2009

How to make an 80m Rx coil and band spread the tuning range if you wish.

Here are the details of the receiver coil used in an 80m band spread TT Paraset. Varnishing the coil affects its' self capacity and so alters the tuning range. Get the coil and its' trimming capacity roughly right for the tuning capacitor you have fitted. A tuning capacitor of about 25pF should be fine which will need a trimming capacitor of roughly 150pF across it. Take one turn off the 25 turn winding at a time until with the tuning capacitor unmeshed your RX tunes to roughly 3.7MHz. Now varnish the coil......

When the varnish is dry adjust the value of the trimming capacitor until the RX just tunes above 3.6MHz (say 3.63MHz) with the tuning capacitor fully unmeshed. When fully meshed you should easily tune below 3.5MHz

Now if you want to band spread 3.5MHz to 3.6MHz over as much of the dial as possible the next bit has to be carried out with care as it involves guessing...if you get it wrong you might need another tuning capacitor!

Always place the receiver in its cabinet before noting and making any adjustments as this does affect the tuning range because of residual capacity.

Now tune to 3.5MHz...look at the tuning capacitor position....if you think removing one moving vane would still leave you enough moving vanes to make up for the lost capacity, then take one moving vane off (like pulling teeth so take care not to damage the tuning capacitor) and then try tuning to 3.5MHz again. You will notice that 3.5MHz has moved closer to the end of the dial. Note how far 3.5MHz has moved from last time and guess if you could afford to take another moving vane off. Repeat this process and eventually you will have a receiver that tunes 3.5MHz to 3.6MHz with just a little overlap at each end.

Band spread has been achieved. Once the receiver is placed in its' cabinet, since this affects the coverage slightly, you can download a chart from this website and use it to calibrate your tuning dial.