dishes will give greater range at the expense of a narrower beamwidth (harder to set up).

  5. #5

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    entertaining the satellite idea, would I just be able to put on the ends I want on each side and and away I go? Do I need some sort of power supply for the dish etc?

    Would a grid antenna like this one be useable as well? http://www.ebay.com/itm/Laird-GD5W-2...item519ce1fb46
    I heard dish and thought of this too.

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    The reflector is all you need, the dish itself, the antenna feed (on the arm) is exchanged for your little box. A solid dish will have a little more gain too...

    The only power you'll need is for the box at the focal point.

    Finding the focal point on a mesh dish is not as easy as placing your ear around the focal point (and jangling keys, turning a ratchet spanner etc on the other side of your head) on a solid dish to find it.

    Mesh dishes are best in windy locations.

    The mesh' minimum dimension (wavelength) will allow higher frequencies to pass straight through the dish, so can be less noisy in crowded locations.

    That antenna you pointed out is just a regular dish with a wifi dongle at the focal point, we can do the same for your video link...

  7. #7

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    Ok. This all makes sense. I got a killer deal on two grid antennas so I figured I had plenty of time to test so that's what I'll be doing. My little devices, isn't so little, it's about 8x6x1 so I'll try using it in the center and solder on a shielded cable and try that and compare the differences. Now, strictly out of curiosity, why would a yagi be a bad idea? Make your post as long as possible. I admit that I'm the guy that will find out for himself in almost every situation, but I'm educated enough about antennas and signal propagation that a good explanation will satisfy my curiosity.

    Another thing I thought about, If I added an antenna through wires I would be changes the resistance value of the antenna that came with it. If I go higher I lose power yes? and If I go lower, I increase the power output? By increasing the output I will most likely need to place a heatsink on the main chip, yes? I currently do not know the output of the device. I can only speculate that it's around 1watt, or less. I believe that sending HDMI at 5.2-5.9 ghz takes a lot of power to reach it's destination (being not more than 100 feet) at a useable signal. I'm pretty sure I would be pissing off the FCC with my little project, but hey, who cares right?

  8. #8
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    A Yagi beam for 5GHz would be too small to make accurately, any wander in the elements would send the RF off axis. You could laser print one on a PCB, but a dish is just easier and cheaper.
    Also, feedlines for that frequency are very sensitive to loss.

    Changing the output power would need another module, changing component values to tweak the default output might gain you a few percent, at the risk of burning something.

    Not quite sure what you are proposing in the second paragraph to be honest...

  9. #9

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    Well I was following how we hook up speakers - as we decrease resistance we increase power output at the risk of heating the mosfets, I was wondering if the same occurred. If I decrease the resistance of the antenna, say it was above 50ohms, would I increase power output? Assuming I was actually decreasing the resistance.

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    50Ohms is measured at a particular frequency, not at DC, so, no, it's not quite that simple...

    A whole topic on Standing Wave Ratio begins... aieee!!!...

  11. #11

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    For the sake of moving forward "I understand." I'm getting two dishes tomorrow and will jingle my keys, that makes sense. Now, how will I know where to put my antenna relative to the center of the dish? Is there an equation? Also, with RF is it essentially one wire that carries the signal? I really appreciate your help.

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    When you jangle your keys, you will find the focal point has the loudest return. You can do this with a microphone to get a better idea of exactly where it is.
    Put the mic at the focal point and align the dish towards a noise source, traffic, birds, keys etc.
    Fiddle with the position of the mic (or move your ear) until you have the most gain, now swap the mic for the built-in antenna.

    There is an equation for "finding the focal point of a dish", it depends on quite a few things, you'd be better off looking up your manufacturers data for that info...

  13. #13

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    ok, so I understand the jingle technique and I get that it works for reflecting the sound and directing it, but how exactly does this work for my 4.9-5.9ghz range? I was under the impression that how far out the arm is on the satellite is dictated by the frequency. Again, long explanations will be read well. I am an audio tech, so I have quite the array of microphones and spectrum analyzing programs.

  14. #14
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    Only the shape of the dish affects the focal point, flatter dish, further focal point. Lots of TV dishes have an offset feedpoint (not in a logical position).
    Mesh dishes have an upper frequency limit dictated by the minimum size of the mesh (the holes let the smaller wavelengths through).

    It is quite amazing how much sound you can collect with a small dish, a high-gain amplifier and fairly decent microphone will detect anything from a whisper over hundreds of metres.

    Take some blu-tak and a lot of small (1") mirrors, tak the mirrors to the dish and it will work for light as well as sound and RF. Good laser/infra red preamplifier!

  15. #15

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    haha i was soo thinking that if this doesnt work the way I wanted to, I would coat the entire dish in high reflective silver paint. haha. Hm, ok. Well, let the testing begin. People in my complex are going to be looking at me very odd like. The guy who i bought the dish from raised a brow. I suppose I would too had I not the information I have.

  16. #16

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    Oh, I posted in the propagation section too, but no one has responded. Care to take a look at that question?

  17. #17

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    Is 5ghz microwave?

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    Yes, 5GHz is microwave, or SHF

  19. #19

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    I was thinking, and I think I may have talked myself into something inadvertently - Does the angle of the dish or grid antenna have anything to do with the frequency? I was thinking yes, but when I actually thought about it, I said no. Depending on the focus point the angle of the dish doesn't matter, it just changes the focus point. I could see how a very shallow parabola makes for better fine tuning, but from what I can gather it's the feed and location of, that makes the reflector enhance.

    I haven't done my tests yet, but I just need to get the feed, or whatever is emitting the frequency in the focus point for the reflector right?

    Can I make a good feed for 4.9-5.9 ghz? I've seem some very interesting stuff on youtube with people in classes, home brews etc, and wondered if I could help myself dial in what I'm trying to do. Is it safe to assume that where the original feed was, from when the satellite was in operation for directv, is where I need to place mine?

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    Quote Originally Posted by dinosquirrel View Post
    where the original feed was
    - yes, try it and see!

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