i've also read else where they would use a special aerial with a capacitance hat on top, it did look a bit like those unmanned monitoring stations, you'd definitely started to wonder what was going on if you saw one of those.AM/MW Unlicensed/Pirate stations
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radionortheast
- proppa neck!
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Re: AM/MW Unlicensed/Pirate stations
i've heard of some hospital radio stations been heard in Scandinavia, think this was 1w in the past, i've also read else where they would use a special aerial with a capacitance hat on top, it did look a bit like those unmanned monitoring stations, you'd definitely started to wonder what was going on if you saw one of those.
i've also read else where they would use a special aerial with a capacitance hat on top, it did look a bit like those unmanned monitoring stations, you'd definitely started to wonder what was going on if you saw one of those.-
jvok
- no manz can test innit
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Re: AM/MW Unlicensed/Pirate stations
Here's an example hospital radio site: https://tx.mb21.co.uk/gallery/gallerypage.php?txid=2477
The complete installations (transmitter, antenna and ATU) were supplied by Radica, usually with an Inovonics 222 processor. The antenna is a vertical wire hidden inside the fibreglass pole (to prevent short circuits and keep it touch safe) and the ground system was 6 foot lengths of copper tape arranged into an H shape, buried just below ground. There were copper earth rods at each corner of the H, mainly for lightening protection (the rods won't do much for RF). The top hat was there to electrically lengthen the antenna, which was very short for medium wave (LPAMs typically uses 1386khz in the UK, so a full size monopole would need to be 54m tall). Even with the top hat they were very inefficient - you usually need 40-50w input to hit 1w ERP. The supplied transmitters were 50w units with a power adjust, and would be trimmed by the installer to hit exactly 1w radiated.
The matching unit lived in the fibreglass box at the base of the mast. It basically was a big loading coil to cancel the capacitance of the short antenna, with selectable taps for tuning. Then there was a Pi or L network (don't remember which) to match the remaining resistive part to 50 ohms.
These installations were very common on hospital and university stations in the 90s/00s, and were also used by a few community LPAMs. I used to have a service manual which had photos of the matching unit but I can't find it anymore - if I do find it I'll post a copy. If you google "Radica AM50" you can find photos of the transmitter.
I once built a clone of the antenna in my back yard, with smaller top hat but more extensive ground system. I never found it radiated that well so impressive if anyone managed to hear one abroad.
The complete installations (transmitter, antenna and ATU) were supplied by Radica, usually with an Inovonics 222 processor. The antenna is a vertical wire hidden inside the fibreglass pole (to prevent short circuits and keep it touch safe) and the ground system was 6 foot lengths of copper tape arranged into an H shape, buried just below ground. There were copper earth rods at each corner of the H, mainly for lightening protection (the rods won't do much for RF). The top hat was there to electrically lengthen the antenna, which was very short for medium wave (LPAMs typically uses 1386khz in the UK, so a full size monopole would need to be 54m tall). Even with the top hat they were very inefficient - you usually need 40-50w input to hit 1w ERP. The supplied transmitters were 50w units with a power adjust, and would be trimmed by the installer to hit exactly 1w radiated.
The matching unit lived in the fibreglass box at the base of the mast. It basically was a big loading coil to cancel the capacitance of the short antenna, with selectable taps for tuning. Then there was a Pi or L network (don't remember which) to match the remaining resistive part to 50 ohms.
These installations were very common on hospital and university stations in the 90s/00s, and were also used by a few community LPAMs. I used to have a service manual which had photos of the matching unit but I can't find it anymore - if I do find it I'll post a copy. If you google "Radica AM50" you can find photos of the transmitter.
I once built a clone of the antenna in my back yard, with smaller top hat but more extensive ground system. I never found it radiated that well so impressive if anyone managed to hear one abroad.