Receiver antenna

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From 'ragmop':

Naw, naw. C'mon. The antenna length is not THAT critical, and can vary a few inches either way with no ill effect. Nor is an RC receiver's front end that tightly peaked. If it were, our planes would be falling out of the sky right and left. Then there's the base loaded antenna which is only a few inches long, with a loading coil which electrically *approximates* the 39 or so inches of a regular antenna. Bill(oc)

(snip)

You are wrong. Even an inch change in the antenna length will affect reception and MAY detune the receiver input, depending on the specific design of the stage.

Don

There MAY be an asteroid headed our way that is going to wipe out all life on earth...... or that MAY also be wrong.

I would gamble good money on not noticing much difference if the last inch of my receiver antenna was mysteriously vapourised, I reckon my plane would still respond as far out as my eyes could see it.

By all means replace the antenna with one of the same length but don't lose sleep over an inch if you didn't measure the entire thing, including the length of antenna wire that is out of sight in the case. These receivers are not that critically tuned. Now if you were to leave

6 inches off...... there would no doubt be some impact on range.

Don't forget the receiver is not critically and precisely tuned to one frequency. The receiver is designed to work across a frequency range and its actual operating frequency depends on the crystal inserted, the antenna and its coupling will work admirably across the range of crystal frequencies that are likely to be used.

Apart from thermal freaks, such as myself, who normally flys a plane that far away from themselves that it is only just visible ? Most model planes are flown closer in and never realy get to fully test the range of the radio gear. Even a very big model glider flown at extreme limit of visibility will be well within the maximum range of a decent transmitter and receiver..... even with an inch missing off the antenna.

Reg

| > Naw, naw. C'mon. The antenna length is not THAT critical, and can vary a | > few inches either way with no ill effect. ... | You are wrong. Even an inch change in the antenna length will affect | reception and MAY detune the receiver input, depending on the specific | design of the stage.

He's not wrong. Gaining or losing an inch in the RX antenna length is unlikely to make a big difference in the range. It may even

*increase* the range if the antenna length wasn't quite right before, though I certainly wouldn't rely on that, and I'd suggest instead trying to be close to the original length.

And please, tell us how exactly a not-quite-resonant-anymore antenna will `detune' a receiver? What does detune even mean in this context?

Ultimately, our gear is generally good to about 1.5 miles or so, but most people don't often fly more than 1/5th of a mile away, and even the glider guys only rarely go more than 3/4 of a mile away. (And even a 3m plane at 1/2 mile is just a speck.)

Ultimately, our gear gives us some room for things that aren't quite perfect. Yes, it's best to keep the RX antenna to be the exactly right length, but an inch or two either way isn't going to make a really big difference.

From Don:

What you are saying would be true if, and only if, the "specific design" of the receiver's front end is acutely peaked at a specific frequency within the 72Mhz band. And it would be true only if the antenna _is part of the resonant circuit determining that frequency_.

Fact is, our RC receivers' inputs are designed to have a sufficiently wide passband to accomodate the whole 72Mhz slot without attenuation. And the antenna does not participate in 'tuning' the receiver. It is simply a 'capture' element, in this case cut to

*approximately* 1/4 wavelength at 72Mhz. Ever wonder what happens when the Tx and Rx antennas are not perfectly parallel to each other? There's a drop in signal strength going into the Rx. In fact there'e a null if one of the antennas is oriented perpendicular to the other. Yet nothin' happens (like kissin' your sister

No, you are wrong. Antennae on receivers cannot be highly tuned, otherwise the proximity of any metallic objects like servos, landing gear and the like, would cause them to go off tune enough to dramatically degrade performance, and changing a crystal in a receiver would require careful tuning of the input stage with expensive equipment.

Tests have shown that at least on some receivers the optimum length is about 1.5 meters. On others, its about a meter. None showed any huge variation in sensitivity with lengths around the meter mark though.

And the same length antenna is used on OUR *35MHz* equipment.

You can get a fair bit of range increase by making all sorts of weird arrangements of wire and especially by exactly tuning the front end of the receiver to YOUR transmitter. If it HAS any front end tuning, that is. Many cheap receivers do not.

Yeah, y'alls' Rx antennas are at approximately 1/8 wavelength, half the 'capture area' of ours, apparently with no reduction in range.

There's also an interesting tidbit familiar to all hams and radio engineers called 'velocity factor', the speed of charge in a conductor. It's a little less than the speed of light in free space ('c'). Depending on the material, it's generally about 80%). So assuming a velocity factor of.8c, an antenna's electricaly-resonant length would be 8/10 (or 4/5) the free-space wavelength. In practical terms, whazzat mean in relation to our

72Mhz Rx antennas (which are ALREADY cut to approx. 1/4 wavelength)? Simplty this=96 Free-space wavelength (1/4 thereof) is 1.04 meters, or about 40 inches. Assuming a velocity factor of .8, that means the antenna's electrical length will be 32 inches (!). Apparently the mfrs. have standardized on 39 inches a compromize. It sure seems like a randomly-selected compromize beween electrical length and 'capture area'. So the notion of 39 inches as 'critical' is pretty much urban legend. The same generalities can be extrapolated to 35Mhz antennas, which are already at half the capture area. oc

I love the threads that crop up occasionally about the radio equipment. Especially the antenna threads. I think Futaba having Hi and Lo band on the 72 MHz equipment tends to make people think the frequency band is wider than it really is and the "tuning" is much more critical than it really is.

Charlie

That and experience with very old receivers where there was no buffering of the oscillator will have people thinking that you can drag the receiver's tuned performance peak away from the optimum setup by varying the antenna length. Modern receivers are more or less immune from this problem.

Ed Cregger

A good receiver input stage (antenna and L/C net) is vital to having a good signal to noise ratio. Nothing about the radio can be better than however poor this is. Some radios (the better designs) have more selective RF circuits than others so the affects of changing frequency without re-tuning, and length changes to the antenna, will be more noticeable.

I would be very interested to hear how modern receiver design makes them immune to physical laws.

If you guys wish to believe old-wives-tales about RC receiver and transmitter things, that's fine, but it can be hard on airplanes.

Don

From Charlie:

Yeah, since Futaba is the only mfr. that does it, it kinda looks more like a marketing ploy to discourage users from mixing brands. If there were a technically valid basis for having "hi band" and "low band"-optimized receivers, you'd think all the mfrs. would do it. As you said, the 72Mhz slot is *not* all that wide, occupying less than 1Mhz of bandwidth (72.01 to 72.99).

"Tuning" of the antenna being related to its length is vastly overblown. By analogy, take a TV that's set up with rabbit ears. The set works fine irrespective of the length of the rods (sometimes working *better* with the rods completely collapsed). The rods play no part in "tuning" the TV; that's all done within the receiver itself. Likewise our RC receivers selectively 'tune to' a specific frequency irrespective of antenna length. As Ed Cregger said, the antenna does not "pull" the receiver off-channel if its length is not perfect. oc

P.S. Where the discussion thus far has dealt strictly with Rx antennas being fairly non-critical as to length, nothing was said about transmitting antennas. A Tx antenna *does* need to be resonant, either at the fundamental wavelength or a fraction of it, like 1/2, 1/4, or 1/8 (as with the 35 Mhz rigs which radiate at 1/8 wavelength). Maybe the critical-ness of Tx antenna length has created the perception that Rx antennas are equally critical.

oc

I understand that the transmitter antenna has to be cut fairly close to the length required for maximun transfer of power (impedance match) to the air, but anyone who has worked in radio for a living, as I have, will tell you that the orientation of the receiver antenna (longwire in this case) to the transmitter antenna (base loaded) will have a thousandfold more effect on the received signal level than the cut length. To add to that we usually hold the transmitter in such wasy as to present the poorest transmission pattern to the receiver, since the pattern for a base loaded whip is like a donut with the axis along the antenna. Now consider how much control you have over orientation of the receiver antenna in flight, and you will see that within practical limits, receiver antenna length is not a very big factor. Still the system works pretty good.

Phil AMA609

| There's also an interesting tidbit familiar to all hams and radio | engineers called 'velocity factor', the speed of charge in a | conductor. It's a little less than the speed of light in free space | ('c'). Depending on the material, it's generally about 80%). So | assuming a velocity factor of.8c, an antenna's electricaly-resonant | length would be 8/10 (or 4/5) the free-space wavelength.

Yes, but in the case of most antennas, the dielectric is air, so the velocity factor is far closer to 1.0 c. 0.95 c is the figure usually used for antenna work, as it's the usually used velocity factor for `bare wire'.

(In the case of a transmission line like coax, the velocity factor is much lower. But with ladder line, where the dielectric is mostly air, it's close to 1 c.)

| In practical terms, whazzat mean in relation to our 72Mhz Rx | antennas (which are ALREADY cut to approx. 1/4 wavelength)?

... but we're talking about antennas, and so relevant term is usually `electrical length' (though of course they're all related) --

And the usual formula for the length of a 1/4 wave antenna, taught to all beginning hams in the US, is `234 feet / ( frequency in MHz)', and for 72 MHz, that gives us a value of 39 inches. (Outside of the US, they probably go for meters ...)

If you calculate the wavelength of a 72 MHz signal in a vacuum, it's just shy of 41 inches.

You are correct that the length of each leg of a dipole is a tad shorter than that of a 1/4 of the wavelength it's meant to detect, but the difference is much smaller than 20% -- it's more like 5%.

| Simplty this? Free-space wavelength (1/4 thereof) is 1.04 meters, | or about 40 inches. Assuming a velocity factor of .8, that means | the antenna's electrical length will be 32 inches (!).

Sorry, but that's wrong.

| Apparently the mfrs. have standardized on 39 inches a compromize. It | sure seems like a randomly-selected compromize beween electrical | length and 'capture area'. So the notion of 39 inches as 'critical' | is pretty much urban legend.

Well, a resonant antenna is more important for a TX than a RX, because any signal that's reflected back can damage the TX. But in the case of a 1 watt transmitter, it's not that difficult to make sure that it can handle the reflection of the entire signal (which would come pretty close to happening if you took the entire antenna off) for a long period of time. They wouldn't want their transmitters frying just because somebody left them on with the antenna down, would they?

| The same generalities can be extrapolated to 35Mhz | antennas, which are already at half the capture area.

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. Perhaps for 75 MHz it's not a big issue, but for the other bands the difference is large enough that they'll want to adjust for it rather than lose that much 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.

To Doug M.: Thx for the clarifications on velocity factor, as i was still thinkin' of it as related to coax feedlines rather than open air. oc

Also bearing in mind that the radiation patterns for Tx antennas are only ,normally, relevant to an antenna in free space, or at some multiples of wavelength above ground. Therefore the "donut" shape is far from that normally described for a Tx antenna because we are holding it at some height (depending on the individuals height) and at some angle over some sort of surface that may or may not be a good ground.

Taking the above into account then the radiation pattern may resemble a very out of shape "Doughnut" (English spelling). There are too many variables to describe the radiation pattern of an RC Tx held by whoever, wherever of whatever stature and whatever angle.

Just to confuse the matter further... the term "long wire" is normally understood to apply to an antenna which is at least 1.5 wavelengths long. Our model plane Rx antenna is not a "long wire" it is indeed somewhere around a 1/4 wave whip (1/8 in UK). Just 'cos it's normally horizontal doesn't make it a long wire and since when did 39 inches (or so) class as long ?

(Why do I think I may regret that last question ?) >:-)

Given all the vagaries of uncertain height, uncertain ground effect, uncertain angles of radiation and uncertain interaction with other metallic objects that may be in the vicinity then I don't see how it can possibly work at all !!!

In retrospect... they were not crashes... it's just that the science doesn't stack up >:-)

Reg

Reg,

Bearing in mind that the most of the audience would be psudo-tech, had to keep it simple,.....but I agree with you and to actually take the variables into calculation would be an enormous task. Good reply

Phil AMA609