[cma-l] AM v FM

[Two Lochs Radio]

The top horizontal section is an active part of the deisgn as well as being a support for the main vertical radiator Ian. It essential forms one plate of a capacitor with ground.

It's a traditional (since before radio broadcasting began) design of shortened LF antenna. As I have always understood it, it's is essentially a vertical radiator, with a horizontal ladder across the top forming capacitive loading that allows the vertical radiator to be more efficient than it otherwise would be without being a full quarter-wave high. The radiation pattern is pretty much omnidirectional, regardles of the azimuth of the horizontal section over the top.

They are a good solution if you happen to have a pair of reasonably tall structures between which the horizontal section can be strung, such as Capital Radio's Lotts Road power station chimneys, or the Titanic's funnels.



Alex
  ----- Original Message ----- 
  From: Ian Hickling 
  To: The Community Media Association Discussion List 
  Sent: Saturday, December 19, 2015 10:10 AM
  Subject: [cma-l] AM v FM


  By a "T-Antenna" are we referring here to a vertical radiator acting as something approaching a bottom-fed quarter wave radiator held in place by a horizontal "dead" wire on either side - or a horizontal dipole fed by a vertical wire or line - or a combination of these?
  It's important - as propagation will be different in each case.
  The antenna we will be using for this project (dictated by land and funding available) will be a simple 6m vertical pole using insulating stays, a capacitance hat and a loading coil in the ATU at the base.
  Efficiency will be about 8% - hence 1kW port output to give 70W EMRP.


  Ian Hickling

  Partner

   
  Office: 01635 578435  (7am-11pm UK time)
  Carphone: 07530 980115 (only responds when driving)
  6 Horn Street, Compton, NEWBURY, RG20 6QS




------------------------------------------------------------------------------
  Date: Tue, 15 Dec 2015 22:26:56 +0000
  From: tlr at gairloch.co.uk
  To: cma-l at mailman.commedia.org.uk
  Subject: Re: [cma-l] AM V's FM

  Sure David, that's been a standard design for LF and MF antennas for over a century. It is a vertical radiator with a capacitive top load to allow it to be shorter, but it still depends on significant height to be efficient, and is still a very big unwieldy structure compared to a half-wave VHF dipole stuck up on a pole! 
  As you say, the T ideally needs wetlands and in any event a good earth system below. As I recall, when Capital Radio started up in London they strung one between the chimneys of Lotts Road power station, and had the advantage of being on the bank of the Thames for their earth. Most town centre community stations won't have such a situation to hand! 

  I think the same sort of antenna design would have been on the Titanic - now that did have a good earth plane below it!
  Alex


    On 15 December 2015 at 14:55 Info <info at transmittersrus.com> wrote:


    Ian, Alex,



    Look into the T-antenna.



    They are relatively low profile, do not requires huge swathes of the greenbelt and are economically priced if you go DIY.



    Peter ‘Chicago’ Murtha installed one just outside Walthamstow for the N. London bible basher. One can see it a couple of minutes up the road from the dog stadium.



    SMC among others supply the hardware.



    Pitch it up over some wetlands with a good ground system, buried or elevated and I reiterate, you do not require anything like 800W of RF to meet that EMRP quota.



    BTW, make sure you have designed an efficient antenna tuning unit which can maximize the bandwidth so you don’t sound like you have a pillow over the mic.



    David




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