Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 25

Thread: Which antenna have more coverage?

  1. #11
    Flight attendant Strix technica's Avatar
    Join Date
    Apr 2017
    Posts
    94
    Quote Originally Posted by abcd567 View Post
    DIPOLE PATTERN vs LENGTH
    Also Applicable to Monopoles with Ground Plane, Length 1/2 of Length for Dipole
    It should be noted that anything larger than 1/4w or 1/2w has funky impedance characteristics and will require a matching network. Without it, you'll lose any benefit you might get from increased power collection ability, if not actually do much worse than a plain 1/4w or 1/2w.

    Colinears are (or so I understand) a different story because they're made up of 1/2w elements put end-to-end in opposing phase. Though I don't think I've ever seen anyone put a colinear on a VNA to test that, commercially made colinears don't seem to bother with matching networks so I guess their impedance is comparable with that of coax.

  2. #12
    Captain abcd567's Avatar
    Join Date
    Sep 2013
    Location
    Toronto CYYZ
    Posts
    2,046
    Some one (TomMuc) has tested 5 different antennas, and has posted his conclusion here:

    http://discussions.flightaware.com/p...2.html#p209072

  3. #13
    Captain Anmer's Avatar
    Join Date
    Apr 2010
    Posts
    1,215
    Quote Originally Posted by abcd567 View Post
    Some one (TomMuc) has tested 5 different antennas, and has posted his conclusion here:
    He says he's tested " five prfessional (sic) made ads-b antennas" of which one is "the best". No description of his tests or which antennae he used, except the one.
    Mike


    www.radarspotting.com

    Radarspotting since 2005

  4. #14
    Flight attendant
    Join Date
    Jul 2012
    Location
    Burnie Tasmania
    Posts
    59
    Its my experience that you will most likely fail in attempt to make an antenna for this freq - unless you have an antenna analyser. Most of my own attempts failed. I have just purchased this analyser
    https://www.amazon.com/N1201SA-Vecto.../dp/B06VVVCKN8

    Below are a few results so far

    http://www.users.on.net/~silver.eagle/ADSB.htm

    Mike

  5. #15
    Flight attendant Strix technica's Avatar
    Join Date
    Apr 2017
    Posts
    94
    Quote Originally Posted by YWYY View Post
    Its my experience that you will most likely fail in attempt to make an antenna for this freq - unless you have an antenna analyser. Most of my own attempts failed. I have just purchased this analyser
    Good job, and I'm surprised to see a VNA so cheap, considering. Still not cost effective unless you plan on making a lot of antennae but having fun is not always cost effective

    I'd also note that the VNA's measurements are going to be thrown off somewhat by impedance mismatches. The VNA will have a 50Ω input where your SDR has a 75Ω input. The difference will not be significant, but it's worth remembering that when you get a perfect VSWR of 1.0 at 50Ω, you may get a warm fuzzy feeling but actual performance will not be exactly as measured.

    Either I just got lucky with my 8-segment colinear design/build or building your own is not as hard as you suggest provided one is exacting with segment dimensions. FR24 reports my 30-day-average max range at 160 nm in public (ie filtered) stats, and my private FR24 dash routinely reports 200–250nm, occasionally 350 nm. My own records report a 30-day max range of 290 nm. (Why the discrepancy between FR24 dash and my own data? Good question. I sometimes wonder about the reliability of FR24's filtering and processing of range data. I can understand mine > fr24, but I don't see how fr24 reports > dump1090, unless dump1090's max range processing is also weird.)

    Considering that the antenna is mounted inside the loft of a mid-terrace brick house with slate tile roof, that's not bad going for a first attempt.

    As I've noted before, it's easy to obsess over antenna design, but the differences between any two antennae, and especially the differences one can achieve tweaking home-built antennae, are probably dwarfed by the differences achieved by height and clearance of local obstructions, remembering the vast majority of a/c are essentially on the horizon. At FL400, for example, an a/c 350 nm away (line-of-sight) only makes about 0.5° to the horizon after you allow for the curvature of earth (1° if you don't). That means that diffraction and neighbours' rooftops are going to be highly significant in practice.
    Last edited by Strix technica; 2017-08-23 at 12:04. Reason: add 50Ω/75Ω caveat

  6. #16
    Passenger
    Join Date
    Jan 2017
    Posts
    14
    What about powered antenna/signal amplifiers?

  7. #17
    Flight attendant
    Join Date
    Dec 2014
    Posts
    70
    Quote Originally Posted by Pyrojim View Post
    What about powered antenna/signal amplifiers?
    A 'powered antenna' is just a normal antenna with an amplifier included. The main benefit is that it may increase the gain of the signal and help to overcome any losses on the feeder cable to your receiver. Down side of any amplifier is that it will amplify all signals within its bandwidth; so if you have strong interfering signals within the bandwidth it will amplify those as well. The better amplifiers will have narrower bandwidth, but that means higher cost. Add on filters to limit the bandwidth will help, but the better the filter, the higher the cost.

    The starting point is a low loss feeder with an antenna in the highest and clearest position. Then look at the add-ons such as amplifiers and filters.

  8. #18
    The difference will not be significant, but it's worth remembering that when you get a perfect VSWR of 1.0 at 50Ω
    As far as I know, in Rx SWR is not affecting. Tx does.
    Even you can use 75 OHm coax in a 50 OHm Rx port with no issues.

  9. #19
    Flight attendant
    Join Date
    Dec 2014
    Posts
    70
    Quote Originally Posted by EA1DDO View Post
    As far as I know, in Rx SWR is not affecting. Tx does.
    Even you can use 75 OHm coax in a 50 OHm Rx port with no issues.
    VSWR is a good indication of whether the antenna is well matched at the desired frequency, even if only used for receiving. What is not as critical is the actual figure.

  10. #20
    VSWR is a good indication of whether the antenna is well matched at the desired frequency
    Just my point of view:

    It is a good indication indeed but... in Rx only system, you don´t need "to match" anything. What you need is the best S/N.
    For example, to use 75 Ohm coax in a 50 OHm system, it is 0.17 dB loss. But the cost of a good 50 OHm coax, comparing with a average TVSAT 75 Ohm coax is not worth.

    So, use good 75 OHm. It will has much less loss and much cheaper than 50 OHm equivalent.

    Spend you effort in a good LNA and good filter.

    Cheers
    Maximo

Posting Permissions

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