[cma-l] AM community radio

Fri Sep 18 09:38:26 BST 2015

Celtic Music Radio broadcast for 5 years on 1530AM, before moving to FM in 2014.

The tx site for us was a challenge. Where do you find somewhere big enough, and secure enough, near Glasgow city centre, where you can put up a mast without too much planning hassle, and with an affordable rent?

The solution, for us, was Shieldhall wharf. 

Shieldhall treatment works is a large sewage treatment plant, a couple of miles downriver from Glasgow city centre. These days, of course, it’s all treated in an environmentally friendly way, but back in the old days the solution was much simpler - you store the stuff in a tank until the tank is full, then pump it into a ship, sail it down the river and out into the open sea.  Then you dump it over the side! 

(The SS Shieldhall and SS Garroch Head, the Glasgow “sludge boats” apparently used to also take passengers - taking parties of pensioners for a “sail down the river” while dumping untreated sewage!)

Shieldhall wharf, where the boats were loaded with their precious cargo, is now largely unused. It’s not exactly a prime development site for residential use - lovely riverside views, of course, but it’s surrounded on 3 sides by a sewage farm!

Conveniently for us, there is a 22m navigation beacon on the waterside. And as well as a conventional ground mat, we could bond onto the network of underground pipes that formed the sewage farm, and the metal structure of the dock wall. That in itself was interesting - we could see the antenna efficiency changing with the level of the tide!

Antenna was a top loaded monopole - basically a big “arrowhead” pointing skywards, with the radials (about 15m each if I remember correctly) guyed outwards with nylon rope. Looked a bit heath-robinson, but worked well.

Sonically, our transmission chain demonstrated that AM can be surprisingly good. We were quite pleased with the sound we achieved - nowhere near as good as FM of course, but to our biased ears, it was much better quality than the “big” am stations in the area.

We’ve still got all of the kit -  the transmitter (initially a Redifon BT500, latterly a BT1002), antenna tuning unit, optimod etc. Might be able to persuade Alex to sell - but tuning it away from 1530 would be a challenge.

Bruce.

> On 17 Sep 2015, at 18:44, Ian Hickling <transplanfm at hotmail.com> wrote:
> 
> If only it were as simple as that!
> I must admit that I hadn't looked at it like that - 531 to 1,611 kHz gives a quarter wave range of of 141 metres to 46 metres - yes - three to one!
> But do you fancy buying/siting/building even a 46m guyed vertical radiator - 152 feet high?
> Apparently there are only 9 AM frequencies that Ofcom feels able to allocate to LP operation - I'll see if I can find out what they are.
> 
> 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: Wed, 16 Sep 2015 21:28:28 +0100
> From: tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk>
> To: cma-l at mailman.commedia.org.uk <mailto:cma-l at mailman.commedia.org.uk>
> Subject: Re: [cma-l] AM community radio
> 
> Given that the MF AM band covers a 3:1 frequency range, is Ofcom at least able to allocate a frequency at the high end of the band, making any given given aerial design a third the size it would be at the LF end of the band? Just wonderin'!
> 
> 
> 
> Alex
> 
> On 16 September 2015 at 20:20 Ian Hickling <transplanfm at hotmail.com <mailto:transplanfm at hotmail.com>> wrote:
> 
> All perfectly acceptable - but as an experienced  propagation engineer my concern is that any departure from a vertical end-corrected quarter wave radiator operating on a groundplane of at least .55 wavelength diameter, the  energy propagation pattern is uncertain and effectively impossible to either determine or plot.
> In order to take his order - and his money - I have to give the Client an assurance that he will be able to cover his audience effectively - and that simply isn't possible with a cut-down compensated radiator on a much smaller earth mat.
> 
> Ian Hickling
> Partner
> 
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