Absolute Radio on 1215 / 1197 to be switched off.

TAFKAlawhec

All transmitting aerials have their radiating elements correspond to a large extent to the transmission frequency they are used for - this is no different for MW & LW broadcast transmissions. Ideally for good groundwave coverage from a site at such frequencies you're looking for a radiating mast, tower or some other element (e.g. sloping wire, like is used for the MW transmissions on Sherrif's Mountain in Derry or Crystal Palace in London) correspond to a length between 0.25 to 0.62 of the wavelength to be broadcast. Usually it is the case outside of this that the radiating element(s) are less than 0.25 of a wavelength long, but at long as the mast isn't too short to use it can be used at the expense of some efficiencies e.g. the main radiation lobe being a bit higher than ideal, and less TX power received at the antenna being radiated** - however a radiating element beyond a length of 0.62 wavelengths is to be avoided at pretty much all costs because beyond this the lobes start to become "broken up" with more of the power directed skywards and less hugging along close to the ground, hence the likes of the Blaw-Know mast at Lisnagarvey getting decapitated - even on the link on my last post, they mention that even going to 0.62 (or 5/8ths) of a wavelength is getting uncomfortably close to having a significant high-angle component reducing the fading-free area, and that the ideal would be just over 0.5 (or 1/2) a wavelength long. Then you might introduce complications where multiple transmissions might use the same radiator - some setups in the UK used to have up to five services broadcast from the same antenna (this included the Derry & Enniskillen sites in the past), but if they don't have services using tens of kW of power each and they aren't too close in frequency to each other, then it's less of a problem.

Again, as the linked article mentioned, the ideal behind the Blaw-Knox design was one of marketing & hype than any actual breakthrough RF engineering discovery, and the BBC weren't going to get fooled again after Lisnagarvey was in service. And while some Blaw-Knox masts are local iconic landmarks in other parts of the world, there's no such sentiment for the Lisnagarvey one that I'm aware of. Once BBC Radio 4 is switched off on 720 kHz eventually, I reckon the mast will eventually be dismantled with little fuss.

(** A lot of the low power RSL or MW community stations in the UK use or used to use quite short towers for the frequency the broadcast on, IIRC the 1 watt BFBS long-term RSL MW transmitters that they used to use in NI needed around 30-40 watts of transmitter power to give 1 watt ERP output, even when using an inductive loading coil to "tune" the antenna and a fairly modern & efficient AM transmitter.)

Declan A Walsh

https://www.boards.ie/discussion/comment/120359925#Comment_120359925

Copied into this thread:

https://www.boards.ie/discussion/2058268430/what-can-be-picked-up-on-long-wave-around-ireland/p4

tinytobe

That "horrible echo" is a well known issue. It's about several AM transmitters not in sync. One doesn't notice the problem from within the UK, but the further one is away, the more the echo is noticeable.