Best external antenna/cable for Moteino?

[Humancell]

I'm curious ... do you think these helical antennas would work well if put directly onto the Moteino PCB?

I've been looking for a way to bend or "coil" the 80mm wire antenna in a more optimal way to fit into an enclosure ... I had not yet run into these.  Are these supposedto be coiled as some optimal size?

The link to the 'antenna' in the ebay page you mentioned is known as a helix antenna and it's only one half of what you need; it is not a complete antenna and I'm not surprised if you have poor performance if you just solder that to one end of a coax cable. What you need for that antenna to work is a ground plane (aka counter poise) and is somewhat provided by soldering the the helix immediately on the antenna port of the PCB radio module. The GND tracks of the circuit board provide a poor, but often sufficient ground plane.

Antennas also have a property called Resonance. That is, they work specifically well around the resonant frequency but can be very poor performers if you go too far away from the design frequency of the antenna. I would therefore not recommend to use a 2.4GHz antenna for a 915MHz radio. Some GSM/3G antennas could have a working range that covers your 915MHz radio, but I would only use these for receive and not for for transmitting. Not even transmitting an ACK packet.

When you say "best variant" and "works good" you may need to specify which area is most important to you, as 'best' for one is not 'best' for all.

- least losses
- furthest range
- smallest antenna
- most immune to interference
- cheapest

TV cable is usually 75 ohms characteristic impedance, but if you got 50 ohms, see if you can find a datasheet for it and check the losses per meter/foot at 915MHz. If you want least signal loss RG-58 is not the answer.

The length of your cable can be chosen arbitrarily if your antenna has a perfect 50 ohms feedpoint impedance, matching the radio and the feedline. If your antenna is not 50 ohms then you may want to calculate the length of cable by taking into account velocity factor and increments of half-wave electrical lengths.

For connectors I would stay away from PL-259, but N-type, BNC and SMA are all 50 ohms at UHF. The N-types are just too big and bulky for me. I have used some SMA connectors and they are nice and small, fairly robust and not too expensive. I like BNC too because it attaches with only a small twist and any strain from misalignments in the two connecting sections can be easily taken out because the cables still allow for rotating while mated.

[captcha]

Without hooking these helicals up to a vector network analyzer it is anybody's guess whether these things are properly made and tuned for the advertised frequency. However, since many people report reasonable levels of success with all sorts of antenna's it may be worth a try.

put directly onto the Moteino PCB

That would be my preferred placement for the helical.

Are these supposedto be coiled as some optimal size?

Very much so. I would not suggest to further bend or stretch or otherwise manipulate the shape of the antenna and only use the antenna for the designated frequency.

Just out of curiousity, what range are you hoping to achieve for your project? Are you very limited to the enclosure size?

[perky]

Speaking in very simple terms, reducing the length of an antenna decreases capacitance, so to make it resonate at the same frequency as a dipole of 1/4 wave monopole you have to add inductance. A helical antenna is basically a coil, get the dimensions of that coil and number of turns right and you can add just the right amount of inductance to resonate at the same frequency as a dipole or 1/4 wave monopole. However shortening the antenna also reduces its radiation. You'll see some compromises that have bottom coils and the rest is straight, but the result is the same, the coils (and the helix) add inductance.
Mark.