Dear Mike, LF Group,
As regards older receivers, I have tried the HRO, CR100 and AR88LF on 136kHz
- they all suffer the problem of poor skirt selectivity, which, combined
with the lack of a proper product detector, gives problems with adjacent
signals like DCF39. The sensitivity is good however; the input impedance of
this type of RX is designed to be quite high on the LF bands, so a wire
antenna can be connected directly without additional tuning components. If
one were to fit a modern IF filter, the performance would be quite good.
I also have had good results using a Telefunken ELK639 - this is an early
solid-state marine receiver, with two separate gang-tuned front ends which
tune 9.8kHz - 30MHz. This has good mechanical filters, and also works well
with a directly connected antenna. Still hasn't got a product detector
though...
Mostly I use the Racal RA1792, which gives good performance at 136k, and is
usable to below 15kHz. The only problem is that the IF filters are not as
good as current receivers, so there is a noticeable, although not serious,
leakage of adjacent strong signals. The front panel controls are a bit
idiosyncratic too. The older RA1772 is similar. These receivers have the
advantage of very good frequency stability due to the internal crystal oven,
and the facility to connect external frequency standards, e.g. GPS-derrived.
Also, the RF and IF signal paths are easy to get at, so I would recommend
these for the more experimentally minded.
Cheers, Jim Moritz
73 de M0BMU
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