Friday, 25 January 2019

Es’Hail -2 on 2m

I’m currently working on a 739MHz to 145MHz down converter so that I will be able to hear downlink signals on my 2m band transceiver.
432MHz will  be used to drive the 2.4GHz uplink transverter/PA/antenna. A dual band, ‘satellite’ transceiver can then be used on Es’Hail-2 in full duplex mode.  The propagation delay will make sending CW and voice difficult, but at least you will know you are getting in and not qrm’ing someone else.
As PA5Y has pointed out, the latency of the typical SDR would make this even worse, suggesting an analogue receiver might just be preferable!

The downconverter is based on my Iceni transverter board, receive section and LO, with 99MHz (lockable) crystal oscillator. I’m using a up mspare (redundant) Iceni V1.0 PCB.

The first problem is that the original LO could not be persuaded to generate any worthwhile power on 594MHz (739-145MHz). Instead I opted to tune it up to half frequency (297MHz) and try using  sub-harmonic mixing. After a lot of work I am achieving a conversion loss of about 20dB through the mixer in this mode.
With a 735MHz Toko helical filter (tuned to 739MHz) on the RF input and suitable filtering/ mismatching on the IF to try  and ensure that the mixer was seeing unwanted frequencies reflected back in the correct phase to minimise conversion loss, i was able to add a post mixer amplifier and filter to achieve  an overall insertion gain of -10dB. As the LNB has massive gain, the downconverter noise figure is not critical.
I have more work to do on the converter before I am happy with it. More anon.

I subsequently found that the level of noise from the LNB, down at 739MHz, was considerably down compared to at 1GHz. Consequently my plan to add a passive three way splitter after the bias tee feeding the LNB resulted in a poor overall system noise figure when feeding the downconverter. I may have to add the PSA4-5043 RF stage back into the Iceni-based downconverter....


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