Receiver antenna

Oh indeed, but all transmitter antenna are bottom loaded these days anyway..least ways on 35MHz they are.

But there is a distinct difference between a transmitter with a whip that is more ore less clear of any obstructions being held by a person who is more or less grounded...and a little receiver in a plane full of metal and carbon bits at unpredictable locations, that is not even able to be extended in a straight line.

Transmitter antennae are tuned because they can be. Receivers are typically not, because they can't be.

Phil. Design me an arrangement of wires to go in a scale plane that has least nulls in it..I was thinking of down the fuse and up the tail, and then from wingtip to wingtip..and then solder the receiver antenna (cut back) to the junction of them all.

TX yes,bottom loading coil.

Rx, no. Its loosely coupled into whatever comes next. On GWS parkfly receivers, its a few pF into the mixer straight. Not a resonant circuit in sight.

Others use a single RF coil, with again the antenna VERY loosely coupled (so that what you do to the antenna doesn't affect the tuning).

Some people use very loose coupling and pretty high Q..these sets benefit from being peaked up to the exact frequency in use. But again its not the antenna that is being tuned, its the coil assy after it. This helps rejection of image freqs on SC sets.

I suspect futaba 'high band/low band' stuff is like this.

Perhaps

Indeed. At least one kit manufacturer simply says 'somewhere between 12 and 50 inches..not critical, tune input coil with receiver in place in model...'

Those sets are very very sensitive once tuned up.

I was about to say, check with your girlfriend..

Couple of weeks ago i was helping a friend outfit his Rogue (a new bipe from Fliton). He was gonna use the same antenna he'd been using in several other high-end planes, tranfrerring it from plane to plane. It's base loaded and the whole works, loading coil and main element, are

From pcoopy:

In any case the 'donut' or toroidal pattern will be pretty distorted unless the whip is vertical to the ground. So far nobody's talked about the helical-wound (or 'rubber ducky') Tx antenna. Maybe NP could elaborate about its unique radiation pattern and polarization of the radiated wave. oc

| > I would guess that 35 MHz and other bands (75 MHz, 50 MHz, 27 MHz, 40 | > MHz, etc.) generally just use a matching network to make their | > antennas approximately resonant, both on the TX and RX ends. | | TX yes,bottom loading coil. | | Rx, no. Its loosely coupled into whatever comes next. | On GWS parkfly receivers, its a few pF into the mixer straight. Not a | resonant circuit in sight.

The GWS park flyer receivers aren't really the best example of a receiver designed to give good range, as they're only meant to provide

| Others use a single RF coil, with again the antenna VERY loosely | coupled (so that what you do to the antenna doesn't affect the | tuning).

RF coil? Wouldn't that be a ... matching network? (even if a poor one?)

The 27 MHz RX's that I've opened up have all had a coil between the antenna and the rest of the receiver. (I haven't messed with 35/40 MHz equipment much.) I doubt they go to any signifigant pains to make sure that the antenna/coil is just right for the chosen frequency, but they do seem to at least try to be somewhat close, since by failing to do so you'll signfigiantly reduce your range.

| > But I've seen little evidence that most R/C manufacturers carefully | > tune their radio gear -- they just cut antennas and such that are | > about right, and leave it at that. | | Indeed. At least one kit manufacturer simply says 'somewhere between 12 | and 50 inches..not critical, tune input coil with receiver in place in | model...'

But that input coil is the matching network. Without that, going from a stock 40 inch RX antenna to a 12 inch RX antenna will greatly reduce your range.

You assume too much jerk.

I've designed and built several of my own RC receivers, tested them and a variety of commercial RC gear, with "long wire" antennas and with loaded whips and loaded tails.

Is your knowledge of electronics so sparse that you need to shift the topic in order toattempt to make your smarmy points?

I really don't care what anyone does to their antennas, but I do want modelers to know there is a risk in tampering with them.

Not me.

Not necessarily.

Depending on waht you mean by matching..

No...what is generally done is to lead the antenna in by either a very small capacitor, or into a single turn of wire round the coil..the main tuning *is* tuned to the band, and is fairly high Q, but the effects of the antenna on it are reduced by the very weak coupling.

Typically there is then another step down winding to impedance match to the mixer.

No, its not.

The tuning is broadly unaffected by changes to the antenna. A little, but not markedly.

In the set I am thinking of, the antenna goes via a 4.7pF capacitor to a tap on the coil primary, the whole of which is tuned, and a secondary winding feeds the mixer.

Now that capacitor has an impedance of about 1k at 35Mhz.

Compare with the impedance of a quarter wave whip of about 50-70ohms, and you will see that there is bugger all matching POSSIBLE via such an arrangement. and in fact in 35MHz is about an eighth wave, so likely even lower.

The input impedance presented by the coil assembly is probably somewhere in the 50-200ohms range. A complete mismatch.

However it doesn't matter, since impedance matching is not the only way to get the most out of a system..Its easier to build in gain, and theory shows that the S/N ratio of a mismatched system is nearly as good as a properly matched one.

Matching is only really important when you are talking POWER. Or when you are pushing the theoretical limits of a system to the absolute limits.

Nope, I had assumed that that was *your* game.

Sure, but not half as much as some people like to claim there is.

I have a "park flyer" rx - one of these micro things (35MHz) and fin the range minimal. I measured the antenna to be about 18" and wondered if I increase it t around 40" if it would help ?

I measured my other receivers on the 35MHz band and they vary betwee

I guess starting at 45" and then trimming it down doing range check all the time would eventually get there?

I use this rx in a rubber model "Sparky" converted to electric for SO event so I need to get it up pretty high but currently am not able t get too much height or drift away before I lose signal on it.

Mar

-- zl3vm

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Probably yes.

If there is a tuning coil near it, re-adjusting that may help too.

If its a hitech feather nothing will help. ;-)