Inductors have several different and important properties besides
inductance -- loss (Q), stray capacitance, and temperature sensativity
among them. If the coil is wound on a magnetic core and DC is passed
through it, core saturation can be an important consideration. Which
properties are important depends on the application. So you'll very
often get badly reduced performance, or a non-working circuit, if you
substitute one type of inductor for another without knowing much about
them. Consequently, a decently written article should include details
about how each inductor is made. If that information isn't provided, I
suggest contacting the author of the article to find out what he used.
That's just about the only way of insuring that you'll get the same
performance the author did.

If you're interested in building QRP receivers, there are a number of
sources where you can find good circuits which include the necessary
information to build them. The very best, although perhaps a bit
expensive for a beginner, is _Experimental Methods in RF Design_,
available from the ARRL and other dealers. Less expensive, and a
tremendous bargain, is a CD containing the contents of 109 issues of
_Sprat_, the magazine of the G-QRP club. For only $25 + $4 shipping ($15
+ $4 for G-QRP club members), you get an amazing amount and variety of
information. It's available in the U.S. from Kanga US, contact Bill
Kelsey at . To order from other contries, go to
http://www.gqrp.com and click the "Club Sales" link.

Roy Lewallen, W7EL

Qas wrote:
Hi,
I am new to ham radio. Well I found lots of QRP receiver circuit on
web, I tried to build some. Everything going to be OK, except making
coil inductors. I don't know how to calculate them, with type I need
(air core or slug tuned), what a difference between coil and RF choke.
Let's assume I need a 100uH coil. How to star making it. I looked for
factory made coil at my radio shack store, but I only found bobbin for
coil.

If you know any interesting link on this topic, please let me know.
I'll appreciate any comment or help.

Thanks in advice,
Edgar
73