Converting from MW to Lw

[Pete_kaye]

I have been building various simple one transistor sets using crytal set ideas from various forums.I want to extend coverage on my present home made coil to cover 1500m or 198 KHZ.Presently It covers 100m ~600m with using only one gang of 2X500pf tuner.I have other tuner and fixed caps in my 'bits and pieces'.Is it possible to extend the coverage by adding in extra caps in parallel to bring in Lw or do I have to make up a bigger coil?I have done some calculations and it seems that the 'LC' product must be about 20 times larger or another 2000pF adding in .Is it as simple as this in practice ?Are there other ways?
TIA

[Steve_P]

Every single set design I've ever seen involves a bigger coil for Long Wave. Certainly that's the case in the Practical Wireless designs book.

Cheers,

Steve P

[paulsherwin]

It's possible to do it by padding with a cap and a few commercial sets did do it this way. You do lose sensitivity compared with a coil though. You'll need about 1000-2000pF.

If you don't want to wind a coil you could always use one from a scrap transistor set.

Paul

[Ian - G4JQT]

Easy way is to put a fixed cap across the tuning cap, although as Paul says you will lose sensitivity. If posible use a silver mica cap, or failing that polystyrene to keep the Q as high as possible.

Ian

[oldoddbod]

I would definitely go for the increase in inductor value, particularly in simple receivers. Just adding a cap. across the existing coil gives an inefficient L to C ratio and poorer results as has been mentioned earlier.
Brian

[Pete_kaye]

Yes my experiments confirm what has been said here.I am using a home made coil presently on a 3" tube with 10 turns of enemelled wire as the aerial /earth coil and on the same former 45 turns with 3 taps across the tuning condensor.The former is now full up .I have plenty more of the wire and wound this on a separate 4" plastic pipe but not sure how to add it in ,Does it need to be inline /below the original one?The circuit is found on http://www.crystalradio.net/beginners2/index.shtml as Oatbox set #2

[paulsherwin]

The coils just have to be in series. not physically above one another.

I'm no expert on this RF stuff but I think PVC piping (sink drainage pipe etc) isn't too good as a coil former because it lowers the Q of the coil. Cardboard is better, the stuff from inside rolls of foil or clingfilm is ideal, or you can use a small glass jar such as a meat paste jar. Something to try maybe?

Paul

[Ed_Dinning]

Hi Gents, cardboard is definately better as it has a lower Er or permitivitty than plastic, so the Q reducing self capacitace is reduced. If you want to be a purist air is the best, but failing that by spacing the winding off the former by means of several matchsticks round the diameter of the coil it can be improved further.

Ed

[jim_jobe]

Some of Sir Douglas Hall's designs used a single coil for both LW and MW. He achieved this by putting a fairly low-value fixed capacitor in parallel with it and then using permeability tuning.
Jim

[Biggles]

I must admit I have never had much success with long wave crystal sets. I think it is because we have a high powered transmitter (Stagshaw, ERP 50KW)on Medium Wave a few miles away and this tends to break through due to the low selectivity of the set. You don't really need any components apart from a diode and headphones to pick up the transmission at our house! Most of the LW sets I have seen put the LW and MW coils in series when switched to LW.

Biggles.