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Building a VHF receiver 108-136 Mhz

  1. Jan 21, 2008 #1
    Hi all,

    I have a plan of building myself a VHF receiver that can pick up the air band (108-136 Mhz). I want to connect it than with an antenna that I plan to set it up on my rooftop.

    Can anybody give me some working schemes and tips how to build such thing?

    Thanks in advance.
     
  2. jcsd
  3. Jan 21, 2008 #2
    Last edited: Jan 21, 2008
  4. Jan 21, 2008 #3

    berkeman

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    The ARRL Handbook has receiver circuits that you can build for your application:

    http://www.arrl.org/catalog/index.php3?category=Technical%2C+Electronics%2C+and+Communications+Reference [Broken]

    Also check out the QEX collection near the bottom of that page -- it might be of interest to you.

    You might look up the local HAM radio club in your area, and stop by one of their meetings to ask about your project. They should be able to offer you some helpful advice.
     
    Last edited by a moderator: May 3, 2017
  5. Jan 23, 2008 #4
    I appreaciate the comments. They helped me a lot. Now, a friend of mine adviced me to get a old VHS VCR, and he will adjust it and modify it so it will be able to work as a receiver of that band. Is this possible? He sounded like he knows what he is doing, but I am a bit sceptical.

    Any advices?

    Thanks in advance.
     
  6. Jan 23, 2008 #5

    berkeman

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    I'm skeptical as well, but maybe he knows some trick about the VCR pickup amp? I doubt it works in this frequency range, though.

    I'd check out the local HAM resources, and look for an inexpensive kit to build. Also, ask about whether there are any local HAM flea markets -- you can pick up some amazing bargains there, as well as ask all sorts of people for their ideas.

    BTW, be careful not to broadcast on those aircraft bands -- you generally need a special license to broadcast (pilots get those special radio licenses).
     
  7. Jan 23, 2008 #6
    No, my idea is only to pick up the freq and not broadcast anything on those freq. I talked with another guy who said that it might be possible, so tomorrow I will search for a used VCR that we can work on. My local area doesn't have HAM flea markets at all, so it is very difficult to get my hands on things like this.

    Anyhow, about the VCR, he said that with turning off the video signal, and leaving the sound (the band is VHF, Channel 3 or Channel 4 which converts to around 112 Mhz and more) so this way this band would be in the same range. Well, if anything, I will let you know if this works.

    Thank you.
     
  8. Jan 23, 2008 #7
    If the VCR has a cable tuner then you can modify it for a receiver from 50 to 550 MHz. For that you would have to construct a voltage variable power supply from 0 to 30 volts to tune the tuner.
     
  9. Jan 23, 2008 #8

    NoTime

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    The FM radio band and the Aircraft band fall between Standard TV channels 6 and 7.
    I know you can modify a FM reciever and the VCR idea seems viable.
    The Aircraft channel spacing is small 25Khz, so if you are in an area with a lot of air traffic the IF circuit bandwidth may be too wide.
     
  10. Jan 25, 2008 #9

    sas3

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    Last edited by a moderator: Apr 23, 2017
  11. Mar 24, 2008 #10
    I'm trying to receive the same frequency's, I have an old Sony Air 7 Tuner/Receiver, with a 1' or so Helical antenna(RX only I assume), problem I'm having is hearing both sides of the conversation, I've moved the antenna outside onto my deck, with some BNC to RJ45 connectors and a bunch of cat 5 cable, so its at about 10' off the ground, in a fairly open area. My reception is much better, very clear, but still no ground level traffic on the air band, and minimal range on the P.S. Band, about 5 to 10km at most.

    I'm going to mount it on the roof, and see what, if any gain I get, but that is at max 10 more feet, and I know the terrain in the area is probably not optimal, so that is a factor I am looking to overcome as well.

    Does anyone have a design for a simple antenna that I could make around 6 to 8 feet high or less, that would extend my listening range??(something smaller, don't want to piss off the neighbors) Maybe something directional for the Air Band, I know the exact direction from my area, its roughly 30km away.

    Or is this something that is just a matter of height and distance, I'll string the antenna to a tree if it makes a difference :)
     
  12. Mar 24, 2008 #11

    berkeman

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    Can you try a simple TV antenna? Do you have one on your rooftop already? It might have enough directional gain and is close to the frequencies you mention. You might need to rotate it vertical, though -- what polarization is used for aircraft communications?

    If that doesn't work well enough, you can make yourself a multi-element Yagi antenna to boost your gain (but you will need to narrow your bandwidth requirement). You can pick up good home-brew antenna books at your local HAM radio store. The HAM Radio Outlet has stores in many areas.
     
  13. Mar 24, 2008 #12
    when you say a TV antenna, you mean a single mast, say like off the back of a late 80's model tv, or stereo, straight, extend-able, kinda like a professor's metal pointer, or are you talking something like rabbit ears or something else?

    as for polarization, I'm not really sure what that is :) If you explain it, maybe I can answer.
     
  14. Mar 24, 2008 #13

    berkeman

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    Yeah, like this one on a chimney:

    http://www.granbytv.com/3671__Antenna.jpg

    That is the classic multi-element TV antenna, covering the VHF and UHF bands (the longer elements are for the VHF band). The spacings are meant to give the antenna good gain in the front-to-back direction. Since the elements of the antenna are horizontal in the picture, that antenna is meant to receive horizontally-polarized signals (the electric field of the plane electromagnetic wave is horizontally polarized).

    You can tell the polarization of the transmitting antennas that you are trying to receive by the orientation of the antenna. If it is like a vertical stick on the plane, then it is transmitting a vertically polarized main signal. Reflected signals can have pretty much any polarization, depending on what they reflect off of.
     
  15. Mar 24, 2008 #14
    Also remember that VHF signals are pretty much line of sight... it may be that you are just too far away from the control tower to receive the signals at all.

    The height of your receiving antenna can also have a profound effect on the amount of signal you receive, higher is usually better... (but not always so).
     
  16. Mar 24, 2008 #15
    well, I don't have one of those great huge things, I thought you meant the small antenna's that would come out of the back of the TV like this :

    http://www.thesourcecc.com/images/Online/27/2701402l.jpg

    Still waiting on an 8' ladder to get up on the roof, to mount my original helical antenna that came with the radio, I'm hoping that helps. Link to the radio :

    http://www.rigpix.com/sony/sony_air7_manual.pdf

    Is it helpful/possible to have 2 or 3 small antennas all fed in thru Coax splitters, to one tuner?

    Also, I ran some new cables today, all 75ohm Coax, the piece I have is about 50' too long, but now with the antenna placed in a temporary location, well below the horizon, and the excess cable just laying on the lawn, I seem to be getting much better reception, and I think I'm getting some of the tower chatter, does that make any sense???

    and thanks for the response's, and ideas, if you have a computer/network question, I'm your guy :)
     
    Last edited by a moderator: Apr 23, 2017
  17. Mar 24, 2008 #16

    berkeman

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    Are you using matching networks/baluns between your antenna and the coax? The manual is not very clear whether the antenna input jack on your radio is 50 or 75 Ohms -- do you know? You should try to use matching networks wherever there is a transition (like form 75 Ohm cable into a 50 Ohm input jack).

    The antenna shown in the manual for Aircraft is vertically polarized (the one on the left), which makes sense. What frequency are you picking up the tower on? 150-ish MHz or so?

    The best and simplest directional antenna that you can make is a 3-element Yagi, which is basically a simplified version of the multi-element antenna shown in the chimney picture that I linked to. It looks like this:

    http://www.mikesfalconry.com/images/products/R-400%20receiver%20w%20yagi%20open%20lg.jpg [Broken]

    The actual antenna is the middle pair (like two sides of the simple TV antenna you showed, with the telescoping length of the two sides set based on the frequency you want to tune to. The spacings to the front "director" pair and the back "reflector" pair, and the lengths of the director and reflector elements can be calculated using this calculator:

    http://www.csgnetwork.com/antennae3ycalc.html

    You would build the 3-element Yagi to the specs from the calculator, and then mount it up high in the vertical orientation like is shown in the hand-held version picture. Aim it at the airport, and you should get a pretty good signal.
     
    Last edited by a moderator: May 3, 2017
  18. Mar 24, 2008 #17
    Ok, lets see, tower is 118.4mhz, plus Im trying to get ATC for the area on 119.2mhz, I'm pretty sure the jack on the radio is 75, but not certain, when I switched over to all 75ohm coax i did see a decent improvement in quality of sound, and it seem to pick up more traffic.

    Also, while waiting earlier, I went ahead and built this antenna(2nd one on page, link below), without the fancy coil, and actually I'm picking up alot more then I was with the factory antenna, I can't wait to try it on my roof. Its very simple, but it seems to be working so far, have a look.

    http://users.belgacom.net/hamradio/schemas/on6mu_vhfrubberduck.htm

    I basically added a cheap old stereo antenna like in the pick in my last post, from an old boom box I had kicking around, did a bit of soldering, and bango, now I have tower talk, and its a CHICK LOL :) Would the coil he uses help me at all ??

    I've been playing with the angle and direction, and so far it seems to be working the best when the antenna is straight up and down, as high as I can get it, for now... I asked this before, does the length of the coax feed line from the hand-set to the antenna make any difference??
     
  19. Mar 24, 2008 #18
    Would it make a difference putting the antenna on the chimney like in that picture?? Or would the furnace create interference ?? I live in a cold climate, furnace runs a lot :)
     
  20. Mar 25, 2008 #19

    berkeman

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    So many questions -- you need to link up with a local HAM radio operator -- that's the easiest and quickest way for you to get a gangbuster antenna setup going. Do you know of any HAM radio operators in your area? See any fancy antennas sticking up in your area?

    The "rubber duck" antenna in the picture is not meant to be remoted off of the handheld transceiver (HT). It needs some ground reference plane, which is supplied (sort of) by the HT itself. If you want to use it as a "monopole" antenna, you should mount it to a piece of metal to serve as the ground plane. The "mag mount" antennas that are stuck onto car roofs use the car as the ground plane, for example. And yes, the coil does serve a purpose -- it is called a "loading coil", and makes the antenna resonant at a lower frequency than you would get from just its physical length, thus allowing a shorter (but less efficient) antenna versus the correct dimension of a quarter wavelength. It is better for you to use a quarter wave (0.25 * 1/118.4MHz * 300m/MHz = 0.63m) element if you are making your own monopole, or use two quarter wave elements to make a dipole.

    The input impedance of a quarter wave monopole antenna is about half of 75 Ohms real and unbalanced, so you should use a matching network to connect to your 75 Ohm coax. The input impedance of a half wave dipole antenna is about 75 Ohms real and balanced, so you need to use a "balun" to do the unbalanced (coax) to balanced (dipole) conversion.

    The monopole and dipole antennas are omnidirectional for vertical polarization, so you are not getting any gain. If you want to use gain to help you pull in the signal from a particular direction, the 3-element Yagi is still your best bet.

    Yes, longer coax feeding from the antenna will have more loss, so only use as much as you need to reach the high antenna. If you don't hear audible noise from the furnace, then you are not having any interference problems with your radio reception.

    If there are no HAMs in your area that you can talk with (they generally are a very friendly bunch), then consider getting a book or two on antennas to learn from. The ARRL has several very good books:

    http://www.arrl.org/catalog/index.php3?category=Antennas%2C+Transmission+Lines+%26+Propagation [Broken]

    These books are written for both technical and non-technical readers, and I think you'd get a lot out of them.
     
    Last edited by a moderator: May 3, 2017
  21. Mar 25, 2008 #20
    books....man, what century do you live in :P I was actually thinking of going to the library tomorrow and getting a couple books they have, on antenna's and radios. So that should help a whole bunch.

    And I know I ask alot of questions, I really really appreciate your help berkeman, you are the light at the end of a long dark tunnel :)

    I don't know of any HAMs in the area, plus I'm not what you would call a social person, but I'm gonna look for some other sites and people to ask, hopefully some locals at that.

    As for my "rubber duck" antenna, its the weirdist thing, I had it set up down low, mounted on my deck, and it seemed to work better then it does way up on the tip of my chimney, so now I'm all messed up. I soldered the coax "stinger" center to the antenna, it was screwed down before with the screw in the antenna, and seemed to work a whole lot better, so I'm gonna go back to the screw deal.

    My only question for this round ..... you said it needs a Ground Plane.... so would that be what you connect the outer braid of the coax to ?? Cause as of right now, the braid isn't connected to anything, and I would have thought it should. Part B, does the ground plane need to be grounded??

    Ok, sorry, 2 in one :) Thats all for me, THANKS AGAIN BERKEMAN, does the link you gave me have forums?? Looking now...
     
    Last edited: Mar 25, 2008
  22. Mar 25, 2008 #21
    you gotta pay to join, more then it costs to get a ham license, what gives :)
     
    Last edited: Mar 25, 2008
  23. Mar 25, 2008 #22

    berkeman

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    Not sure what that means. Is it saying you have to be a member of the ARRL to buy their books? That's weird (but I'm a member, so I wouldn't see those messages). The books are available at many bookstores as well, or just off of Amazon.com. The nice thing about the ARRL antenna books, is that they are written with experimenters and people who want to build their own antennas in mind (like you). If you just get a straight antenna theory book from the technical library, it will be much less construction-oriented.
     
  24. Mar 25, 2008 #23

    berkeman

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    Yes, your coax and your ground plane need to be connected correctly. If you were using 50 Ohm coax (which is closer to the Zin of the monopole antenna's 75/2 Ohms), then you could just connect the braid of the coax to the ground plane plate, and feed the center conductor through the plate to the feed of the quarter-wave element. The ground plane should be about as wide as the monopole element is tall, and does not need to be actually grounded to anything. When I'm using my magmount antenna somewhere other than my Suburban, I just clonk it onto any old piece of metal that is handy.

    Since you are using 75 Ohm coax, and probably don't have a balun or matching transformer handy, the best omnidirectional antenna you can make for yourself is the one that is shown in your radio manual, the one on the left that is labeled for aircraft band use. It is made with a quarter wave element vertical out the top, and a number of radial elements sticking out at an angle down, acting sort of like a ground plane.

    Like the antenna on the right in this picture:

    http://en.wikipedia.org/wiki/Image:136_to_174_MHz_US_base_station_antennas.jpg [Broken]

    The input impedance of this unbalanced antenna (which means it can be fed directly by coax without a balun needed) varies with the angle of the bottom elements. If you had an SWR meter, you could bend the bottom radial elements down from vertical until you had the best SWR closest to 1:1, which would mean that your coax was matched well to the antenna input impedance. Except, you'd have to transmit to do that, and you don't have a license to do that. Scratch the SWR meter part. Anyway, just stick some radials on your next antenna, soldered to the coax braid, and point them down about like in that picture. The center of the coax feeds the top quarter wave element. Stick that antenna up as high as you can, and that's about the best omnidirectional antenna you will get for a particular frequency in the band you are experimenting in. Post a picture of your next antenna!


    BTW, the wikipedia.org entry on antennas is a pretty good overall introduction, no books required! :-)

    http://en.wikipedia.org/wiki/Antenna_(radio)
     
    Last edited by a moderator: May 3, 2017
  25. Mar 25, 2008 #24
    I have a Balun I think, its a coax to 2 prong screw on cable feed thing for old TVs, the ones where you screw this adaptor to the back of the tv, 2 screws, it doesn't say anything on it except the name... its a Magnavox CATV systems T-26.

    So if this T-26 is the Balun, how would I hook it up, one end to the actual antenna, and one end to my ground??

    Also, I have my Coax grounded thru an F-86 male to male coax splitter, attached to a 2.5' ground post, does that do the same as grounding anywhere else?? Or would I need to install a seperate ground for the balun?
     
  26. Mar 25, 2008 #25

    berkeman

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    That sounds like a TV 300 Ohm balanced to 75 Ohm unbalanced (coax) converter, which won't be of use in this project. There's a picture of one of these part-way down this wikipedia page:

    http://en.wikipedia.org/wiki/Balun

    If you were using a half-wave dipole antenna, you would use a 75 Ohm to 75 Ohm balun to convert from the balanced antenna to the unbalanced coax (or 75-to-50 Ohm, if the coax were 50 Ohm coax).

    That wikipedia page does talk some about how to wind your own balun, and you can find similar helpful pages on the net (many of the HAM sites), but unless you have a nice selection of RF ferrite cores in your parts bins, it will be a bit hard for you to make your own balun. The simplest route for you in the short term is probably to stick with unbalanced antennas with around a 75 Ohm input impedance, so that you can connect your coax directly to them.
     
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