Small as possible 30.8 MHz antenna

[Berni]

We use RF garage door remotes for a lot of things and i started designing my own to merge all of them in to one.

To do all the things i want it needs to be able to transmit on both 433MHz and 30.8MHz. Im not too worried about 433 since i can get a reasonably compact chip antenna. What i am worried about is the 30.8 MHz. Antennas that go this low tend to be huge. Even if i could find a chip antenna for this frequency it would still likely be too big. What makes it worse is that one of the doors that it opens has crappy sensitivity on its receiver, so i also want to have a reasonably high transmit power as well.

For inspiration i have looked at how these remotes look inside and i found a ferite rod with a few turns of wire around it. A rough schematic of what seams to be going on is attached below. The way i understand it is that the top part of the inductor is tuned to resonate at the correct frequency while the bottom inductor pumps it with energy. Once it gets in to the swing of things the top inductor probably starts oscillating with a amplitude many times the input voltage while bleeding some of its energy out in to the air as RF. Im guessing its a really crap antenna at its working frequency but because the amplitude is so large it still manages to get enough power out there.

Is this a good way of doing it and simply pump more power in to it?

[Berni]

The largest ferite rod i could fit in there is 4cm long and about 7cm thick since i (perhaps stupidly) decided to make it fit in to a rather compact case as these sort of remotes tend to be.

My power output is obviously limited by the also tiny battery that i can fit in here. Perhaps by adding a tiny supercap i could mustard up to 1W of power in the transmitting pulses(Input power, not RF output power) but id say up to 100mW is more reasonable.

Hopefully i can find a suitable rod that is small enough. The rods used in AM radios are too big and id imagine most ferite cores used in transformers and inductors are unsuitable for use at 30 MHz. But i will probably use one of those AM radio rods as a proof of concept on the bench to see what sort of RF power it can put out since i can always break the rod in to shorter chunks.

[cdev]

Magnetic loop? They are very simple, very small and really can't be beat for their size. They are semi-directional, with a sharp null. Read up on them. Very easy to make. Also, bandwidth at resonance is very narrow. But for a single frequency application thats also an advantage.

[Co6aka]

Look at the methods used to make antennas for 13.58 MHz. The same techniques apply to your freq.

[Berni]

Thanks for all the tips.

I was never all that familiar with antenna design so i decided to play around with it a bit. I did some trial and error in LT Spice by modeling the loop antenna as a transformer. I did notice that the coupling between the small feeding coil and the large resonating coil has to be weak, otherwise the tank circuit bleeds energy back out trough the feeding coil.

I also played around with coils of wire around my spectrum analyzer. To get a resonable amount of inductance in a small coil i did make multiple turns but later read that this is bad due to making the Q factor worse. Attached a picture of one of the test examples i built. I didn't worry about getting the resonant frequency spot on, but instead just changed the input frequency up and down a bit to fine tune it, usually i landed within about 1 to 3 MHz of my target. The tank circuit did work and i could pick up its signal across the room with a few dBm of input power. The frequency response is indeed sharp, but not too ridiculously so.

I did also feed in a square wave signal as the input to see how that goes. Does still radiate some harmonics but at much lower powers so driving it with a saturated switching transistor is not that bad of a way to do it. Playing with a piece AM radio ferrite rod did not show good results tho. I used a slow square wave on extra coil to excite my tank circuit and watch it ring. Air core inductors seamed to ring for quite a few periods before fading out out while using a ferite core would damp them out in just a few osculations. Could be that my ferrite is too lossy at 30MHz or i need to drive it diffidently or something. So for now i have stuck with air core inductors. But i am also using just general jelly bean SMD ceramic or film caps. They have no Q factor rating at all so they could also be lossy at these frequencies. I should probably order some proper RF grade high Q caps.

Was quite interesting to discover this loop antenna design. The HAMs seam to be capable of getting really good transmit performance out of them, tho running 100s W of RF power in to those leads to some rather scary currents and voltages on that loop.

My first plan is to try make the remote and see what sort of range i can get out of it. All the other things it operates could welcome better range, especially when used inside a car where you need some extra oomph to get outside when you don't point it out the window. If that badly sensitive door still doesn't have satisfactory range then i might crack open the self contained motor mechanism and see if i can extend the antenna out of it on to a long wire.

All this RF stuff is a field i am getting in to. I been designing digital and IC based analog for a long time while never really getting in to any of this RF voodoo. So i hope i don't cause too many facepalms here in the RF section of the forum (I don't think i even read more than 1 or 2 posts from this part of the woods)