Page 2 of 2 FirstFirst 12
Results 11 to 18 of 18

Thread: Optimize the Whip Antenna Which Is Supplied With DVB-T Dongle

  1. #11
    Flight attendant
    Join Date
    Mar 2016
    Posts
    82
    Quote Originally Posted by abcd567 View Post
    Your both points are worth considering.

    (1) Antenna length: length inside the base is a variable factor, depending on the manufacturer/design & worker who assembled the antenna (how much cable he left without braid). In the antenna I have, the total VERTICAL distance from the point where whip is screwed to base, up to the cable entry point is about 25 mm. If it is assumed that antenna length starts from cable entry point, then the whip should be chopped another 25mm i.e. 67-25=42mm. Since somewhat longer antenna is better than somewhat shorter antenna, I wont chop off entire 25mm. In fact couple of months ago I did the exercise below:

    Starting at 67mm whip length (as in the photo n my first post), I observed the maximum range this antenna brings in for few days. I then chopped it off by 5mm, again observed for a day, found improvement. I did a 2nd chop of 5mm, and observed for a day, found improvement. I then did a 3rd chop of 5mm, and observed for a day, found performance slightly decreased, so I stopped and did not chop any further. For my antenna 10mm chop seemed good, i.e. whip length 57mm instead of 67mm.

    Since your manufacturer may have a slightly different arrangement inside the base, you have also to repeat the stepped chopping exercise I have done to find best length for your whip.

    (2) Touching metallic base of antenna with the metallic can or plate is worth trying, and may improve performance. I have never tried it. If you peel off the circular plastic tape from bottom of antenna, bare metal will be exposed. Placing it on metal can or plate will make direct electrical contact. This is easy and worth trying.
    Would't it be much easier to trash completely the whip antenna and build a dipole with 65 mm of the core wire at one side and 6,5mm of the shield at the opposite?
    Last edited by RIN67630; 2016-03-06 at 13:56.

  2. #12
    Flight attendant
    Join Date
    Mar 2016
    Posts
    82
    By the way: did someone measure the velocity factor of the supplied coax cable?
    I can imagine, that a λ/4 T stub could well improve the overall sensitivity by short-circuiting the lower-band signals and avoid useless load on the RTL chip.
    Do you agree?

  3. #13
    Passenger
    Join Date
    Dec 2015
    Posts
    27

    Sleeve antenna

    Quote Originally Posted by RIN67630 View Post
    Would't it be much easier to trash completely the whip antenna and build a dipole with 6,5 mm of the core wire at one side and 6,5mm of the shield at the opposite?
    1 In case you are willing to increase your targeted antenna segment length 10 fold, you might have a chance. The length of both the active elments should be less than 69mm.
    2 The design you are targeting probably is just one kind of a sleeve antenna. Turning back the cable screen around the feeding coax with coating after stripping the cable coating for a magnitude of less than 65mm - around 45? depending on cable type - and using around 65mm inner conductor as whip. Theoretically you might get rather good results with this antenna. In practice the characteristics are defintitely sensitive to tiny changes in geometry. Further, you have to optimize length of shield folded back and whip experimentally. The screen has to be shorter than wavelentgh in free air by a factor near the velocity factor of the cable and so on.
    3 In case you will try this design, use very thick cable and a rigid sleeve. This will somehow make your design more robust. Probably you could choose Ecoflex 15 plus copper tubing 15mm for the sleeve, connected to the shielding.
    4 In case, you won't have success, go back to the cantenna design, turning out to be a somewhat sleeved antenna with dimensions such big, gaining much more robustness against mechanical variation.

    However, a not optimized sleeve antenna definitely will outperform the funny whip antenna provided with the electronics as long as you go close to half wavelength with both the active elements - in other words: do not try 6.5mm but less than 69.

    Good luck

  4. #14
    Flight attendant
    Join Date
    Mar 2016
    Posts
    82
    Oops, you ar right: I meant 65 mm or 6,5cm. It was a typo.

    I did not mean turning the sleeve back, but in a T-form.

    That is what i've made with regular Sattelite antenna cable, since I needed a longer cable:
    Beast4.jpg

    The wire and sleeve are just placed under the duct tape. This is a dirty construction placed inside a wardrobe under the roof. I will make a better antenna later, when the wheather permits.
    You may use other non conductive supports as e.g. gardening rods to support the construction outdoors.
    Using the original cable without the losses in the intermediate connectors shoud be good as well.
    Last edited by RIN67630; 2016-03-06 at 17:27. Reason: fixed broken attachment

  5. #15

  6. #16
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,722
    Quote Originally Posted by rueckwaertsflieger View Post
    1 In case you are willing to increase your targeted antenna segment length 10 fold, you might have a chance. The length of both the active elments should be less than 69mm.
    2 The design you are targeting probably is just one kind of a sleeve antenna. Turning back the cable screen around the feeding coax with coating after stripping the cable coating for a magnitude of less than 65mm - around 45? depending on cable type - and using around 65mm inner conductor as whip. Theoretically you might get rather good results with this antenna. In practice the characteristics are defintitely sensitive to tiny changes in geometry. Further, you have to optimize length of shield folded back and whip experimentally. The screen has to be shorter than wavelentgh in free air by a factor near the velocity factor of the cable and so on.
    3 In case you will try this design, use very thick cable and a rigid sleeve. This will somehow make your design more robust. Probably you could choose Ecoflex 15 plus copper tubing 15mm for the sleeve, connected to the shielding.
    4 In case, you won't have success, go back to the cantenna design, turning out to be a somewhat sleeved antenna with dimensions such big, gaining much more robustness against mechanical variation.

    However, a not optimized sleeve antenna definitely will outperform the funny whip antenna provided with the electronics as long as you go close to half wavelength with both the active elements - in other words: do not try 6.5mm but less than 69.

    Good luck
    The statement "The screen has to be shorter than wavelentgh in free air by a factor near the velocity factor of the cable" is a misconception.

    The coaxial cable's Velocity Factor is applicable to the circuit formed by inner surface of shield and core wire of coax with PE or FPE etc insulation completely filling the gap between core and shield.

    In case of sleeve, the circuit condutors involved are the sleeve's inner surface and shield's outer surface. The insulation between these two conductors is composed of an air gap and the outer plastic covering of the coax. Since the sleeve is loosly fit, the air gap is much bigger than the thickness of coax's outer plastic cover. Hence effectively the velocity factor is close to 1, and in most cases safely ignored.

    Translation from German to English (in red ink) added by me




  7. #17
    Passenger thehague's Avatar
    Join Date
    Aug 2017
    Location
    Randstad
    Posts
    17
    Quote Originally Posted by abcd567 View Post
    FURTHER IMPROVEMENT
    Things to be tried:
    1. Remove black plastic circular adhessive cover from bottom of antenna, and check electrical continuity between exposed metal base plate & the outer metallic part of MCX connector at other end of antenna's coax.

    2. Place exposed base on a metallic can and check if direct contact gives any improvement.
    Did you already try this?

  8. #18
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,722
    Quote Originally Posted by thehague View Post
    Did you already try this?
    Yes, did try. There is not any noticeable difference between placing on metallic can with plastic cover, and without plastic cover

    Further reading on whip antenna optimization:

    Trial Run Results for Three Types of Whip Antennas

    .
    Last edited by abcd567; 2017-08-29 at 22:04.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •