Receiver antenna

There are many different combinations of parts that will achieve the same end result. While 72 MHz does follow the same rules as real RF the wavelength is so long it takes a lot of change to notice much of a difference. I have not looked at the schematic for the RX we use but if I were laying it out I would make the front end pretty forgiving to the antenna impedance. The antenna is subjected to all kinds of abuse in the normal everyday world from getting a few inches chopped off, to sitting next to a steel pushrod, to being less than fully extended. It really is very forgiving when you think about it. The last thing most of us need is a RX that is tuned so tight on the front end that chopping off a few inches of antenna wire will cause a significant change in range and if the front end is peaked like that you can forget about changing out the crystal. As far as believing old wives tales I don't think it is me that is buying into them. The evidence is there to observe, the RX front end is tuned to accept a relatively wide band width.

Here is a link to an easy to use and understand calculator for anyone that wants to play around with wavelengths and frequencies.

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Have fun, Charlie

Reply to
Charlie H.
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The art of engineering design is to maximise the effects of what you are trying to achieve, without adding any influence from things you don't want to affect performance.

This is why cars have pneumatic rubber wheels. These work with physics, to have low resistance in the direction of travel, and high resistance against the direction of travel. They support the car,but absorb shocks from small stones in the road.

But I guess you are the sort of guy who takes a look at them and sniffs and says 'I'd like to know how modern wheel design makes them immune to physical laws'

Straw men as a way to divert attention from a losing argument is pretty sad.

Reply to
The Natural Philosopher

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.

Reply to
The Natural Philosopher

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.

Reply to
The Natural Philosopher

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.

Reply to
The Natural Philosopher

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

Reply to
The Natural Philosopher

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

*printed* two-dimensionally on a short strip of phenolic board about 6" long (!). The main element is a flat, zig-zag foil trace, evidently intended to mimic a helical element. I said, "let's check this thing for continuity" and stuck the ohmmeter on it. Well, it had continuity through the loading coil and thru the first "zig" and went open. Then it had a second break about an inch from the far end. Two opens in that cheesy thing! And he had been flying it in other planes for God knows how long with no range problems. Needless to say, he junked it anyway. Bill(oc)
Reply to
Bill Sheppard

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

Reply to
Bill Sheppard

| > 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

1000 feet of range or so. These receivers just aren't typical.

| 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.

Reply to
Doug McLaren

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.

Reply to
Don Bowey

Not me.

Reply to
The Natural Philosopher

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.

>
Reply to
The Natural Philosopher

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

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

>
Reply to
The Natural Philosopher

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

40 & 45".

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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Reply to
zl3vml

Probably yes.

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

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

Reply to
The Natural Philosopher

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