[cma-l] AM v FM

Sun Dec 20 02:50:23 GMT 2015

Just joining the discussion here on AM aerials.

You could also use an inverted L antenna, tuned to a quarter wave and an
ATU to match it to the transmitter output.

These were very successfully used by the pirates of the 60s, 70s and 80s
when strung between two high trees.  The vertical bit should be as long as
possible and the earthing system was essential.  Rods and radials were the
order of the day.  50 watts of RF power up the aerial used to get very good
coverage up to about 15 miles if lucky.

Just my 2d worth.

Geoff

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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-- 
Geoff Rogers
Programme Director
Susy Radio, Local Community Radio for Redhill and Reigate
On-air across Sussex and Surrey on 103.4FM NOW
Web: susyradio.com <http://www.susyradio.com>

Susy Radio Ltd. A company registered in England and Wales.
Registered Office: 54 Nutfield Road, Merstham, Redhill, Surrey, RH1 3EP.
Registered Number: 06748586
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