Best Antenna Solution

[Gary Mortimer]

What would the best way of getting a 900/1800/2.4 Mhz signal back to a fixed point??

Should the aircraft have the best antennas or would it be better to have a very high gain receiving solution??

Would making my own antennas for the aircraft be any use???

What do all the grown ups out there think?

G

Still as all of what we are doing should be within sight of the aircraft, should it matter.

[zik]

This stuff is more complicated than it sounds, but the simple rule of thumb is to use a decent antenna whenever you can. For the frequencies, power and range you're interested in most reasonable quality commercial antennas will be ok.

Don't bother trying to make your own - antennas must be resonant to be any good at transmitting and you need good test equipment (or dumb luck) to make a good resonant antenna.

If you want to learn more about making antennas I'd suggest you look into doing an amateur radio course - this is the kind of thing ham radio is all about.

[Gary Mortimer]

Perhaps then a source of good antennas??

I keep seeing rubber duck antennas sticking out of aircraft and can't help thinking they must be knocking aerodynamic efficency fairly dramatically.

Something along the lines of a dipole of the correct length embedded in the fuselage is what I was thinking. Although I realise that might bring directional problems. Perhaps not so bad if vertical.

Does anybody put groundplanes under the rubber ducks they use??

Would it make a difference?

Would it be better to fit antennas to the underside of the aircraft if possible??

Has all this been asked and answered in other places on the forum!!!

G

[zik]

Well let's see... For a maxstream modem running on ~920MHz you could make an antenna out of foil tape stuck on to the surface of your plane.

Let's say a 915MHz 5/8 foil whip stuck on to your vertical stabiliser. That'd give you a good omnidirectional radiation pattern. By my calculations it'd have to be about 19.5cm (7.7") long so you could probably fit that on most decent sized planes. You'd need an antenna analyser or at the very least a SWR meter to get it right though.

Alternatively you could use a sleeve balun to get a balanced output and feed a center-fed dipole. You could make it a bit shorter (15.6cm / 6.1") and it'd probably perform a little better than the 5/8 whip but it might be physically awkward to center-feed in the middle of the stabiliser.

[lucent]

I would at least try a bi-quad for 900mhz. It shouldn't be too large and you won't need a matching balun. It is balanced by design.

[Gary Mortimer]

Thats two squares basically is it not.

Good for picking up odd signals from different angles??

[lucent]

Thats two squares basically is it not.

Good for picking up odd signals from different angles??

Yes, Gary it is two squares or two circles. A circle is marginally better, but I wouldn't take it that far unless the square works.

Edit: Adjust the distance in the middle for best reception.

[Gary Mortimer]

Thanks Lucent, that does not look too onerous for airframe dimensions either.

I'm wondering how whacked out the SWR might if one just built one.

Found this interesting design...

http://www.qsl.net/n9zia/wireless/pics/900quagi.gif

[zik]

Yes, Gary it is two squares or two circles.
What kind of radiation pattern does that have lucent?

[lucent]

This is a wifi antenna for 2.4ghz. http://www.lecad.uni-lj.si/~leon/other/wlan/biquad/index.html

Probably, best used with horizontal polarization for our needs.
RC-Cam thread. http://www.rc-cam.com/forum/index.php?showtopic=28

[kd7ost]

If you're flying within the range of the standard transmitter, I don't think you need to get too elaborate. Here's a couple of pictures from my own set up.

I did put a ground plane on my fuselage underside on my larger sized Ag AP plane. The ground plane is cut from some really thin aluminum sheet from the local hardware store. The metal case of the 2.4 Ghz transmitter is mounted to it inside the fuselage to make electrical connection. You can see the air cooling scoop underneath as well. This serves to direct the radiating pattern downward as well as out from the antenna to keep RF from radiating skyward and out. I don't need RF going above the plane. Just down to the ground.

As an aside, I have accidentally broken off the outer case on one of those 2.4 Ghz plastic housed antenna's before. The outer end piece is glued on to the antenna base. Inside is a short wire about 1/3rd the length of the plastic housing. Its also pretty small in diameter. If you wanted to reduce drag on a small plane you could pull that plastic end piece off and put a plastic rod like a nyrod scrap on the wire antenna to keep the antenna stiffened yet flexible in case of impact. The antenna supplied with those little 2.4 Ghz transmitters is in a pretty large housing for what the actual radiating element needs.

On the ground I simply use a patch and keep it aimed at my plane. I'm also using 500 to 600mw at the transmitter. I don't think I need the high gain patch on the ground. It's overkill but works well in my missions. I think a standard Omni directional antenna on the ground would be fine unless I'm directly overhead and am working off the antenna nulls. But I don't have a need for flying overhead like that.

Dan

[Ceros]

Gary - there are so many issues to consider here mate, the issue is not that straightforward, but all things equal:

1) your best freq choice will be 900Mhz - because it requires the least power for the same gain, and suffers the least free space loss for a given rf power/gain, and is least susceptible of the 3 to propergation problems.

2) max out as much as you can on the ground station antenna - construct a multi-turn helical for 900Mhz - it will give you the most gain possible for a given size, and in terms of Rx ability has one unique ability that most other antenna's don't have - it is not fussy about the UAV TX signal polarisation.

3) use a omni directional single element on the UAV - and again the same rules as in 1) apply here - 900Mhz will be your best choice of the 3.

One thing you havent said - is how big the UAV is?

[Ceros]

Perhaps then a source of good antennas??

I keep seeing rubber duck antennas sticking out of aircraft and can't help thinking they must be knocking aerodynamic efficency fairly dramatically.

Something along the lines of a dipole of the correct length embedded in the fuselage is what I was thinking. Although I realise that might bring directional problems. Perhaps not so bad if vertical.

Does anybody put groundplanes under the rubber ducks they use??

Would it make a difference?

Would it be better to fit antennas to the underside of the aircraft if possible??

Has all this been asked and answered in other places on the forum!!!

G

Gary

Nothing wrong with an antenna "pole" of any length - so long of course as it is tuned correctly to frequency and ratio'd (i.e. 1/2 wave, 1/4wave, 5/8's etc etc ...) for the freq it is going to be used at/for.

Dipoles, or any poles (of any length or ratio) should be embedded into a fuselage as your absolute last choice.
The reasons are, amongst others, also the one you correctly noted - that of directionality. Poor groundplane is another of the problems you'll run into - in fact you'll have 50% of the transmission lobe running right into the groundplane - not good at all.

Your best option on (on balance) is in most cases a single element tuned length vertical antenna - and vertical can mean upside down as well (meaning underneath the fuselage) - with a ground pane.

Tin foil with a spray on adhesive works very well indeed - because of its thiness it is easily cut and "molded" to curve with wing and fuselage contours, it adds no significant mass and will adhere well with any decent spray on adhesive - and can then be painted over.

Antenna tuned element length is not much as an issue as is width. There will always be a trade-off one way or the other with small UAV's when it comes to antenna's, and the laws of nature are such that they are impossible to beat - so just accept them. The problem with antenna width is that is really adds considerably to drag, so keep it as thin as practically possible - although you can actually get to thin - a consideration with spread spectrum and frequency hopping Tx/Rx modems, as well as broad band video Tx modems which "spread" the video over a single carrier that maybe as much as 4MHz - 8MHz wide - as the diameter of an Tx element is related to its resonance at any bandwidth.

Don't be to concerned about the deviation of the E and H plane (polarisation) of the Tx signal from an antenna that is mounted verticaly on a fuselage, but which then bends back at ay 20 - 40degrees from vertical during flight. Yes the propagation lobe (doughnut shape, figure of eight shape - changes depending on wheather you use a 1/2 wave, or 1/4wave, or 5/8 or whatever tuned length) will bend to the back slightly, but so long as you are within range, if you are using a ground/recieve antenna (like the helical design I described earlier), this will more than adequently compensate for the distortion - so long as you are within range.

In summary: use a single element pole type of antenna if you wish - thats fine, keep it thin, don't worry about it bending during flight (so long as the recieve antenna is of a decent design), and do build a decent groundplane which is incorporated into the fuselage and wings -spread out equaly each way as much as is possible.