Not sure if title is correct.

I have been working on a 1 tube Reflex radio from the mid 20's posted on Radio Broadcast.
Before I proceed I would like to research the lack of selectivity and selection I obtained today when I hooked up the radio.
I already have another thread regarding the variable condenser posted.
The coils were supposed to be wound so many turns on a 2 5/8" OD form. I had a 3" OD form and keep the turn count the same.

How would a larger diameter coil form with the same turn count affect the receiver?

Should I have done the math for say 60 turns on a 2 5/8 dia and reduced the turns for the 3" form to keep the wire length the same?

Or is the coil turn number critical?

This fascinates me - sorry for maybe dumb questions.

But before I move onto the upgraded 2 tube reflex circuit on Radio Broadcast magazine I want to be sure I have the basic correct.

Here is a link to the publication. It is from Nov. 1923.

http://www.vacuumtubeera.net/RadioBroad ... azine.html

So if you used the same number of turns but on a larger form you used more wire. This means that the coil will now resonate at a lower frequency, all other things being the same. So the answer is Yes.

In order to cover a specific frequency range, a coil is designed to have a particular inductance. Inductance depends on the diameter of the coil, the length of the coil, and the number of turns. The wire diameter plays a role, also, but usually not critical.

http://www.crystalradio.net/cal/indcal2.shtml

If you know the dimensions of the coil specified in the article, then you can use the above calculator to get its inductance. Knowing that, you can change the diameter and number of turns to see if you can get the same inductance with a different combination.

If you choose a larger inductance (more turns or more diameter) the tuning range will be lower than originally specified. If the inductance is smaller (fewer turns; smaller diameter), then the tuning range will be higher in frequency.

Generally, it's easier to put some extra turns on a coil, then "prune" to get the range you want. It's hard to add wire to a coil.

Rich

From my 1965 Allied Electronics Data Handbook:

For a single layer cylindrical coil:

L= (r*N)^2/(9*r+10*a) where: r=radius, l=length, N=#of turns. r & a in inches, L in uH.

Can also be found in your ARRL handbook. Screwing with any physical dimension, or the number of turns will alter the inductance. Higher inductance -> lower frequency, and the other way around.

Thanks for the link - I plugged in my numbers and there are big differences. Will need to find smaller coil forms or remove some turns.