[cma-l] AM v FM

Sat Dec 19 17:54:57 GMT 2015

T aerials worked pretty well for Laser, and Caroline after the big mast
fell down.  And I recall R4 Droitwich is a T as well (although that one's
pretty big!).

If you can put-up with some directionality, probably one of the most
cost-effective solutions is a sloping wire radiator from a block of flats.
Radio Rag in Manchester used to use that solution in the mid 80's.  It was
an inverted arrangement - transmitter on 11th floor connected to wire
radiator stretching down to ground.

Glyn

-- 
Glyn Roylance - Principal Consultant
Associated Broadcast Consultants <http://www.a-bc.co.uk/>

On 19 December 2015 at 16:05, Two Lochs Radio <tlr at gairloch.co.uk> wrote:

> 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 <transplanfm at hotmail.com>
> *To:* The Community Media Association Discussion List
> <cma-l at mailman.commedia.org.uk>
> *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
>
> <http://www.transplanuk.com/>
> *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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-- 
Glyn Roylance - Principal Consultant
Associated Broadcast Consultants <http://www.a-bc.co.uk/>
<http://www.a-bc.co.uk/index.html>
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