Radio Road

• RE: thanks for the good wishes, but .... - JRMA 11:56:51 02/13/16 (3)
  In Reply to: RE: thanks for the good wishes, but .... posted by Timbo in Oz on February 05, 2016 at 21:34:06

  Timbo - thinking of making a rhombic for my roof. Question about the shape of the 4 sides. Can the angles be adjusted to give more rejection on the sides? Problem is there is a large hill across the street that causes some multi path. what do you think?
  

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  • Have you read the article in FAQs here at AA? Print it out and read it a few times. - Timbo in Oz 14:12:27 02/13/16 (2)
    In Reply to: RE: thanks for the good wishes, but .... posted by JRMA on February 13, 2016 at 11:56:51

    Yes, it is possible to build 'doubled and spread' versions, and you can narrow the Rhomboid shape to tighten the pattern.

    Remember that only a truly directional antenna can reject multi-path.

    Once you find the straight line direction between you and the transmitter, mark it on a map of your town and environs. Use a school geometry half circle compass to read off the bearing. Then print-out one of just your house on its block and mark on it the bearing. For a compass use one of the GPS based ones downloaded to your smart phone, as it won't be affected by large ferrous-metal objects.

    Given the hill you need to test the 'straight-line on the map' bearing, from you to the transmitter.

    Most stations use a mixture of vertical and horisontal polarities to assist car reception. So, use a ribbon T-antenna on a vertical board to find the best bearing. Then try if the same bearing is best with the T antenna horizontal.

    Do remember to get away from the antenna yourself to minimise your own effects on it.

    Outdoor wire antennas? You'll need to look at a book called 'Joe Carr's receiving antennas' published in the USA for guidance for wire antennas. Including earthing, static charges and lightning strikes. It doesn't have any stuff for (FM) rhombics but the principals are the same.

    Local 'hams' might be a good resource.

    See if you can fit it into your loft space. Because you can use indoor rated 300 ohm ribbon as the article assumes. This doubles the gain without any increase in size / area, as you then have two rhombics 'stacked.'

    Rhombics are by nature wide-band-width antennas. IE the shorter the wavelength you are picking up the greater the gain. For that reason alone a loft DIY rhombic had a lot of appeal to me.

    It would have covered VHF digital TV (BndIII) and FM (BndII), with lots of gain, almost no maintenance, long life, etc. Everything I want is in band II (FM) and band III DTV, all from the same mast.

    But, the floor of my loft is the ceiling of the house - plaster-board nailed to timber beams - and all covered by very thick fibreglass insulation, well above the beams. So I can't see where to walk. End of that idea. :-)!

    LBNL, given the hill, you might find it works better aimed up a bit, like my new VHF-DTV item is.

    
    Warmest
    
    Tim Bailey
    
    Skeptical Measurer & Audio Scrounger
    
    
    
    

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    Edits: 02/13/16   02/13/16   02/14/16

    • RE: Have you read the article in FAQs here at AA? Print it out and read it a few times. - Tin Ear Bob 13:36:48 04/11/16 (1)
      In Reply to: RE: Have you read the article in FAQs here at AA? Print it out and read it a few times. posted by Timbo in Oz on February 13, 2016 at 13:12:27

      I heard a rumor that mounting an antenna inside am attic or loft ensures reflections off anything metal and loading by near by metalic objects.
      
      If you happen to have line of site, or close to it and you need to reduce multi path from a mountain, in theory you can install the rhombic in your yard.
      
      Best from T'son
      bob
      Never assume anything I post is accurate.
      
      

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      • Yes if you have metal tanks etc in the roof that could affect any antenna up there. Foil insulation, too. - Timbo in Oz 14:51:59 04/11/16 (0)
        In Reply to: RE: Have you read the article in FAQs here at AA? Print it out and read it a few times. posted by Tin Ear Bob on April 11, 2016 at 13:36:48

        Though it depends; if the tank will be behind where the directional antenna is pointing it isn't an issue.
        
        I now have a whole bunch of foil-coated ducting in the roof space and that finally killed my plans for a large rhombic up there.
        
        That's one of the reasons why we decided on a new pair of antennas on the old mast array for FM and DTV. The other being the SO's attitude to a ceiling rhombic! ;-)
        
        Yes you can put a rhombic wire antenna in the yard but then you need to deal with static charges and lightning.
        
        And, UV stable 300 ohm ribbon might be unobtainium these days, so you might need to build two single wire rhombics to maximise the gain. Depends on how long the sides will be.
        
        There is a good book on 'receiving antennas' by Joe Carr published in the USA, which covers static and lightning.
        
        
        
        
        
        
        
        
        Warmest
        
        Tim Bailey
        
        Skeptical Measurer & Audio Scrounger
        
        
        
        

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