The AM Forum

in my other thread, i was asking W4AMV about this topic and he recommended i make a new thread to get more opinions.

what are the advantages of one coil form versus the other. i had assumed that solenoids are preferrable due to Q values being higher but looking through the ARRL Handbook and Experimental Methods In RF Design i noticed a lot oscillator circuits using toroids.   

so what are the pros and cons of each coil type and what do you guys prefer to use?

Advantages - toroids are self-shielding, *can* be higher Q, and can be more easily made mechanically rigid.
Cons - toroids are much harder to wind, not adjustable, and generally (unless you choose the core and capacitors carefully) are less temp stable.   

I've measured a Q of over 400 @ 1 MHz on some lackluster solenoid, slug-tuned coils from the 3.58 oscillator in TV sets.  Some of those AirDux coils would certainly be higher.

Try both and see what works best for you.  There is no "right" or "wrong", but in my experience it's easier to build a stable oscillator with a solenoid-wound coil, and a slug-tuned coil will allow you to set the oscillator's frequency range.

Ive built many SS VFO's using 6 and 7 mix toroids and they have been amazingly stable, even at 17 and 20M.
Since fewer turns are needed as you go with a larger diameter, fine tuning is quite easy by adjusting spacing. The larger core is more stable as a bonus. Ive used T-68 thru T-94 sizes where many stick to T-37 and T-50 and have difficulties. Since those larger sizes are commonly used for switched input networks on amps with 100-200W of drive they are far from stressed in a VFO.

Also use the largest wire size that fits well on the toroid.

Carl

the May and September 2011 ER issues respectively show a 3.5 and a 7 Mc vfo using the T-68-6 core and a J310 (both employ diode tuning and use a trimmer for bandset).


 

thanks for all the great information. i have been using ceramic coil forms in my experimental oscillators thus far but i recently started testing T94-6 toroids with success. the size difference is so big that i could imagine simply running multiple switchable tank toroid based tank circuits and still use less space than a tapped solenoid coil.

KC2ZFA, are any of those J310 articles available online? i just bought a bunch of J310s and would be very interested in any articles that specifically document their use.