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G4EBT > OLDSET 08.10.09 20:22l 67 Lines 2550 Bytes #999 (0) @ WW
BID : 191393G4EBT
Read: GUEST
Subj: MW modulator for AM radios
Path: IZ3LSV<IK2XDE<ON4HU<ON0BEL<GB7FCR
Sent: 091008/1753Z @:GB7FCR.#16.GBR.EU #:33115 [Blackpool] FBB-7.03a $:191393G4
From: G4EBT@GB7FCR.#16.GBR.EU
To : OLDSET@WW
I have a collection of vintage valve radio dating back to the 1930s, the
restoration of which for many years has been my only involvement with
radio.
The constructional aspects of the hobby was always my overriding interest
and the fascination for DX soon wore off. (Anyone sat where I'm sat at the
same time using the same equipment would get the same results).
Although my AM valve radios receive many overseas AM stations on MW and
LW, before too long, there will be no UK stations to listen to due to the
"Digital Britain" moves to close down AM/FM broadcasting by 2015.
With that in mind, I've just built an AM modulator which enables inputs
from any audio source - ipod, DAB radio, whatever, to be played on 430 -
490 Metres in the Medium waveband via the radio's aerial socket.
Basically, it consists of a VFO with variable tuning to cover 430 - 490
Metres, which is then modulated by the incoming audio, and fed to the
radio at a vacant spot on the dial in that waveband.
Adjust the tuning of the modulator till a signal is hears on the radio,
then adjust the depth of modulation adjusted to avoid distortion by
over-modulation.
All built on three little PCBs - one for the PSU, one for the VFO, one for
the modulator. (I etch my own PCBs using self-sprayed UV lacquer and a
light box).
It was a magazine design from some years back, and recently the editor
commented that due to the switch off of AM/FM stations the magazine may
look again at the project to see if it can be updated, as the Toko tuning
coil is obsolete.
Apart from that, it uses readily obtainable components.
I worked out the inductance needed and wound my own coil on a scrap
transistor radio IF former. The sine wave was excellent on the scope,
and the VFO didn't drift even a few hertz over several hours.
The modulator works a treat and there's really no substitute for
the mellow sound of a valve radio. "Real" radio glows in the dark!
I'm sure there must be similar designs out there in cyberspace.
You can get "micro transmitters" which are legal in the US and meet FCC
regs, which will transmit signals at short range to the radio - low power
devices rather like so many others from intruder alarms to car remote
keyfobs.
But I've not checked if those are legal in the UK.
I suspect not, or those who sell them here would surely say so.
Best wishes
David, G4EBT @ GB7FCR
Cottingham, East Yorkshire.
Message timed: 18:50 on 2009-Oct-08
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