Page 182 of 284 FirstFirst ... 82132172180181182183184192232282 ... LastLast
Results 1,811 to 1,820 of 2838

Thread: best antenna

  1. #1811
    Passenger
    Join Date
    Nov 2014
    Posts
    37
    abcd: have you tried a 1/4 decoupling sleeve above the feeder? Looks like you're attempting something like it without the sleeve

  2. #1812
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,826
    Quote Originally Posted by hikeofyourlife View Post
    abcd: have you tried a 1/4 decoupling sleeve above the feeder? Looks like you're attempting something like it without the sleeve
    Decoupling Sleeve, as it's name shows, decouples antenna from unbalaced (common mode) currents on the outer surface of sheath of feeder coax. It helps preventing feeder coax's sheath to distort the radiation pattern, reducing gain & directivity. It does improve performance to some extent, but not fully as it does NOT reduce SWR.

    It is the high SWR which is the real culprit in case of CoCo and most of other antenas whose elements are considerably longer than1/4 wavelength (coco's elements are 1/2 wavelength, twice of 1/4 wavelength).

    The piece of twin-lead transmission line on the other hand reduces the SWR considerably. This increases signal flow from antenna to receiver, and far off planes whose signal is weak due to distance, start showing up on the rtl1090 & adsbScope.

  3. #1813
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,826
    Quote Originally Posted by hikeofyourlife View Post
    abcd: have you tried a 1/4 decoupling sleeve above the feeder? Looks like you're attempting something like it without the sleeve
    What you are refering to is a balun (balance to unbalanced) with no impedance change i.e. it is 1:1

    Please see my post #1208 about sleeve baluns "Sleeve (bazooka) balun 1:1", Source: ECE Dept, McMaster University, Ontario, Canada. Antenna Lectures by Prof. Natalia K. Nikolova

  4. #1814
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,826
    4-Element CoCo (made from RG6 coax) with Twin-Lead Transmission Line piece for Impedance Matching


  5. #1815
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,826
    The beauty of Twin-Lead SWR reduction method is that it requires only changing the length of Twin-Lead, while observing the max range & max number of aircraft picked by rtl1090/adsbScope/VRServer.

    With push-pin construction in above sketch, Twin-Lead's length adjustment is easily achieved by pushing the feeder coax towards CoCo to reduce the length & pulling feeder coax away from CoCo to increase the length.

    Another beauty of this arrangement is that even if the RG6 coax's exact VF is not known, and the CoCo is made with the element length 114 mm, the SWR can still be reduced to below 2, by adjusting the length of Twin-Lead between 30mm & 60mm.

    For guidance, a table is given below which shows the optimum length of Twin-Lead for different VF (obtained by simulation).

    4-Element CoCo, made from RG6 coax, Element Length = 114 mm
    Coax V.F. Twin-Lead Length
    (shield-to-shield)
    SWR Gain dBi
    0.80 30 mm 1.01 5.5
    0.81 33 mm 1.14 5.5
    0.82 37 mm 1.30 5.5
    0.83 42 mm 1.50 5.5
    0.84 48 mm 1.71 5.5
    0.85 57 mm 1.89 5.5
    Last edited by abcd567; 2014-11-08 at 19:30.

  6. #1816
    Purser
    Join Date
    Feb 2014
    Location
    South Oxfordshire
    Posts
    141
    Hi ab cd, I must have missed your calculation for the element length of 114mm in post #1811. I thought with a VF of 0.83 we would be looking at an element length of 124mm. Is the difference due to the effect of the matching element

  7. #1817
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,826
    Quote Originally Posted by trigger View Post
    Hi ab cd, I must have missed your calculation for the element length of 114mm in post #1811. I thought with a VF of 0.83 we would be looking at an element length of 124mm. Is the difference due to the effect of the matching element
    Length is calculated by standard formula:
    Element length = VF x 1/2 wavelength = 0.83 x 1/2 x 275 = 114 mm

  8. #1818
    Purser
    Join Date
    Feb 2014
    Location
    South Oxfordshire
    Posts
    141
    Quote Originally Posted by abcd567 View Post
    Length is calculated by standard formula:
    Element length = VF x 1/2 wavelength = 0.83 x 1/2 x 275 = 114 mm
    Ooooops ! I obviously made a mistake a few months ago when I worked out the element lengths and wrote them down. I referred back to those notes this morning. This mistake will invalidate all the CoCo experiments I've done in the past

  9. #1819
    Passenger
    Join Date
    Nov 2014
    Posts
    37
    I'm wondering how much the outer jacket brings down the VF, since you're using the outside of the shield as the actual antenna elements and not the inside of the coax. I'm thinking the twin-lead tuning method works because the VF is not 83 percent anymore and you're actually correcting for this error in VF and phasing. With my CoCo, I stripped off the outer jacket and shield and put the dielectric and inner core sections into 1/4-inch refrigerator tubing (fits perfectly). I assumed the same VF. The antenna was useless without a decoupling sleeve but works excellent with it, I used 3/4-inch copper pipe for the sleeve. I may try your twin-lead tuning method with a spare antenna and see what happens. Unfortunately I'm in Reno and surrounded by high Sierra mountains so I don't really have a clear view to the horizon in any direction. However, I'm able to pick up as far away as Fresno. I'm also picking up TIS-B in San Francisco with planes that are as low as 2,000 feet but that's a whole nuther story

  10. #1820
    Purser
    Join Date
    Feb 2014
    Location
    South Oxfordshire
    Posts
    141
    Quote Originally Posted by abcd567 View Post
    SUCCESS!!!! - 450 kms+ : The result of 23 hrs trial run of 4-elements, 124 mm element-length, open top CoCo
    Cable used: RG6, Foamed Polyethylene (FPE) insulation, VF=82%
    Element length = wavelength x VF/(1+VF) = 275 mm x 0.82/(1+0.82) = 124 mm
    I went back over this thread and found where I got my value of 124mm

Tags for this Thread

Posting Permissions

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