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Model Forum / Radio Controlled / Air Models / February 2007recieving antenna.
When I used to play around with CB radio we matched the physical antenna length to the frequency we were working i.e 27mhz was 11.1 metres so a 1/4 antenna was about nine feet, would it improve the reception and in turn the range oof a transmitter if tranny and receiver had 1/4 wave antenna? they would be easy enough to manufacture with the one in the a/c being coil wound and hidden in the rear fuselage, regards, Terry > When I used to play around with CB radio we matched the physical antenna > length to the frequency we were working i.e 27mhz was 11.1 metres so a > 1/4 antenna was about nine feet, would it improve the reception and in > turn the range oof a transmitter if tranny and receiver had 1/4 wave > antenna? they would be easy enough to manufacture with the one in the > a/c being coil wound and hidden in the rear fuselage, The transmitter IS already tuned to a quarter wave by bottom loading. The receiver aerial is not tuned at all and feeds such a completely mismatched impedance that tuning it would have little effect..and anyway its pretty hard to complete the 'earth' part of a 1/4 wave whip in a plane.:-) A dipole would be a better choice, but they are very directional. The reality is that in an aircraft, you want an aerial that won;t be massively detuned by running near pushrods and servos, so its just a random bit of wire that is 'waved in the vicinity'* of the receiver front end. Its easy enough to build a receiver that is sensitive enough to be fully saturated by the other electrical noise around, without resorting to tuning the aerial anyway. * its usually coupled in via a very small capacitor or s low turn on a tuned coil. > regards, Terry >> When I used to play around with CB radio we matched the physical antenna >> length to the frequency we were working i.e 27mhz was 11.1 metres so a [quoted text clipped - 25 lines] >> regards, >> Terry ------------- As Terry pointed out, it is better to have as strong as possible a signal emitted by the transmitter with the receiver just able to receive said signal and no more. If you have ever operated a high quality CB set, a good ham receiver or a good broadcast band receiver, you will notice that they have a knob labeled RF or sensitivity. This basically allows you to attenuate the incoming signal so that other signals will be suppressed. This assumes that the signal that you want to receive is the strongest signal present to your receiver's input. Some model grade receivers utilize a circuit called an AGC (automatic gain control). The latter technique is so effective that JR has been building and selling single conversion receivers for a long time and utilizing the AGC to suppress other, weaker, signals. No one scheme of design is satisfactory for every possible situation that one can encounter, but JR's design has worked very well for the majority of model uses. Ed Cregger >> When I used to play around with CB radio we matched the physical antenna >> length to the frequency we were working i.e 27mhz was 11.1 metres so a [quoted text clipped - 24 lines] >tuned coil. >> Dunno how it is on your side of the pond, but in the US where the R/C allocation is in the 72 MHz band, Rx antennae are generally pretty close to 1/4 wavelength, about 1 meter (figures to 1.04 M at 72 MHz with c=3.0xE8 for approximation). Abel >>> When I used to play around with CB radio we matched the physical antenna >>> length to the frequency we were working i.e 27mhz was 11.1 metres so a [quoted text clipped - 29 lines] > close to 1/4 wavelength, about 1 meter (figures to 1.04 M at 72 MHz > with c=3.0xE8 for approximation). So they use little or no bottom loading. Unsurprisingly the sets on sale here on 27 MHz,35MHz, and 40MHz use exactly the same aerial as you do...Just different electronics. The receivers likewise. Transmitter aerials are tuned, as it gets more power into the air so to speak, and the conditions - being held by the pilot with his feet plonked on the ground - make them amenable to it. Receiver aerials are not tuned *as aerials*..though the better sets do have a tuning circuit before the mixer, and sometimes a second one if they have an RF stage..the main design goal is to get enough signal in without tuning so critical that installation issues will throw the whole thing off tune. The way to achieve that is to ensure a huge mismatch on the aerial to receiver coupling..tests have shown that the aerial length can be varied by very large amounts without making a huge impact on the range. > Abel >>>> When I used to play around with CB radio we matched the physical >>>> antenna [quoted text clipped - 52 lines] > >> Abel Once you have enough signal, anything extra has to be attenuated anyway to prevent distortion. Ed Cregger >>>>> When I used to play around with CB radio we matched the physical >>>>> antenna [quoted text clipped - 57 lines] > > Ed Cregger I would be shocked if the receivers did not have an AGC circuit that feeds back from one of the IF stages to reduce the gain of the first RF stage. They could not operate reliably without it. If one had the patience, they could install a small transmatch between the receiver and the antennae and tune the specific installation. It wouldn't make much difference, though, except in rare fringe cases where the airplane got a little too far away. Anyone that flies their planes that far away on a regular basis is going to loose one sooner or later, with or without a tuned antennae in the receiver. I have designed and built several transmatches and spent quite a bit of time experimenting with them. I can transmit into a bedspring or an iron fence, and the transmitter sees perfect matching impedance. However, most of the energy is dissapated by the transmatch, and little makes it to the bedspring, fence, etc....that is the problem with matching networks. If the anennae is badly mismatched, most of the energy goes into the matching network, and reception and power out is poor. Hi Terrence, IMHO it wouldn't really matter because I don't know of any brand name rig that won't give you OOS range. Therefore any increase in range becomes moot. However, my layman's intuition is telling me that some sort of general tuning exists in both Tx and Rx antennae because of the explicit warnings to insure that the Tx is fully extended and that you do not cut excess Rx antenna. >>>>>> When I used to play around with CB radio we matched the physical >>>>>> antenna [quoted text clipped - 78 lines] > the anennae is badly mismatched, most of the energy goes into the matching > network, and reception and power out is poor. > Hi Terrence, > IMHO it wouldn't really matter because I don't know of any brand name rig [quoted text clipped - 3 lines] > insure that the Tx is fully extended and that you do not cut excess Rx > antenna. True for a TX, but the RX situation is more complex. Tests on a GWS receiver revealed it did best with about 1.5 meters of antenna, wasn't bad on anything between 0.5 and two meters, and just lost range progressively and smoothly below 0.5 meters. That is simply showing that its tuning (if it has any at all), is phenomenally broad. The impact of reducing the aerial size is far more consistent with simple loss of area covered by it. IIRC range is simply proportional to the length of the aerial, tuning issues aside. The message <1170758459.8795.0@proxy02.news.clara.net> from The Natural Philosopher <a@b.c> contains these words: > IIRC range is simply proportional to the length of the aerial, tuning > issues aside. This is what I was getting at in a way in that if you fittes a n ine foot open coil antenna inside the fuselage which would only be nine feet of copper wire wrapped around a thin plastic tube would it help increase reception and by way of this improve things like aileron flutter etc, it is different with CB in that the receiving antenna is also the transmitting antenna so has to be specifically tuned to the fequency being worked but I know the bigger the antenna the better the reception and wondered if it was also the case with r/c aircraft, regards, Terry On 6 Feb, 08:38, Terence Lynock (CSD) <customshipy...@zetnet.co.uk> wrote: > The message <1170758459.879...@proxy02.news.clara.net> > from The Natural Philosopher <a...@b.c> contains these words: [quoted text clipped - 13 lines] > regards, > Terry The only reason we have an external antenna on a trany is because of the stupid FCC reg that says the signal is supposed to be vertically polarized. Then no one holds the trany so that such a signal is transmitted in violation of the law I suppose. You can fly a JR trany just fine with the antenna unscrewed and not even attached. More then adequate range in the air. I have flown Futaba tranys with the plane pretty far out with the antenna down all the way and seen no real problems. I can only conclude that in general we have way more signal then we need if the antenna is installed properly. So why waste time fooling with the rx antenna when it is already more then sensitive enough to do the job? I even heard of one over water range check on a slow stable model that almost flew itself. Trany was on shore. A chase boat followed the plane and via marine radio told the pilot what to do. They were miles out and still had control. I will admit over water is a best case and over land you likely would not get out miles. But over land you never run out of range as long as you can see the plane unless someone turns on a second trany on your frequency. In that case more sensitivity will make no difference anyhow. > On 6 Feb, 08:38, Terence Lynock (CSD) <customshipy...@zetnet.co.uk> > wrote: [quoted text clipped - 22 lines] > adequate range in the air. I have flown Futaba tranys with the plane > pretty far out with the antenna down all the way Mine got to 20 meters and crashed. I think you are talking through your anus. Even the best reciever I have only has a 60 meter ground range with the futaba antenna retracted.. with no antenna at all I would expect no more than 15m. Maybe with a retarcted aerial, and a top line reciever you would have 120m in the air. Whereas with it extended its probably 2km. and seen no real > problems. I can only conclude that in general we have way more signal > then we need if the antenna is installed properly. So why waste time [quoted text clipped - 9 lines] > on a second trany on your frequency. In that case more sensitivity > will make no difference anyhow. You can cut yor receiver antennae down a lot..you will get reduced range. I fly loads of stuff with JETI REX 4 receivers - they come with about 12" of lead. They do about 600 meters more or less. Likewise a short leaded futaba feather. Decent enough, but bu no menas full ranege. My best range currently is from futabas with full length antennae..they run into issues at 400-500m where I have multipath issues, but are fully controllable out to 6-800m or more. The one that lost about 8 inches of its end is not perceptibly worse,.. > b...@scn.org wrote: > > On 6 Feb, 08:38, Terence Lynock (CSD) <customshipy...@zetnet.co.uk> [quoted text clipped - 61 lines] > > - Show quoted text - Never said it was recommended practice. How far you get depends a heck of a lot on ground conditions. Wet soil and not far. On the other hand in extreme dry soil conditions I once ground tested with the antenna down and got 1200 feet with full control. Same trany, same rx and wet soil and maybe 80 feet. At 20 meters you likely were not high enough off the ground to get out of the ground soaking up your signal. On the other hand of the two times I flew that way the first was a brain fart and I forgot to put the antenna up and took off and flew fine. On the second I took off with the antenna up and pushed it down slowly and was fine. I both cases I pulled it up before landing. I saw the JR experiment done. Pilot took off and his dad slowly unscrewed the antenna while he was flying. Replaced it before landing. They are JR reps so were advertising. If you are getting any problems at less then a mile in the air your equipment is crap or you have horrid interference. We have put altimeters (one of the fairly inexpensive watches will record max altitude, just put it in the plane) and flown gliders well over 5000 feet above ground level with no problems other then you get tired watching it through binoculars. Have to do it in a reclining lawn chair. You also need a 4 meter wing span otherwise it is hard to see. You can see a 3 meter wing span that high. Even naked eye. But when you turn the plane will disappear and you must not take your eyes away from where it should reappear in the sky or you will never see it come out of the turn. An awful lot of people fly a lot farther out then they think they are. I know of cases where the pilot was positive he was within the fly zone and not more then 600 feet away when in fact by actual measurement he was over 1500. >> The only reason we have an external antenna on a trany is because of >> the stupid FCC reg that says the signal is supposed to be vertically [quoted text clipped - 6 lines] > Mine got to 20 meters and crashed. I think you are talking through your > anus. I agree with your comment... I would advise anybody, with any sense at all, to ignore the statement that you can use the transmitter with no aerial attached. Apart from anything else, there is the slight matter of the aerial being needed to provide a suitable load for the output stage of the transmitter. You would have NO range and would also be doing your damndest to cook the power transistor. Anyone who doubts this should take their oldest and most beat up plane, go well away from anything damageable, remove the transmitter aerial, kiss the plane goodbye and then launch it ! The guy can quote whichever book he wants.... fact is... he's either never read it, or didn't understand a word of it ! On one occasion I launched a large glider which promptly went completely out of control after 20 to 30 yards or so and crashed. I was distracted by several people around me and while talking to them launched with the transmitter aerial down.... the glider was repairable ! This glider with the same radio gear normally flys right out to the limits of my visibility. To the guy who is wondering about coiling up a longer length of wire... If you have a 9ft aerial (for example) that works on whatever frequency you use it for then.... to coil up the aerial and get the same effect you actually need about 18 ft of wire. To make a helical antenna requires more wire than if you are just making a straight antenna such as a 1/4 wave whip..... you don't want to know why ! Antenna and feedline theory is a fairly complex subject and you have to have a damn good reason for ploughing through it to gat a qualification. For model aircraft there is no need to know any of it. The radio gear supplied for use, apart from short range stuff for park and indoor flying, will adequately control a model much further than you can see it. There is no need to alter the length of reciever aerials and if you must have shorter Tx and Rx aerials then buy ready made helically wound ones from a good source. Beware the guys who reckon to have read a book and can do magical things which directly contravene the laws of physics !!! Reg The message <531cl1F1pr3aiU2@mid.individual.net> from tux_powered@nowhere.at-all.net contains these words: > the aerial being > needed to provide a suitable load for the output stage of the > transmitter. You would have NO range and would also be doing your > damndest to cook the power transistor. Sounds like the same as CB rigs where you have a SWR (Standing Wave Ratio) to match to your operating frequency, the idea being you lengthen or shorten the antenna until you have a SWR as close to 1:1 when transmitting as possible, disconnect the antenna and key up and you will hit 10:1 and after about 30 seconds watch the smoke start to rise, regards, Terry > The message <531cl1F1pr3aiU2@mid.individual.net> > from tux_powered@nowhere.at-all.net contains these words: [quoted text clipped - 11 lines] > > regards, Terry Basically you are on target.. However it has been found not to be so clever to make transmitters that burn out when the aerial is retracted - most modern designs have enough margin to survive, tho mostly they do get hotter. >>> The only reason we have an external antenna on a trany is because of >>> the stupid FCC reg that says the signal is supposed to be vertically [quoted text clipped - 27 lines] > the same radio gear normally flys right out to the limits of my > visibility. I was distracted,. and totally pissed as well. It was a great afternoon..no damage after we got it out of a hedge, a short flight, and then...We thought we were hallucinating - a hot air balloon landed in the next field. >>> The only reason we have an external antenna on a trany is because of >>> the stupid FCC reg that says the signal is supposed to be vertically [quoted text clipped - 15 lines] >transmitter. You would have NO range and would also be doing your >damndest to cook the power transistor. AFAIK this really has to be an old myth. Now back in the 60's and 70's there were some transmitters that saved a dollar, and used a marginal output transistor, they were at risk, although I can ever only recall replacing one or maybe two in that era. Reputable manufacturers generally used 2N3866 or 2N4427 transistors or one of the equivalents which were pretty bullet-proof. As one example I have a service manual for a JR Galaxy, dated 1986. There is no warning whatsoever about operating without an antenna, but there's plenty of other warnings about stupid things. Back in the early 90's I did some trials with a thermocouple on a 2SC1973 (equiv to MRF229) output transistor, after 1 hour of operation with no antenna, the transistor case temp increased 18 degrees, hardly a risk. However,I'd like to hear about *documented* cases of such damage in a modern reputable design. Barry > >> b...@scn.org wrote: > [quoted text clipped - 40 lines] > > - Show quoted text - You will not hear of any documented cases because there are none. After all, it does not take much to dissipate the tiny amount of heat in the transistor vs sending it out the antenna. We are not exactly sending kilowatts out of our toys. If the trany is so poorly designed that it is even possible for you to fry the final with the antenna removed you should put the whole thing in the trash as who knows what else is wrong with it. But you can rest assured it was designed by an idiot so something serious is likely wrong. Or are you going to argue that because of a 20:1 SWR the output stage has to absorb 20 times as much energy? If so go patent a perpetual motion machine. Back to antennas. The only reason we have an external antenna is because of the stupid FCC rule on polarization of transmitted signal. Get rid of this stupid rule that no one complies with anyhow because of the way they hold the trany and you can put the antenna inside the case. Dirt simple and gets rid of the most breakable part on the trany. Probably saves a buck or two in cost of the unit also. And the FCC rule can not be important as I have yet to see a single pilot fly with the trany in a legal position. I suppose there is one someplace as I have not seen every single pilot fly. It probably is worth noting also that with the antenna down on a Futaba you are far short of zero antenna. Now what is impressive is the JR experiment where the matching network and leads are all the antenna you have and the plane was still under control. Tells you how little signal you really need once you are out of ground effects and wire fences and metal sheds. > Back to antennas. The only reason we have an external antenna is > because of the stupid FCC rule on polarization of transmitted signal. [quoted text clipped - 12 lines] > little signal you really need once you are out of ground effects and > wire fences and metal sheds. Hmmmmmmm, I guess you are a troll.... surely no one is actually this daft ? If there is an insistence on a certain polarisation of the Tx antenna then doen't that only make sense if the receive antenna also has the same polarisation ??? Doesn't a simple whip antenna have both vertical and horizontal radiation components ? You are however correct when you state that the Tx antenna is held at random angles by whoever holds the Tx. What is the basic radiation pattern of a simple whip ? What happens to that radiation pattern when the antenna is lowered from the vertical? What effect does height above ground have ? If your statement was correct... we only need Tx antennas because the FCC insists on a certain polarisation of the transmitted signal... Then those holding the Tx at a slight angle to the ground would actually be using the optimum method to ensure best contact between Tx and Rx ! You seem to have the impression that the ruling is to ensure that the TX antenna is held vertically.... now why would you think that ? I lost control of a large glider while using a Futaba Tx with the aerial accidentally left retracted. Control was lost within some 30 yards. By this I mean... the glider went straight in BUT it had not been responding properly before it got anywhere near that distance. I was flying part way down a sloping hillside which is at an angle of about 50 degrees and is a very high hill. I was some 1/3 of the way below the summit. The surface vegetation is mixed - rough grass and heather. The soil covering is thin and overlays limestone and slate. Now why is my description of the site relevant ? Why does it show you are spouting rubbish ? Note.... I was using a Futaba Tx which as you correctly state still has a certain length of antenna even when it is in the retracted state. Reg snip >Hmmmmmmm, >I guess you are a troll.... surely no one is actually this daft ? Could be >If there is an insistence on a certain polarisation of the Tx antenna >then doen't that only make sense if the receive antenna also has the [quoted text clipped - 4 lines] >to that radiation pattern when the antenna is lowered from the vertical? >What effect does height above ground have ? The importance of polarisation in RC is another myth. By arbitrary convention it's the direction of the electric vector, but that changes all the time in an RC environment. The transmitter is all over the place, as indeed is the model, and then there's a number of mechanisms that confound polarity. If there is indeed an FCC reg that transmitters shall be vertical pol, then it's one of their most breached regs, and 99.9999% of RC modellers should be prosecuted. Even in a carefully constructed (and expensive) Outside Area Test Site (OATS) it's not always easy to tightly control polarity. A typical RC site has zero chance. Barry > I lost control of a large glider while using a Futaba Tx with the aerial > accidentally left retracted. Control was lost within some 30 yards. > By this I mean... the glider went straight in BUT it had not been > responding properly before it got anywhere near that distance. My experience exactly. About 30 -60 yards with a retracted antenna. I use it as a range check all the time. If I get 15 yards I will fly, but only to arioud 300 yards out. If I get 30, I can fly to around 600 yards and if I get 60 yards I am safe to over a kilometer. Thats on a Futaba set with the antenna as retraced as it will go. > I was flying part way down a sloping hillside which is at an angle of > about 50 degrees and is a very high hill. I was some 1/3 of the way [quoted text clipped - 6 lines] > Note.... I was using a Futaba Tx which as you correctly state still has > a certain length of antenna even when it is in the retracted state. Yup. > Reg Not really a myth. However, most r/c equipment does not put out enough power to damage the output amp. Any ham operators out here? Know what happens when you tune up your transmitter, disconnect the antennae, and hit the key or mike? :-) I'm not sure if "modern" designs have overheat protection for this. I'm too old fashioned to use modern gear - I'm rather found of my old Drake, Heath, and Swan gear. I used to work on a radar transmitter that put out a peak power of over 2MW (2 million watts). The power transformer put out 50KV at a half amp continuous. And the maggie would fry real fast if something went wrong with the waveguide. > AFAIK this really has to be an old myth. Now back in the 60's and > 70's there were some transmitters that saved a dollar, and used a [quoted text clipped - 16 lines] > > Barry > Not really a myth. However, most r/c equipment does not put out enough > power to damage the output amp. Any ham operators out here? Know what [quoted text clipped - 30 lines] >> >> Barry I believe you, Zootal. I was there too. <G> Blowing the final transistor in R/C transmitters with no or collapsed antennas was warned against in some R/C system manuals. These were the ones provided by the manufacturers - so it was no myth. I even knew a few modelers that had to return their sets for repair for just this particular problem. Early Futabas (MRC) seemed to be the worst for losing the final transistor, though other brands did too. Later on, the problem seemed to virtually disappear. But out of habit, I still, to this day, extend the antenna at least partially when turning the Tx on. Ed Cregger >> Not really a myth. However, most r/c equipment does not put out enough >> power to damage the output amp. Any ham operators out here? Know what [quoted text clipped - 43 lines] > still, to this day, extend the antenna at least partially when turning the > Tx on. Early on people used little plastic cased transistors. Possibly with marginal voltage ratings (you CAN get very high voltages on unmatched stages) Also I think you can adjust the output coil design so that current falls on an unmatched aerial. Then the manufacturers realized the extra cost of a more sturdy output transistor was worth the reduction in warranty claims. > Ed Cregger Call it entrenched fuddyduddy-ism from the old days, but I still cringe at the thought of running a Tx with the antenna down. Sure, nowadays it's "safe" and people do it all the time.. like back-driving a servo and it doesn't strip the output gear, but evokes the very same "yeeee-ech" feeling. In the old days, transmitter antenna matching was a major deal. And when solid state CBs came along, i worked replacing a ton of fried finals in those, and that's where this latter-day "yeee-ech" factor comes from. Bill(oc) > Not really a myth. However, most r/c equipment does not put out enough power > to damage the output amp. Any ham operators out here? Know what happens when [quoted text clipped - 8 lines] > (2 million watts). The power transformer put out 50KV at a half amp > continuous. And the maggie would fry real fast if something went wrong with I dunno what peak power is for the USA, but in this country it used to be about 500mW. You can get a TO5/T)39 type transistor with a heatsink that will absorb ALL of that and more and not fry. I built a 27Mhz SC transmitter years ago..pulled about 280mA off 18v of dry cells. The output transistor got hot, but it never blew. I estimated around 600mW of heat..so the rest of the 5+W must have been going up the antenna..I used to enjoy flying my superregens in the company of superhets.. >>Apart from anything else, there is the slight matter of the aerial being >>needed to provide a suitable load for the output stage of the [quoted text clipped - 7 lines] > manufacturers generally used 2N3866 or 2N4427 transistors or one of > the equivalents which were pretty bullet-proof. Old myth or not.... the fact is that RF energy is usually matched into the Tx antenna. This way you get MAXIMUM output power and a minimum of reflected power. With CB and ham radio gear the output stages are powerful enough to self destruct if abused. RC transmitters are relatively low power and will almost certainly survive some abuse. However.... why chance it ? There may be only a slim chance of damaging the PA transistor due to overheating but there is still a chance. It may be a long slow degradation process that finally reveals itself when your highly prized model goes in at top speed, as long as it doesn't go into the other guys fiddling around in the pits then it won't really matter ! You could run your car without an air filter on the engine and go round boasting that you have done 1,000 miles in it and NEVER had a problem. Using air filters is therefore not necessary but is just an urban myth. I have seen recommendations for removing the crystal, and thus disabling RF output, when using the transmitter on flight simulators with the aerial retracted for long periods. Output transistors are matched into the antenna so that they "see" a suitable load. Cause a mismatch, or heaven forbid, remove the antenna and there is NO suitable impedance match.... the outgoing RF will either be far less than it should be, or in the case of no antenna at all.... virtually none ! There have been fatalities already with guys using good equipment, the authorities are going to be really impressed at any fatality caused by some dimwit flying without a Tx antenna ! Reg >Old myth or not.... the fact is that RF energy is usually matched into >the Tx antenna. This way you get MAXIMUM output power and a minimum of >reflected power. With CB and ham radio gear the output stages are >powerful enough to self destruct if abused. RC transmitters are >relatively low power and will almost certainly survive some abuse. Agreed >However.... why chance it ? There may be only a slim chance of damaging >the PA transistor due to overheating but there is still a chance. It may >be a long slow degradation process that finally reveals itself when your >highly prized model goes in at top speed, as long as it doesn't go into >the other guys fiddling around in the pits then it won't really matter ! Agreed, there is no special reason to chance it, but it can and does happen for a variety of reasons. The RC manufacturers have a duty of care to anticipate it. I believe they all do these days, but that was not always the case. I have never found any good documentation that suggests such a failure mode exists, including the superb "RF Transistors, Principles and Applications" by Dye and Granberg of Motorola. However, while such a failure mode COULD exist, it's many orders of magnitudes below the other things that modelers do, and have personal control, and a responsibility, over, like: Lack of care with batteries. Ditto for black wire corrosion Proper freq control Ditto for RC system care and routine checks Ignorance of structural considerations Ditto for aerodynamic loads Stupid things like leaving the servo arm screw out. Flying a model beyond their capability. Airworthy repairs and there's many more >You could run your car without an air filter on the engine and go round >boasting that you have done 1,000 miles in it and NEVER had a problem. >Using air filters is therefore not necessary but is just an urban myth. A silly strawman argument, not capable of withstanding any rational analysis. >I have seen recommendations for removing the crystal, and thus disabling >RF output, when using the transmitter on flight simulators with the >aerial retracted for long periods. A number of transmitters disable the RF stage when the DSC is connected to the sim. That would be the sensible way to go. >There have been >fatalities already with guys using good equipment, the authorities are >going to be really impressed at any fatality caused by some dimwit >flying without a Tx antenna ! Agreed, and I don't accept anybody would ever go flying like that, except for two cases. a. Forgetfulness (let the person who has not suffered this at some stage put their hand up now) b. A carefully staged trial to explore margins However it's interesting to note that of the RC fatalities I'm aware of over the years, the authorities have not prosecuted anybody, even although a couple of the accidents were caused by negligence. And yes, I'm well aware the Hungarian Police locked up Stefan Wurm after the double fatality there in May 2006, but it was a knee-jerk reaction, and he was released with an apology from the public prosecutor's office. Barry No matter what bm459@scn.org says do NOT fly with the TX ant. down. I have done this and it took an Uproar from(I guess I took it from myself). I do not mean to be rude but I certainly do not want anyone to get the idea that flying with the Ant. down is going to work. mk > On 6 Feb, 08:38, Terence Lynock (CSD) <customshipy...@zetnet.co.uk> > wrote: [quoted text clipped - 36 lines] > on a second trany on your frequency. In that case more sensitivity > will make no difference anyhow. > The message <1170758459.8795.0@proxy02.news.clara.net> > from The Natural Philosopher <a@b.c> contains these words: [quoted text clipped - 10 lines] > fequency being worked but I know the bigger the antenna the better the > reception and wondered if it was also the case with r/c aircraft, No ..a helix still only covers the same area as a piece of wire about the same length as the helix. From rusty memory the energy is 'collected' over an area that is about that of a circle constructed in the plane of the total aerial, and of the dameter equal to the largest linear dimension. I.e. folding a long aerial in half effectively is one quarter the area, and therefore one half the range, > regards, > Terry >> The message <1170758459.8795.0@proxy02.news.clara.net> >> from The Natural Philosopher <a@b.c> contains these words: [quoted text clipped - 13 lines] >No ..a helix still only covers the same area as a piece of wire about >the same length as the helix. No that's not quite right, the electrical aperture (which is probably the most important antenna parameter for RC work) is not the same as the physical aperture. It's generally smaller, the classic case being a microwave horn (and one that is easy to visulize) the electrical aperture is usually only 75-90% of the physical aperture. But, OTOH, a narrow-band tuned helix will give MUCH greater range than a straight wire of the same length, try it. However, we have a large margin with modern RC gear, I have often flown a model (with care, at the right height, upwind and an engine failsafe,-- for all the doom merchants) with no antenna and or reduced to one section with good results. Antennas are rather complex, and have many parameters to consider, nearly all of which inter-relate. J Krauses' 3rd edition of his famous text, "Antennas" is more readable than the 1st and 2nd Eds and is an interesting text, if one wants to get into them. > From rusty memory the energy is 'collected' over an area that is about >that of a circle constructed in the plane of the total aerial, and of [quoted text clipped - 5 lines] >> regards, >> Terry |
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