Solent Club for Amateur Radio & Television

Thursday, February 23, 2012

Last Updated on Friday, 07 October 2011 18:03 Written by M6PJD

The inductors were cut from 0.63mm thick copper sheet into approx 3mm wide strips with the ends opposite to the trimmers folded at right angles so that the inductors are 2.5mm above the PCB, this gap is maintained by pads of PCB material. The pitch of the 23cm inductors is 6.5mm (approx 3.5mm gaps) and the 70cm ones are pitched at 10mm (approx 7mm gaps), finally the horizontal lengths are 23cm = 13.5mm and 70cm = 72mm; output tapping points are 1/3rd from the cold end. The choke feeding the Gali-84 is made by winding 3 turns on a 2mm drill on one tail of the 68 Ohm 0.6W resistor.

I have just tried to find out this method’s capabilities (by altering the layout to the one sketched on the circuit diagram) and these are the results (“warts an’ all”, measured on a known to be good Marconi TFT 6460/1):

1) 1249MHz: The most useful outputs at the various gain settings on the SR are: 8 (120uW), 9 (0.4mW), 10 (0.77mW) and 11 (1mW) – The SR goes up to ‘Gain 15’ (1.7mW) but this causes some compression.

2)Typical outputs from the 23cm filter (with gain settings in brackets)are: 3.6mW (8), 13mW (9), 22mW (10), 26.3mW (11).

3) 436MHz: The SR outputs much more on this band: 8 (0.45mW), 9 (1.7mW), 10 (2.8mW), 11 (3.6mW).

4) Typical outputs from the 70cm filter (with gain settings in brackets)are: 25mW (8), 68mW (9), 83.7mW (10), 89.3mW (11).

Mathematically minded readers will see why SR users usually run at about Gain 8 and why attenuators placed as shown are essential to prevent overloading, the chosen values will of course depend on individual layouts. Attenuator selection is easiest if you have access to a spectrum analyser but if not, you can get near to the required levels by calculating with reference to the characteristics of the Gali-84 and the PAs.

Tuning the filters is straightforward if you start at half mesh and remember that altering the capacitance of one trimmer connected to the Gali-84 slightly de-tunes the other one.

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