Dear LF
I've attached the selectivity figures for a "low-end" ADF RX, in this case a
Becker RA3502 from the late 1990s. The IF is quite wide, compared with what
we're used to on LF.
73, Dave G3WCB IO91RM
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of James Moritz
Sent: 16 February 2012 12:57
To: [email protected]
Subject: LF: Re: WRC-12 Approves the New 472-479 kHz Amateur Radio
Allocation
Dear Roelof, LF Group,
> The requirements for the service area of a NDB is a fieldstrength of 70
> uV/m at the borderline.
> So to cause noticeable interference, the offending signal must be rather
> strong.
> Many NDB's share the same channel, apparently without problems.
> Unless you actually live in or near the service area of a NDB in the
> 472-479 kHz frequency range, the NIB regulations won't pose a problem.
The ADF receivers that use these beacons essentially process the carrier
amplitude and phase received using directional antennas in order to obtain
the bearing. So the effective bandwidth of the receiving system can be made
rather narrow; the audio sidebands are mostly there for ID purposes. Since
there are always many potential QRM and noise sources in the MF range, ADF
receiver design has evolved to cope with quite poor SNR. So even if you are
located near or within such a service area, provided you keep clear of the
actual beacon carrier frequency, ADF receivers will probably not suffer
harmful interference. This should not be too hard with 7kHz to play with,
and only a handful of widely scattered beacons within the range - it is only
common sense to avoid using a frequency where there is a strong beacon
signal causing you 24/7 QRM!
Cheers, Jim Moritz
73 de M0BMU
RA3502.jpg
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