Yes, you're definitely on the right track there. And thanks for trying out some calculations, it shows us you're keen to learn!
. RF isn't rocket science but it does have a number of variables that usually don't play a significant role in many other hobbyist projects.
I'm actually open to antenna recommendations, but want something that can attach to a bulkhead connector on the enclosure, and provide decent gain/signal.
The gain and signal properties can work just as well (maybe even better, because you're not putting an antenna right on top of the Moteino) when using bulkhead connectors. In one of my projects (not Moteino based, but it does use a HopeRF radio module), I specifically *want* to be able to easily interchange antennas and I went with a custom pcb design that has pads for putting an sma connector over the edge over the circuit board and the connector just pokes out the die-cast aluminium box.
An important difference between all the various antennas is radiation pattern. Just to get the dialog going, you have three main choices:
- If you want to transmit and/or listen all around you (from above and below, but not straight up or down): go dipole in a vertically polarised mounting position.
- If you want to transmit and/or listen in two opposite directions (e.g. North-South): go dipole in a horizontally polarised mounting position.
- If you want to transmit and/or listen all around you, bu within a limited angle of 'elevation' (and not straight up or down): go quarter-wave vertical
- If you want to concentrate the signal into a narrow beam (hmm.. more fat sigar-like) for listen or transmit: go yagi
If you look closely at a yagi, you'll see that the driven element is just a dipole.
If you look closely at a quarter-wave vertical, you'll see that it's just a dipole with one of the legs replaced by a ground plane.
Check out this awesome link from John K2OX explaining many aspects on the topic of antennas:
https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/antenna-tutorial-or-antennas-in-a-mote/I guess the question is: What's your application?
For coax with Vf = .79, 1/2 wavelength = 12.95 cm
Yep, for 915MHz and that velocity factor that looks correct.
have that much coax from the Moteino PCB to the SMA? Or do I count the internal lengths of the SMA and Antenna SMA in this number?
My suggestion would be to minimise the length of coax where possible. Especially on those higher frequencies the losses in that type of coax is quite substantial.
In your enclosure I would use maybe a few cm's of coax, just to connect the radio module to the sma connector. The sma connector should have PTFE (Teflon) dielectric (same as the RG-316) so it should have a similar velocity factor. Then, add all the lenghts all up and make sure the total electrical length is close to (a multiple of) a half-wavelength.