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LNA for my TV antenna

Discussion in 'Electronic Design' started by Mac, Jan 17, 2004.

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  1. Mac

    Mac Guest

    I live near several strong HDTV broadcasters, but for some reason, in one
    room in my house (the one with the TV, unfortunately) I get very poor
    reception. This is true for regular TV, too. The transmitters for regular
    TV are in the same place as the HDTV ones.

    I made a simple folded dipole antenna, and with it I get two HDTV
    stations. With store bought antennas, I get nothing, so I guess the folded
    dipole is reasonably effective. For the moment, all the stations are in
    the UHF, so having a relatively narrow band antenna is OK, I think.

    It seems like the easiest next solution is to try a low noise amplifier
    between the antenna and the TV. So my question is this: Is there a simple
    and possibly worthwhile way to make a LNA which can span the television
    broadcast range, from 50-800 MHz. Should I just by a cheapo amp from Radio
    Shack and be done with it? I have pretty much ruled out normal op-amps,
    because the fast ones aren't all that low noise, and they don't cover all
    the way up to 800 MHz. (If I'm wrong, let me know, by the way.)

    But it seems that a single transistor set up as a common-emitter amp might
    do the job if it is biased right. How hard would such a circuit be to get
    right without using signal generators and network or spectrum analyzers? I
    have access to those at work, but I'm pretty busy, and don't really have
    time to fool around with extracurricular activities.

    If this is workable, then maybe someone could recommend a specific
    transistor, preferably one stocked by digikey, since I will be ordering
    some other stuff from them soon anyway.

    best regards,
    Mac
    --
     
  2. Robert Baer

    Robert Baer Guest

    Why not put an antenna in a decent place, use a 300 ohm to 75 ohm
    transformer, a low loss version of RG59 to TV (which might accept the 75
    ohm connector directly).
    That is what i did; had about 70 feet of cable; worked like a champ,
    even with weak stations over 100 miles away.
     
  3. Mac

    Mac Guest

    That is on my list of backup plans, but I'm trying to avoid running cable
    between rooms in my house, since I don't have a good way to hide the
    cable, and I don't want to run it on the baseboards. It's nice to know I
    can get away with the long cable run if I have to, though. ;-)

    regards,
    Mac
     
  4. <>) about 'LNA for my TV antenna',
    Almost certainly, but maybe from Newark, for example rather than the
    Shack. These things use hybrid amplifier devices specially for the job.
    While you can buy the devices, the PC layout is critical and you won't
    be able to make a unit for less that double the price of buying one.
     
  5. SioL

    SioL Guest

    <snip>

    You've got the money for HDTV, but are saving a few bucks on a good antenna?

    Get a nice wideband log-periodic antenna and put it on the roof, as high as possible.
    You won't need a LNA anymore.

    You can get a nice wideband log-periodic to cover entire UHF band. But you may
    need a few antennas extra for the lower bands. Perhaps you just need UHF?

    You can't improve your S/N ratio when your signal is below ambient noise. No amplifier
    can change that. A good antenna can.

    Siol
     
  6. Fred Bloggs

    Fred Bloggs Guest

    The only appropriate use for an LNA in this application would be to
    overcome cable feed loss from a remote antenna to the receiver-locate
    the LNA at the antenna. Your reception is bad because the input signal
    power is swamped by the internal noise on the receiver front
    end-assuming the xmit distance is not so great that atmospheric noise
    has swamped it-and you say this is not the case. Any LNA fed by the same
    antenna will also similarly swamp the antenna signal with its noise- so
    you end up piping larger S+N with S/N even worse, because you have added
    LNA noise which should be comparable to TV front end noise, into your
    receiver. You may get the signal power above the thresholds for locking
    in a synchronous frame but the picture noise will still be bad.
     
  7. John Larkin

    John Larkin Guest


    Maybe your problem is too much signal, not too little.

    John
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Close but no cigar- most likely effect of too near would be that ground
    wave overshoots his house before it has developed- no field strength to
    be received. Too much signal would be characterized by reception of
    satisfactory signal with no antenna at all.
     
  9. John Larkin

    John Larkin Guest

    Interesting concept: an antenna that transmits a powerful far-field em
    wave with zero near-field intensity. You should post that idea to one
    of the free-energy newsgroups.

    I guess that explains why Paul's field strength meters don't work:
    they're too close to the antenna to pick up any signal.

    Really, you're so eager to contradict me that you do it without
    thinking first. That's not effective.

    John
     
  10. I read in sci.electronics.design that John Larkin <[email protected]
    techTHISnologyPLEASE.com> wrote (in <[email protected]
    4ax.com>) about 'LNA for my TV antenna', on Sat, 17 Jan 2004:
    It's called 'umbrella effect', and is significant for antennas mounted
    at high elevation on masts. The field strength at ground level close to
    the mast is not very high because those sites are 85 or more degrees
    off-axis of the antenna. The BBC TV antenna (1 MW) at Crystal Palace in
    south-east London provides no more than a 'strong signal' (less than 50
    mV/m IIRC) at the surrounding houses, not 100 V/m.

    I hate to think what would happen to the buses at the terminus in
    Crystal Palace Parade if the field strength were 100 V/m.
     
  11. John Larkin

    John Larkin Guest

    But I doubt that 50 mV/m will require an extra LNA to get decent
    reception.
    My old Fiesta (known in Ann Arbor as the Ford Fiasco) had a cheap
    after-market radio and ratty speaker wiring. When I drove over Twin
    Peaks on my way to work, near the base of Sutro Tower - maybe 25 MW
    total - my speakers would squwak and buzz, with the radio turned off.
    The maximum seemed to be at roughly 45 degrees blast angle, but the
    terrain is messy there so complicates things.

    http://www.jimprice.com/sutro/

    People who live near the base of the tower do have problems with the
    huge field strength, and not just with their TVs. Lots of telephones
    and other gadgets don't work up there.

    John
     
  12. Tim Shoppa

    Tim Shoppa Guest

    I disagree. What you probably want is a Yagi antenna pointed at the
    stations of interest. As you point out, UHF narrow-band antennas are
    small and easy, so building a Yagi for each station you're interested in
    is reasonable.

    The next step is to use better feed cable from the roof to your TV.
    RG-6 is cheap and really quite good.
    If you really want to build a LNA, the NE34018 is cheap and has a very
    very good noise figure. And it's stocked by Digikey.

    NEC has some application notes with suggested
    PC board layouts. It's generally used in the low GHz region, but will be
    great for UHF too.

    Tim.
     
  13. I read in sci.electronics.design that John Larkin <[email protected]
    techTHISnologyPLEASE.com> wrote (in <[email protected]
    4ax.com>) about 'LNA for my TV antenna', on Sat, 17 Jan 2004:
    Was it not much bottle?
    Ah, well, the BBC had to spend quite a lot of effort to meet the
    government's requirements about protecting the surroundings from high
    field strengths, not for any biological reason (it was long before the
    current scares) but simply so that radio, TV and telephones continued to
    work.

    The original separate ITV mast had to have re-designed antennas shortly
    after going to full power (IIRC), because the umbrella leaked.
     
  14. John Larkin

    John Larkin Guest

    The prople who built Sutro Tower found it more efficient to just buy
    off the politicians.

    John
     
  15. Zak

    Zak Guest

    IWell, ISTO that close to an antenna magnetic and electrical fields can
    be 'out of balance'. If you put a 'magnetic' and an 'electrical' antenna
    (a small high-current loop, and a tiny dipole) close together, the
    coupling will be inefficient. At larger wavelengths things will be
    better, relatively. But I guess less distance will always mean a
    stronger signal (minus radiation pattern effects...)


    Thomas
     
  16. Fred Bloggs

    Fred Bloggs Guest

    He's in the far field but on a low point of the elevation gain pattern,
    the ground wave has not formed, or he is in the shadow of a large
    obstruction like a hill.
     
  17. Mac

    Mac Guest

    Thanks. I'll take a look at Newark. I am not totally ignorant of the
    layout issues involved, but I very much appreciate your warning. There are
    times when I don't mind experimenting, but for the moment I am just trying
    to find a solution that doesn't involve putting the antenna in another
    room or on the roof. ;-)

    Mac
     
  18. Mac

    Mac Guest

    I don't think so. Without an antenna I get nothing, and when I tune in
    non-HD stations they look snowy, the classic sign of weak signal. Also,
    the HD receiver has a signal strength meter, which is at about 40 percent
    (whatever that means) on the stations I receive, and at 0 for the stations
    I don't receive.

    Mac
     
  19. Mac

    Mac Guest

    This is probably right. Depending on the exact location of the antenna, I
    may not have a clear line of sight from my house.

    Mac
     
  20. Mac

    Mac Guest

    I live 1.5 miles soutwest of Sutro Tower. But in a room literally 20 feet
    away from the HD receiver, I get all regular (non-HD) stations just fine
    with a folded dipole made from 300-Ohm cable. In the bad reception room,
    the regular reception stations come in but are very weak. Some of them
    don't really come in at all. So I know the situation isn't hopeless. I
    just don't want to lay cable between the two rooms.

    Mac
     
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