Silent-AV8R

I've been sitting here wondering about something. We have all been taught to do a rigorous range check, especially on a new plane. We obsess about the range check. But I have never seen one piece of empirical evidence that relates antenna collapsed ground range to antenna extended airborne range.

I'm not interested in the "theories" and such. Or what "common sense" tells us. What I am trying to find out is there any data anywhere that demonstrates a relationship. Further, is there any data anywhere that show what the minimum acceptable range is?

Again, I am only interested in hard data.

JPMacG

My Feedback: (2)

You may receive anecdotal examples, but you are not going to find what you are looking for. There are far, far too many variables and the relationship between collapsed antenna range in close proximity to the ground and extended antenna range away from the ground is intractably complicated. It would make a good Ph.D. thesis for a graduate student in EM theory.

The range test serves only as a crude go/no-go test.

Silent-AV8R

One of my favorite sayings:

The plural of anecdote is NOT data!!!!

I do not test my ham rigs by collapsing the antenna. In fact, I wonder if the antenna down range check really has any validity. Without hard data to prove it all we have is the tradition of doing it.

I guess my question really, if I see a 50% reduction in range between engine on and engine off do we really know that this means anything?? I know we "think" it does. But where is the data??

dirtybird

My Feedback: (5)

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I guess my question really, if I see a 50% reduction in range between engine on and engine off do we really know that this means anything?? I know we "think" it does. But where is the data??
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I can guarantee that if you have a 50% reduction in range when your engine is on you have a problem. You don't need anymore data.

mr.rc-cam

RF behavior is predictable. The antenna collapsing trickery merely simulates RF attenuation so that you do not have to walk a mile to perform a range test. If collasping the antenna results in unusally short range, with or without the engine running, then the airborne range will be suspect too.

I can assure you that once you have fingerprinted the range behavior of your system, with the antenna collapsed, any changes to the observed data will have value to you. But only you. It makes no sense to use the data collected by someone else that demonstrates the performance of their particular setup. Each one is unique.

CrashGaalaas

My Feedback: (8)

I have measured the signal strength on a separate receiver.

The signal strength is the dark horizontal bar at the bottom of the display.

The receiver is only 6 feet away from the transmitter, first pic shows signal strength with antenna collapsed, second show with antenna extended. Sometime when I go out to the field, I'll try to get some examples at 100 feet to show the difference.

Silent-AV8R

That's silly. You cannot state that something is a fact without a shred of evidence to support it. That is the point I am trying to make. Saying it is so, does not make it so.

Gremlin Castle

My Feedback: (14)

The minimum flying acceptable range is full control for as far as you can see what the plane is doing.
The large body of anecdotal offerings is all that you are likely to find. There is no great economic incentive for any company to attempt a serious study of antenna down performance.
This isn't what you are looking for but that is the real life situation.

Knighthawk2100

Lets say you cut your antenna in half. This doesn't necessarily mean you will get half the range. If you were to add an inch to your antenna your distance would decrease since the antenna length corresponds to the frequency. Rule of thumb is you always go by 1/4. RF is a VERY complicated thing. I worked for an electronics company making magnetic switches etc, and we did a lot of RF studies. I'm sure all of the radio companies have graphs for there radios showing what the range is with antenna at X length. Every channel and every radio will be different though. All electronic components have tolerances and a 1 or 2 percent can change RF dramatically. It will also depend on the angle of the antenna. AM antennas like to be vertical wile FM likes to be horizontal (I think that's right been out of electronics for almost a year so I may have that reversed) All of this is a factor. Now, it's not just people saying to range check 100' with antenna down engine on. It's the manufacturer. They have probably found that with a collapsed antenna the worst possible range you should get is 50' assuming all components are on the "low end". No one here probably has the equipment to do the check and make the graphs. The company I worked for was in the 10million dollar range, and we still went to the local college and used there tester (forgot what it's called, but it's cool). As for you theory on if your range gets cut in half when the engine is turned on common sense tells you there is a problem if your car only goes half as fast when you turn the windshield wipers on is there a problem? Where's the data? It's just the fact that it isn't suppose to happen since it doesn't happen to everyone else it's not the "norm". This would be a good discussion to send to futaba or one of the other companies since they can probably have a graph that shows antenna length to range. I guess the over point is it only takes a minute to do, and I don't know what kind of planes you fly, but I would hate to loose a $1000 plane because I didn't range check. I will stop rambling now hopefully some of that makes sense

just found this article on Futaba http://www.futaba-rc.com/faq/faq-q331.html. I'm sure they have data or they woulden't say it

dragoonpvw

My Feedback: (7)

The only thing that is silly is your reply, the fact that there is reduction of range by 50% when the engine is running, is the evidence of a problem. A 50% reduction in range by turning on the engine is enough data for the most seasoned scientist. Back to the original question, if you want hard evidence of the lack of range obtained by flying with the antenna collapsed just look into the garbage containers at a thousand flying fields around the world. If you haven't seen it happen with your own eyes then you haven't been flying long enough. The sheepish look on the victims face when he sees his collapsed antenna is also I would say serious anecdotal evidence, wouldn't you. If you haven't seen the change in signal strength overcoming an interference problem by extending your antenna then you are lucky. The physical repeatable evidence is there for anyone with a brain, anyone with an ounce of scientific method could prove it to his own satisfaction. The fact of reduced range is not a theory, it is a fact. Lastly, in reference to this gem
come on now, I am sure that anyone could make a determination of what the minimum range would be when the antenna is extended (as far as we are going to fly the plane),, with the antenna extended one segment then 100ft is recomended with no more than a 10% reduction in range when the engine is running.


Good luck Paul

Silver182

My Feedback: (2)

Hello..... I have probably more range checks since I began flying $10,000.00 dollar plus model aircraft than anyone would want to hear about. I can tell you from my experience that the best failsafe free baselines (no on-board RFI emissions operating) range for most of our radios is between 4 tens of a mile and 1/2 mile. Of course that distance is measured with the trans antenna fully extended, and the aircraft on a non-matallic stand about 3 feet above the ground surface.

I can tell you that each trans / receiver combination will measure different for the max baseline distance. As a rule of thumb or pocket book.... I will not fly any expensive (over $2000.00) model aircraft unless it will range check failsafe free at least 3 tens of a mile. I do full antenna extended range checks on all expensive aircraft!! That minimum distance is with all on-board electronics operating, ECU, fuel pump at full throttle, smoke pump if equipped, strobe lights if equipped.... every on-board system operating at full capacity!

I have witnessed NEW trans / rec. combos that will not measure failsafe free distances of > 2.5 tens a mile (1320 feet). Just because you just purchased a new radio doesn't mean it's aligned & operating properly! Through much trial and error, I have been able to reduce measured distances to a consistent 650 feet best baseline when only one stage of the transmitter's antenna is out. So with full on-board RFI emitting.. that no fly range would be 195 feet or less. Remember that must be a failsafe range with full on-board RFI emissions operating, whatever they maybe. Humm maybe there is a relationship here between sections extended and distances that can be demonstrable!?!

I don't believe there is an empirical number per say.... but the best way is to measure several different Trans / Rec combinations as I have... after a while you get to know what is good, better & best. The bad ones stick out like a sore thumb. I always shoot for the best with high cost aircraft.
Regards,
Lee H. DeMary
AMA 36099

PS: Anyone that will accept the manufactures suggested minimum range check distances for the last word before flying is setting himself up for trouble.... Some manufactures state that 60 feet with one section out is OK.. believe me if that is all the range you can get... even with the aircraft on the ground I would hesitate flying it in a kite...

mr.rc-cam

But not having range test data in your hands does not mean that the suggested practice has no value. As has been advised, no firm has publically published the hard data that you are looking for. You will have to make the measurements yourself in order to create the documentation you need to further your cause (whatever that is).

Not that this is much help, but here is some data that was collected regarding R/C Rx antenna lengths: http://www.rc-cam.com/ant_exp.htm . The methods described there can also be used for your evaluation of the range test practice.

Knighthawk2100

Very Nice page Mr Cam Man. RF for dummies. The only think I would like to see is what would happen if you cut the antenna exactly in half or added exactly 1/4 to it. We were always taught in school that you have to use either 1/4, 1/2, 3/4, or full length of an antenna. The question would be what length is the 39" is it full or half or 3/4. Your data seems to show that it wouldn't matter, but now I am curious. I think I might try your little experiment. See text book applies to our radios.

CrashGaalaas

My Feedback: (8)

39" is 1/4 wave length at 72MHz.

JPMacG

My Feedback: (2)

LOL.... I could guess where this thread was headed from the start.

CrashGaalaas

My Feedback: (8)

Now that Mr RC-CAM has explained db I can related some of my understanding in terms of db. And thanks for your signal strength analysis on the receivers. You saved me pulling a receiver apart to do essenially the same tests.

Power density decreases as the square root of the distance. So if the distance doubles, the power density decrease by 2 squared or 4. A factor of 4 decrease in power density equals 6db of signal loss. So if you can measure you signal strength at a fixed distance, you can then compute your signal strength (power density) at any distance. This table shows signal strength loss compared to the signal strength at 100 feet.

Distance(feet) DB Loss
100 0
200 6
400 12
800 18
1600 24
3200 30
6400 36
12800 42

So if you are flying out at 1600 feet away, your signal strength will be 24db less than it was at 100 feet.

Now back to the range check, if I am 100 feet away and reduce my signal strength 36db by collapsing the antenna, it should be the same as if I were 6400 feet away with the antenna fully extended.

Next step for me is to get a better calibrated number on how much the signal really goes down by collapsing the antenna.

And keep in mind that there are many other factors that affect your range even more such as antenna orientation.

73,
Larry

mr.rc-cam

The extrapolated data shows that when compared to the 1/4 wave reference (~39 in), a 1/8 wave has -5dB loss and the full wave has a -3.5dB loss. Not all the data was shown, but my records indicate that the 3/4 wave had about a -2dB loss.

An efficient antenna is a combination of good resonance and impedance matching, as explained in the article. The issue is that the Rx's input impedance is fixed and the impedance of the various "ideal" antenna lengths is not constant. So, "idealizing" the length is not sufficient unless you also deal with the resulting impedance matching issues. That is why "better" is not always better.

But, this thread is really not about antenna lengths. I only offered the web page as an example on how the average Joe can collect "hard data" using affordable RF tools.

JPMacG

My Feedback: (2)

I should point out that as mr rc-cam mentioned in his web page, the other side of the rx antenna is very important to antenna performance. In mr rc-cam's experiments there was a good "counterpoise" made up of power supply leads, meter leads, instrument cases, ac mains wiring, etc. Under those conditions a 39 inch wire will be close to resonance. This is very different than a rx antenna in an airplane. Our airplanes generally have short pieces of wire making up the other side such as the servo, battery and switch leads. Therefore, rx antennas in airplanes generally are not resonant, and in fact they are very reactive (not a good impedance match to the receiver).

I would have to say that those experiments are meaningless with regard to an rx antenna mounted in an airplane. Very sorry.

mr.rc-cam

Sorry, but I do not agree with your conclusion. The counterpoise issue does not change the measured data to any significant degree (it will affect the dBm levels -- that is why it is reported as dB instead). I tried leads of various lengths, a battery supply, and nothing adversely affected the reported dB measurements. The point is that during the RF tests it is important to maintain a constant setup, otherwise the reference measurement will no longer have meaning. So, moving the leads around, as each measurement is made, would definitely be a problem. The setup must remain undisturbed, as described in the article.

I can agree that the antenna's native lengths are not optimized for ideal resonance. The designers usually do what they can to invoke resonance (via reactive components in the Rx's front end) and attempt to impedance match as well as possible. It is at best a compromise. That is summarized in the article too.

I am not sure, but I think that perhaps there is a misunderstanding. I am not pushing resonance wavelength as the critical factor in the article. The published data points are not meant to suggest that 1/4 wavelengths are what caused the best performance at 39". That conclusion was never drawn or stated. It would be myopic to solve the antenna issue by using simple math to determine the antenna's optimum size.

JPMacG

My Feedback: (2)

I should back up and define the term. An antenna is said to be resonant when its input impedance is purely resistive. This condition is of no consequence to the operation of an antenna other than to get power efficiently into or out of the antenna. A non-resonant antenna is just as good a radiator/receiver as a resonant antenna, provided that a proper impedance matching network is employed.

Dipole and ground-plane antennas are resonant when their elements are just under 1/4 wavelength in length, or an odd integer multiple of that length. An antenna consisting of a 1/4 wavelength wire on one side and a few pieces of short wire on the other side is not resonant, and will have an input impedance of (if I recall correctly) tens of ohms resistive plus thousands of ohms reactive.

Your measurements are very interesting. They suggest that the receiver has an input impedance of somewhere around 50 ohms, since you got the best results with the 1/4 wave antenna length (which has a similar impedance). This would make sense as most test equipment is 50 ohms, and the receiver was probably developed using 50 ohm equipment.

If you could repeat your experiment with the receiver completely isolated from the environment, including all of the test equipment and power supplies you are using I think you would get a different result. Unfortunately this is not easily done. I could imagine performing the test with the receiver on a foam pedestal running off a battery pack, but how to make the measurement? Maybe through fiber optic cables or maybe placing ferrite chokes on your test cables to isolate them from the measurement. I don't know, but you get the picture.

Please feel free to PM me. I suspect I am boring the snot out of our audience.

Best regards,
Jon