Total posts : 45366
Here are some differences between an open form coil and a coil such as the SSTRAN loading coil:
Open form coils have:
Bare windings which make tapping easy.
Spacing between the windings which increases the interwinding breakdown voltage allowing high power operation.
Virtually no form material which reduces dielectric losses from the form material.
Lower interwinding capacitance which raises the self resonance frequency caused by interwinding capacitance.
Possibly higher Q (lower resistance) compared to narrower, longer coils for a given L.
Are available as “coil stock”, so the builder need only cut off the coil to get the L needed.
The coil in your picture appears to have shorter and wider dimensions than the SSTRAN coil but I don’t see any advantage of this over the SSTRAN coil with one exception. A coil close wound with insulated wire will have a lower self resonance frequency that the open form type. I don’t know what this is for the SSTRAN coil, but I wound a similar coil using #14 insulated house wire and measured the self resonant frequency at 3.3 MHz. This is about the second harmonic of the upper end of the AM band and may allow second harmonic radiation above desired levels.
Other than this, considering the low power and low frequency of operation for part15 AM, and considering that the resistive losses of the coil are small compared to ground losses and need not be mininmized, I see no advantage of one over the other.
Neil
