Airframe & Radio Question

[challenger_i]

I am contemplating building a stand-way-off-and-sneeze scale model of an ultralight airplane. I have a lot of aluminum tube onhand, and want to use faom board ribs, ali framed wing, tailplanes and (ahem!) fuselage. Covering will be polyester dress lining (this stuff works great for covering!). It will be approximately 1/3 scale, and will have a converted weed wacker engine (I'm hoping to be using the "Gas/Glow" fuel system, so no radio noise from the ignition).

I was wondering just how much radio problems I will have, with an FM radio, and all that aluminum hanging about.

Thoughts? Hints? Suggestions?

[BMatthews]

Just don't put the antenna wire inside a tube.... :D

Seriously if you just run it out of the model as soon as practical and work to keep it decently away from the majority of the metal it should be fine. Today's radios are amazing bits of RF design.

If it means you are better off with a teflon tube inside one of the wings because there's less metal there then do that If you're using metal skins then don't. Also if you can locate it under the model it would be best since the ground link is below the model when in flight.

When you do your initial ground range checks have someone hold it up and present it at various angles from below and see if it hiccups. If not and if the antenna down ground range testing seems comparable with all plastic and wood models or at least close then you're home free.

[challenger_i]

Thanks, for the leg up!

She's going to be fabric covered, but the frame work will be aluminum tubing, with the odd broke channel piece, here and there.

The suggestion about having some one move the plane around to give a different antenna orientation is a good idea, until you consider this beastie will have about a 14ft span, and weigh in at about 30 pounds... Might be a tad awkward! :D

Well, at least I have faith it may work, now. No one has shot me out of the saddle, yet!

[MCarlton]

Why not use something like a deans mini whip antenna mounted on the underside of the fuselage?

[vintage1]

Why not use the frame AS the antenna?

[challenger_i]

Vint:

That is an interesting idea, however, I haven't seen too many full-size planes using that concept, so I'm a bit hesitant to try that. If some one were to show me where it has been used, I'd certainly look into the idea.

Why not use the frame AS the antenna?


Regarding the Deans whip antenna, its a goos idea, but I can do much the same by routing the antenna through a piece of NyRod, facing down.

[MCarlton]

Regarding the Deans whip antenna, its a goos idea, but I can do much the same by routing the antenna through a piece of NyRod, facing down

You can indeed, which would be fine. The only thing is that the deans is shorter, so that you wouldn't need some of the antenna wire coiled up in the fuselage to prevent it being too long and catching on some grass and pulling out of the Rx ;)

[BMatthews]

Vintage, it won't work since the airframe is so big and varied in shape. Antenna theory and practice says "DO NOT DO THIS AT HOME!".

Either a lower surface whip or mount the loose flex wire outside the model and as far as practical from as much of the metal as possible. And do not anchor the end to the metal structure. Instead tie on a non conductive extension that runs to the structure. The longer this extension, within reason, the better.

Something like sticking the antenna out the side and then angling it up to the mid point on the wing sort of like a "flying wire" would work. Just finish the other end with a good foot or more of non conductive line of some sort to the anchor point.

[vintage1]

Vintage, it won't work since the airframe is so big and varied in shape. Antenna theory and practice says "DO NOT DO THIS AT HOME!".


Beg to differ. Our rx antennae are anything but tuned. The only theory that counts is 'induced voltage is proportional to the length of wire transverse to the EM wave field'. They aren't directional either by design.


On HF sets it's just 'about a meter of wire' loosely coupled to the front end.


Could as easily be a mess of aluminium tubing and probably be as good, if not better.

[challenger_i]

Vint:

Would you clip the antenna, before attaching it, or attach it, as is?

Regarding using a piece of Nyrod, I could coil the excess around the base of the Nyrod, outside the fuselage, and run the balance through the nyrod by entering in through a hole drilled "downstream" from the coil.

[Rodney]

A couple of things for thought. If there is any metal to metal contacts in the frame that can vibrate against each other, you could have a lot of noise created that will swamp out your signal. solution: make sure any metal to metal contacts are very firm (can't vibrate or rub against each other) and, best to ground the entire framework to the negative terminal of the power supply. A shortened whip antenna is probably not the best idea, any shortening of the antenna (unless retuning of the receiver is properly done) will seriously shorten your range, at least by 50%.

[vintage1]

A couple of things for thought. If there is any metal to metal contacts in the frame that can vibrate against each other, you could have a lot of noise created that will swamp out your signal. solution: make sure any metal to metal contacts are very firm (can't vibrate or rub against each other)


Yes. Mandatory.

and, best to ground the entire framework to the negative terminal of the power supply.



This seems completely wrong to me.
The metal will, if not used as an antenna, interact with it. Its more likely to interact more if its grounded.

Once you ground it, you have in a sense made it truly part of the antenna structure.

There is no simple answer here. Its necessary to really consider all the metal bits and the antenna as a complex unit.

The mathematics is horrendous.

In reality, I would recommend simply trying all possibilities, and doing range checks.

A shortened whip antenna is probably not the best idea, any shortening of the antenna (unless retuning of the receiver is properly done) will seriously shorten your range, at least by 50%.

Not true. There is virtually no RX antenna tuning on a 27,35,36,40, 50 or 72Mhz set. Only in the case of a short whip is tuning used, to some effect. But even then range is badly affected. But that's because its short, and the tuning is used to try and get the signal back, but the success is only partial.

Generally the range versus antenna length goes up as the antenna gets longer, gradually falling off when it gets too long.