For general reception and for my U3 transmitter I needed a simple ATU. There is nothing complicated about this circuit. It's a simple L-match that uses a cheap polyvaricon capacitor double ganged to make 600pf. The inductance is wound on a ferrite rod with a permeability of about 2000.
The circuit is drawn below. The ferrite rod came from Spiratronics in the UK, see my useful links page or Ebay. The polyvaricon came from the GQRP Club sales, cost about £1.50 (Power handling is easily 5 watts) A simple LED tuning indicator is made from a low current LED and a diode is connected back to back to STOP THE NEGATIVE PART OF THE RF WAVEFORM FROM DESTROYING THE LED. A 30k resistor in series with the LED to make sure that the brightness is not too high, otherwise it's difficult to see the peak. This simple circuit means you don't have to worry about obtaining a VSWR indicator for the LF bands ! It works perfectly.
A little experimentation may be required to optimise the number of turns to tune both bands. It's best to start with 100 turns first and then take off a few turns to get the best impedance match on 475Khz first, and then add the second coil for best results on 136Khz.
THE EXTERNAL VIEW
THE INTERNAL VIEW OF THE MARK 1 VERSION
THE MARK 2 CIRCUIT
(NOTE - u=2000 for the ferrite rod)
475Khz band is tuneable with either 0.5mH or 1mh.
NDB / LW broadcast is split between 1mh and 2mh.
136Khz is 2mh.
I decided to make a MK 2 version. I now wanted to cover the NDB band and the LW broadcast band. So I needed coverage from 136Khz - 600Khz approx. I did not want to start everything again, so I tried to think of a way of re-using the existing box etc. Firstly I rewound the inductance, to make 3 seperate coils. The next problem was the inductance switch. A 3 way rotary switch would be a problem to fit. So I devised a way of getting 3 inductance values from a simple ON-OFF-ON style switch. This will fit into the existing box, with no modification. The switch / band positions do not go in sequential order, but this is a small price to pay. You have to be careful wire the coil and the switch, otherwise you will get strange results. You will then be able to have an inductance that goes 0.5mh - 1mh - 2mh (approx) I measured this on my LCR meter.. A 100pf 50v fixed capacitor across the 600pf variable give me my preferred tuning ranges for the 3 bands of interest. Individual users may leave it out, or experiment a bit. 200pf was too much, and spoilt the 475Khz tuning.
THE MARK 2 INTERNAL VIEW (AND I MADE A NEATER JOB!)
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