On Apr 23, 6:01*pm, "Joel Koltner"
wrote:
Are there some vendors of readily available (stock) inductors that have
reasonably good Q's (100) throughout the HF band?

I'm familiar with something like the Coilcraft "maxi springs"
(http://coilcraft.com/maxi.cfm), and while they hit Q's of 100 by 30MHz, I'd
really like something that's already there by 3MHz or less (...and still have
a self-resonant frequency of 30MHz). *This seems quite doable simply by
making a larger coil with thicker wire, I'd just like to find a place that
offers such inductors off-the-shelf rather than having to wind my own.
Inductance in the ballpark of 1-10uH would be good, although it's not
critical.

The goal here is to try out making some adjustable notch filters for a
receiver (by switching L's and C's in and out, like antenna tuners do).
Hence, high Q is important, but power handling capability isn't so much, and
anything smaller than a breadbox is fine size-wise.

Thanks,
---Joel

You'll be limited by physics, Joel...I know you know that, but I think
it's worth putting bounds on things. If you're willing to use a coil
on a powdered iron core, you should be able to get the sort of Qu
you're asking for in a fairly compact coil. If you want to order
production quantities, you can easily find vendors willing to wind
specific coils for you, but if you want just one or a few, you may
have more difficulty. I've had situations where I wanted to avoid the
distortion of a powdered iron core, and used some "air core" coils
(actually phenolic, I think) from API Delevan and equivalent ones from
Gowanda. They were "stock" values, but I learned that they commonly
build them when they get an order, but try to have a few around they
can ship as samples. But again, they aren't as high Q as you want at
the lower HF frequencies.

For an air-core coil at HF, you can estimate the size required for a
given Qu pretty accurately as Qu = 100 * D(inches) * sqrt(f(MHz)).
That's about right for a coil the same length as its diameter.
There's a chart in the Sams/ITT Reference Data for Engineers book that
gives essentially a correction for the length, if it's shorter or
longer than that. At lower frequencies (AM broadcast band, and up to
a very few MHz) you can do a bit better with the appropriate Litz
wire. Anyway, for example at 4MHz, expect to need an air-core coil
about 1/2 inch diameter to get Qu around 100.

It used to be that you could buy a supply of AirDux or equivalent coil
stock, and just cut off the number of turns you needed for some
specific inductance. My answer to that these days is to thread some
plastic tube on a lathe, and have a few different thread pitches
around, then just wind magnet wire down in the groove to get what I
need "on demand." It's easy to do the winding and makes a nice,
uniform-looking coil. I've used that technique to make sets of coils
for reasonably sharp cutoff bandpass, lowpass and highpass filters
with very low distortion that I've used to characterize the distortion
performance of amplifiers and the like.

Sorry that I don't have a better source of ready-made coils...maybe
someone else has.

Cheers,
Tom