LF: 2xTA split frequency work

[MarkusVester@aol.com]

Hi Joe and group,

even though there was no QSO yesterday, I think that the concept is right, and you will be successful (again) sooner or later.

And I like the split-subband type of operation. Even with simultaneous transmit activity from my nearest neighbours (DJ2LF and DK1IS at 20 and 50km) plus a strong SXV, sensitivity in the top subband was in no way compromized. With an offset RX antenna a couple of 100m away, even full duplex operation would seem feasible in the future.

With my homebrew SSB transceiver, I had at first expected some difficulties to receive both bands at the same time. Normally this is not a problem as I work LF using USB above a fixed 135.5 kHz zerobeat, and generate variable audio signals with DL4YHF's SpecLab Digimode function. But for 135.922, this would mean 422 Hz TX input, which produced unacceptable audio-harmonic sidebands. In the past, I had avoided them by switching the LO down to 134.5, but this then of course forbids reception above 137.2 kHz, and it also caused some desensitizing due to SXV in the middle of the passband.

Yesterday I changed the scheme to 138.5 kHz LSB. The frequency was chosen to have DCF39's carrier suppressed enough to prevent overloading, but still keep it's residual image alias during FSK telegrams below 700 Hz (ie above 137.8 kHz). Then 137.777 becomes a comfortable 723Hz, while the transmit band centered around 2.58 kHz is already attenuated by the IF filter, avoiding blocking by my neighbours. Two separate instances of SpecLab do all the rest of the work, with IQ-downconversion, 21mHz FFT filtering and colour display of received azimuth.

Hope I haven't bored you with these technical details...

73 de Markus, DF6NM


In einer eMail vom 05.01.2004 17:57:18 Westeuropäische Normalzeit schreibt jcraig@mun.ca: 

> Alan G3NYK is predicting favourable condx for possible QSO's
> so starting at 2300 tonight, I'll  TX 137.777 kHz and
> RX 135.922 kHz +- 1.5 Hz starting 2330.  Will
> repeat this at 0000 and activate beacon if no QSO.
>
> Good signals have been received from CT1DRP, PA0SE, DF6NM
> on a 15 turn 5 foot square loop.
>
> Cheers
> Joe VO1NA