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Help with aerial for channel 38

Discussion in 'General Electronics Discussion' started by Miguel Lopez, Dec 26, 2014.

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  1. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    Hello everybody

    I need some help to build an aerial to tune the channel 38. In Cuba, digital TV has started with this channel and althought I can tune it with the legacy aerial (wide band), I find that sometimes reception is not good, so I thought to build, a specially-built antenna for the channel 38.

    Any help most welcome

    Thanks in advance
     
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,559
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    Jun 21, 2012
    Would this be TV Channel 38, broadcasting from Miami FL as WPMF, TV Azteca?

    I would think a ten element Yagi (one reflector, one dipole receiver, and eight director elements) specifically cut for Channel 38 (614 to 620 MHz) would be sufficient to receive TV Azteca. I have attached a representative Yagi antenna design example. The element lengths and spacing for this example are sized for 608 - 614 MHz, so they are a little too long for Channel 38. Fortunately, at this amateur radio astronomy site there is a free program that will allow you to determine element length and spacing for TV Channel 38.

    Construction is pretty simple, and it might even perform well in-doors, which is a big help if you want to "prune" the element lengths for optimum reception sensitivity. However, it is best to install the antenna outside on as tall a mast as you can reasonably accommodate.
     

    Attached Files:

  3. Miguel Lopez

    Miguel Lopez

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    Jan 25, 2012
    It is not intended to see TV Azteca. It is to see Cuban Digital TV broadcastings that began with Channel 38.

    Thank you very much. I will consult that site
     
  4. hevans1944

    hevans1944 Hop - AC8NS

    4,559
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    Jun 21, 2012
    Then you will probably be able to get by with a shorter Yagi with fewer elements... maybe just a dipole and a reflector element. There are advantages to the ten-element design: it is highly directional with a large front-to-back ratio, which helps minimize multi-path reflection reception. Multi-path reception is what caused "ghost" images on analog TV, but it can cause complete dropout on a digital TV signal.
     
  5. Externet

    Externet

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    164
    Aug 24, 2009
    Hey Miguel
    I strongly suggest you build a loop for 616 MHz. It will be ~300 ohm but do not pay attention as the modern TV tuners do not know that.
    I just made one after sick and tired of marginal reception on 516 MHz with a stupid telescopic rod. Now the 8 inch loop is performing great. I installed on top of the TV cabinet using a concentric power jack and the loop on a concentric power plug. It can be rotated and detachable. Ran a wimpy highly flexible coax to the tuner input connector (audio shielded vulgar cable !)
    ----> http://imageevent.com/holl_ands/loops/uhfcircular
    The loop is household solid AWG 14 or whatever non stranded piece in the bottom of your drawer behind the spider webs.
    Try ~7 inches diameter for yours.
    These often ignored crap works superb ----> http://sdpb.sd.gov/reception/images/image002.png

    ----> http://s588.photobucket.com/user/Innernet/media/P1010515_zpsb694a8f4.jpg.html?sort=6&o=88
    ----> http://s588.photobucket.com/user/Innernet/media/P1010517_zps0e80e324.jpg.html?sort=6&o=86
    ----> http://s588.photobucket.com/user/Innernet/media/P1010518_zpsebcc287e.jpg.html?sort=6&o=87

    Will take you 2 minutes, Come back with results.
     
    Last edited by a moderator: Dec 29, 2014
  6. Miguel Lopez

    Miguel Lopez

    252
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    Jan 25, 2012
    Hello everybody. Hope all of you had had a nice time last week. Happy new year for everybody!!!

    Well, the antenna worked!!! I built a 25cm round loop and positioned on the roof of my sister house. The antenna is for my sister as I have not acquire the digital decoder yet :(. The reception is very good. No more pixelation of the image till now.

    Here some pictures:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited by a moderator: Jan 5, 2015
    KrisBlueNZ likes this.
  7. Externet

    Externet

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    164
    Aug 24, 2009
    Great to see the hurricane-proof version. Congratulations !
     
  8. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    Yeah, yeah!!! Hurricanes is something that must be taken into account ALWAYS in Cuba.:D
     
  9. Miguel Lopez

    Miguel Lopez

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    63
    Jan 25, 2012
    In order to increase the gain, I have thought to use two turns instead of one.

    Would this increase gain?

    Would this increase impedance too?

    Is it worth a try?
     
  10. Externet

    Externet

    754
    164
    Aug 24, 2009
    Hi tocayo.
    Two turns should not provide gain in my opinion. Will be detrimental to the frequency of reception.
    I would place reflectors and director loops in front and back to make it unidirectional gain instead of the current bidirectional behavior.
    The spacing is supposed to be calculable ----> http://fermi.la.asu.edu/w9cf/yagipub/

    Escarba aquí :
    ----> http://www.linuxtv.org/wiki/index.php/Shielded_Loop_Antenna_/_Yagi-Uda_array_of_loops

    ----> http://www.ntms.org/files/1296 Loop Yagi Dimensions.pdf

    ----> http://cheatthezone.tripod.com/wireless/loopyagi.html

    ----> http://home.planet.nl/~luijk802/yagi-uk.htm

    ----> http://n5dux.com/ham/files/pdf/902 MHz Loop Yagi Antenna.pdf

    Try to come up with a loop mounting way that the extra loops can slide forth and back in order to evaluate optimal spacing.
     
    Last edited: Aug 18, 2015
    hevans1944 and davenn like this.
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    great response @Externet

    a 3 element loop yagi will make a big difference over a single loop

    Adding elements generally decreases the feedpoint impedance
    But for TV receive only, this isn't really going to be a problem


    Dave
     
  12. Miguel Lopez

    Miguel Lopez

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    Jan 25, 2012
    Thank you very much for your answers. I will try that :)
     
  13. Externet

    Externet

    754
    164
    Aug 24, 2009
    Just in case, worth mentioning...
    If the antenna you built is now marginal or not working, will let you know the one I built has that behavior now.
    Trying to find why, I can only think both, yours and mine where put to work and were satisfactory in winter. Colder temperatures should have some effect with propagation. Mine started degrading at the beginning of warm weather.
    Unless someone else can contribute with other possible reasons...
    Edited- added :

    UHF-temp.png UHF-humid.png

    Shows a sustantial UHF loss above 22C and also, at 80%. Both together can be horrible :eek:
     
    Last edited: Aug 19, 2015
  14. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    The aerial is working but gain has decreased. You should probably be right as temperatures are hell-like by these days in Cuba. And humidity is always high. Signal is lost most of times when it is rainning.
     
  15. davenn

    davenn Moderator

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    Sep 5, 2009
    that shouldn't have happened unless you made errors in the construction
    show us some photos and give measurements of diameters of loops and distance between loops
    I really want to also see the coax feedpoint to the driven loop

    Dave
     
  16. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    Pictures are in Post #6. I have not made any change in the aerial until know what to do first. I want to make changes as with the original design (the one in the pictures) I have noticed that signal level in the TV set has decreased. That's why I want to make changes to increase gain.
     
    Last edited: Aug 24, 2015
  17. davenn

    davenn Moderator

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    Sep 5, 2009
    OK when you had said you will try, I was assuming you were going to try the 3 element style for higher gain

    but those pics in post #6 don't show the feedpoint detail

    As I said earlier .... for receive antennas, dimensions are not wildly critical

    make the reflector loop 10% larger and the director loop 10% smaller than the driven element
    place the reflector and director loops around 1/2 wavelength (of the freq you are interested in)
    behind and in front of the driven element ( the one with the coax feed). you can experiment with
    varying the distance between the driven element and the director loops

    Dave
     
    Last edited: Aug 25, 2015
  18. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    That is exactly what I intend to do.

    Thank you for your advices Dave, I will talk about the results when I try it.
     
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