Hs gets professional antennas

Discussion in 'News' started by Clive G3GJA, 19 August 2012.

By Clive G3GJA on 19 August 2012 at 12:25
  1. Clive G3GJA

    Clive G3GJA Active Member
    Staff Member Website Admin Chairman

    22 January 2011
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    On Saturday 18th August, a working party of five Committee members managed to fit a day in at the repeater site between contests, holidays and bad weekend weather.

    The priority was the replacement of the Diamond X50 antennas previously used on GB3HS with a pair of commercial grade Amphenol J-Beam folded dipoles. The spec. can be found here: http://www.amphenolantennasolutions.co.uk/7050135-omni-center-fed-dipole-vh-pol-135-160-mhz-39-p.asp The X50s are not ideal for a repeater on a hill as reference to their radiation plots show that the main lobe is around 5 deg above horizontal. This is not helpful as most of the areas we need to cover are at sea level or not much above and therefore are below horizontal. The folded dipoles have the classic doughnut radiation pattern so they squirt RF in virtually all directions.

    The main problem with the X50s is that they are made from mechanically jointed dissimilar metals. This causes noise to be generated in the TX antenna, when it has RF applied, which is then picked up on the RX when the antennas are flexed in the wind. We know that there will still be crackles caused by the tower in windy conditions but we hope that overall there will be an improvement.

    We also took the opporutnity to align the new TX and RX antennas to maximise the isolation bewteen them and that has resulted in abolutely no desensitisation of the RX when the TX is on.

    We also cleared out a lot of redundant hardware and equipment from the hut to make way for some new shelving. The shelving will hold new filters for GB7HU and a 2.2kVA UPS. The site was sprayed with SBK to kill off the weeds that seem to have attempted a takeover.

    Thank you to all that gave up their Saturday.

    Clive G3GJA
    Chairman EYRG
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Discussion in 'News' started by Clive G3GJA, 19 August 2012.

    1. Andy G0VRM
      Andy G0VRM
      So far from the brief trips I have made whilst mobile, the antennas seem to have made an incredible difference to the performance of GB3HS. Although the Diamond X-50 antennas may appear to have more gain on paper than the folded dipoles, the signal strength locally has increased dramatically as a result of more signal being radiated below the horizon as Clive describes.

      The repeater's coverage is probably slightly reduced on the fringes due to the lower gain, however areas close in that were hard to reach such as along Hessle Foreshore, along the A63 between the Humber Bridge and Smith & Nephew and Brough are now seeing the benefits, and I am receiving solid S9 signals.

      We look forward to receiving your signal reports.

      73 de Andy
    2. Ron Young
      Ron Young
      Is the Slim Jim a good antenna for repeater use? seems quite useful for simplex FM use.
      cheap, robust, and claimed low angle radiation.
    3. Andy G0VRM
      Andy G0VRM
      The antennas we use are 1/2 wave folded dipoles fed at the centre and have the classic donut radiation pattern - giving some radiation at a downward angle from the hilltop the repeater is on. A slim jim is an end-fed 1/2 wave and is biased upwards, despite the 1/4 wave matching section, so probably not quite as good.

      The antennas we use manufactured by Amphenol Jaybeam, and are meant for long term installations on commercial towers, or the classic guyed pole found on farms - they generally don't use dissimilar metals as it can cause intermodulation products. If you are careful will the mechanical construction, then a slim jim would be OK in that respect - we have found the usual white stick colinear antennas just can't stand up to the increased wind loading and fall apart after a couple of years.

      Hope this helps...
    4. Ron Young
      Ron Young
      Yes true quality pays for itself, unfortunately price does not always mean quality, commercial antennas are best for repeaters on insurance grounds as public safety is always a concern. 73s

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