[cma-l] AM antennae
fantasy office
office at fantasyradio.co.uk
Sun Dec 20 16:56:05 GMT 2015
OK - I'll join in.
I have run several low power AM installations for RSL broadcasts, I come
from an engineering and broadcasting background and do have some
knowledge and experience.
It's true to say that AM presents special difficulties due to the large
scale, but most principles are the same.
I took some valuable advice from a (BBC) qualified transmission
engineer, both in the transmitter design and more importantly the
antenna. As a boy, I lived within two miles of the BBC masts at
Droitwich - and they fascinated me, I could see them from my bedroom
window! So I really do know what that antenna consists of, a T antenna,
strung between two 750ft masts...which is pretty high. (long waves) with
four vertical wires. (that being the active radiator.)
When I came to do my own, I used fristly a T then an inverted L, Unable
to get a great deal of height, I was aware that the Aerial Tuning Unit
would have to make the Transmitter 'see' a much taller antenna. As our
very first RSL, broadcasting from a large garden in Wiltshire, using a
transmitter of 60 watts, into the inverted L, in favourable conditions,
reception in the local area was as solid as a rock. Audio Signal
Processing was very important to ensure it 'sounded' right. And it did!
Further afield, we received several reception reports on the first day
from places much, much further afield, in the UK, Jersey, Sweden and
Finland. To this day, I still have the card, cassette tape and pin badge
sent to me by two listeners working in the VW car factory in Wolfsburg,
Germany.
See, it can be done.
Happy Christmas and a Prosperous New Year to all my radio chums, AM or FM!
Best Wishes,
Phil Dawson
FANTASY RADIO (now on) 97FM
Wiltshire
On 12/20/2015 2:44 PM, cma-l-request at mailman.commedia.org.uk wrote:
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> Today's Topics:
>
> 1. AM v FM (Ian Hickling)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 20 Dec 2015 13:24:19 +0000
> From: Ian Hickling <transplanfm at hotmail.com>
> To: The Community Media Association Discussion List
> <cma-l at mailman.commedia.org.uk>
> Subject: [cma-l] AM v FM
> Message-ID: <DUB125-W358EC2082AE9F1A496AF5CADE30 at phx.gbl>
> Content-Type: text/plain; charset="windows-1252"
>
> I appreciate all the advice and input - thank you.
> I have the huge advantage of having worked in an engineering laboratory that built VHF and UHF antennas and range-tested them so I am very fortunate to have an inbuilt comprehension of how they look and perform.
> Nowhere in all that worthy information is there evidence of energy propagation patterns - actually measured in the field as opposed to calculated or computer modelled.
> Most important - the difference between success and failure.
> I can't guarantee to a Client how well an AM (MF) antenna will perform as I can with FM (VHF) - and that worries me.
>
> Ian Hickling
> Partner
>
> Office: 01635 578435 (7am-11pm UK time)Carphone: 07530 980115 (only responds when driving)6 Horn Street, Compton, NEWBURY, RG20 6QS
>
> From: geoff at susyradio.com
> Date: Sun, 20 Dec 2015 02:50:23 +0000
> To: cma-l at mailman.commedia.org.uk
> Subject: Re: [cma-l] AM v FM
>
> 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
> 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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