23 Jan 2015

70cms AM

As you may recall, I am quite a fan of super-regen receivers. Nothing as simple is able to match their AM sensitivity, but they tend to have poor selectivity.

With the availability of very low cost 433MHz modules (TX and super-regen RX) designed for AM data, these modules could be ripe for conversion to ultra-simple 70cm AM voice transceivers. One local friend, Andrew G6ALB, is currently carrying out experiments to see if this is feasible. I hope to work him on 70cm AM using such a rig in the months ahead. Sadly, I am still too clumsy to do any building work - very frustrating.

I am wondering if anyone else has tried these modules in such a circuit? With a simple MMIC PA (50 ohms in and out) the power output could be lifted to around 25mW carrier or more. With V2000 verticals this should be good for local ranges. Even at the low milliwatt level from these QRPp modules several km range should be possible with co-linear antennas.

UPDATE 2310z:   I wonder what sort of power the TX modules typically produce with linear (speech) modulation rather than 5V square wave data modulation? Presumably much less than their rated power, so an add-on PA may be essential?


G1KQH said...

Prehaps you should watch VK3YE latest Videos:





73 Steve

Roger G3XBM said...

Thanks Steve.

Anonymous said...

Thanks for the links Steve, I was not aware of these.

It looks like the modules I’ve selected are different from the type Peter used.

I’m hoping for linear operation from them, obtainable with no switching approach being required, just the effort of some minor reworking of the modules themselves.

See: http://forum.hobbycomponents.com/viewtopic.php?f=25&t=1324

I will publish the results of my investigations as soon as I have them.

73, Andrew G6ALB

Anonymous said...

They do work.
On TX the modules really need more filtering, as 867MHz and 1299Mhz harmonics are quite strong.
The modulation is also rather funny mix of AM and FM.

I'm yet to play with the original receiver modules, mine seem to use a lm358 op-amp at the output as a comparator to square the output.
So the RX module will likely require some mods.
I had one 70cm AM HT in the works with the modules, but the TX side of it and the case got used for a fox hunt transmitter.