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Thread: best antenna

  1. #2551
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    Not giving up!
    I'm not yet dismissing the previous experiment. One explanation could be that the dongle was overwhelmed by the new signal, hence the poor performance. So I tested with all gain settings and it still performs the same. Putting a 68.8mm wire brings back the good reception.


    Will also try next to build the following antennas as I didn't find any mentioning of them here:

    Slim Jim - http://jeroen.steeman.org/Antenna/Sl...nna-Calculator
    PCB antenna - http://f5ann.pagesperso-orange.fr/An...MHz/index.html
    PCB colinear - http://f5ann.pagesperso-orange.fr/An...MHz/index.html

  2. #2552
    Captain abcd567's Avatar
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    Quote Originally Posted by ccz View Post
    Not giving up!
    I'm not yet dismissing the previous experiment. One explanation could be that the dongle was overwhelmed by the new signal, hence the poor performance. So I tested with all gain settings and it still performs the same. Putting a 68.8mm wire brings back the good reception.


    Will also try next to build the following antennas as I didn't find any mentioning of them here:

    Slim Jim - http://jeroen.steeman.org/Antenna/Sl...nna-Calculator
    PCB antenna - http://f5ann.pagesperso-orange.fr/An...MHz/index.html
    PCB colinear - http://f5ann.pagesperso-orange.fr/An...MHz/index.html
    Great. Happy experimenting!

    It will be nice if you post some photos of your test setup and test antenna.

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  4. #2554
    Captain abcd567's Avatar
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    J-Pole & Variants

    j-pole and variations-b.jpg

  5. #2555
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    Thanks! Will try them asap.

    Yesterday I got some lathe time and finished a collinear project I begun a week ago.
    Some brass tubes cut to 0.8 x 1/2 wavelength with 1/4 base and tip, not shorted. I added a very nice wire ground plane between antenna and cable that is not seen in the pictures.

    It came out looking nice but performance was extremely poor, under 50nm. Even placing the amplifier right after the antenna did not help.


    This tells me that whatever precision I can muster in parts making is not going to help me a lot. I need to tune the antenna and no specialized equipment at hand.

    Considering my current results I will also order one antenna from ebay, to add it as a reference point.

    Until next time, I have stumbled upon this video, tuning the antenna using an oscilloscope and signal generator:
    Attached Images Attached Images

  6. #2556
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    i thought from your previous post you werent going to have any dielectric on the antenna?
    T-EGLF8

  7. #2557
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    That's right, but since I didn't have time to print the spacers, I assembled first the one with dielectric.

    The version without will follow soon. First I want to try the oscilloscope method for tuning, it seems pretty bad that I can only do 1/4 wave antennas. I have to find out where I am making mistakes.


    Are our RTL sticks 75 or 50 ohms? I saw it mentioned somewhere but can't put my finger on it right now.

    Ahh, got it:
    What is the RTL-SDR input impedance?

    Since these dongles are intended for TV, all dongles will have an input impedance of about 75 Ohms. However, the mismatch loss when using 50 Ohm cabling will be very minimal at around 0.177 dB.

    The 75 Ohm impedance for the R820T can be checked on the datasheet which can be downloaded here.
    Last edited by ccz; 2016-08-02 at 21:08.

  8. #2558
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    ALL TV sticks are 75 Ohm... but I wouldnt loose much sleep over the impedance, the rest of the homebrew antennae are going to be nowhere near calculated !!!!
    As for using a 'scope - you are going to need very deep pockets for a 1 gHz version ! not to mention a sig gen that covers the frequencies.

    If you are intent on spending money - you wont go far wrong with an analyser... such as :

    http://miniradiosolutions.com/54-2/
    Last edited by Rooster; 2016-08-03 at 13:53.

  9. #2559
    Captain abcd567's Avatar
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    miniVNA €450
    and a Signal generator, may be same price!

    I can buy 20 very good quality commercial antennas for this money.

    Purchase a 100 ft/30 m coil of cheap TV or Satellite coax, and a tape measure. Then cut 6 lengths of 1.50 meters each from this coil, and start cutting pieces.

    8 pieces x 110mm (vf=0.8),
    8 x 112mm (vf=0.81),
    8 x 113 mm (vf=0.82),
    8 x 114 (vf=0.83),
    8 x 116mm (vf=0.84), and
    8 x 117mm (vf=0.85).

    Assemble the 6 cocos and try. At least one of the 6 will be good! Use the balance coax in the reel as your feeder coax. Cost < €10

  10. #2560
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    your plan wont work, because if you cut to those lengths by the time you have trimmed of the outer needed to push the elements together they will non longer be the right length.

    see image, length.jpg

    that's your corrected wavelength
    T-EGLF8

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