Jonathan Brooks Posted April 3, 2008 Share Posted April 3, 2008 Hello Before I get shot down, i know that aerials have an harmonic length to the wave length transmitted. My question is, can you cut an aerial where .5 mile range is not required. Is it better to route the aerial up the fus and back again? I am building a little depron extra from www.flyelectric.ukgateway.net which has some cool plans on pdf.My aerial is twice the length of the fus! I cant imagine i'll ever fly this plane more than 100m from the transmitter and cant live with all that wire flapping around,. With the low speed of the extra i'll end up doing an aerial cut rather than a ribbon cut.I have heard of shorter aerial rx's, is there a harmonic length i can trim my aerial too? Answers please as due to the forecast for inclement weather this weekend i am hoping to get in some stick time in the boot of the car or maybe the pub- you can get two pints and a packet of crisps on a transmitter tray dont you know! Quote Link to comment Share on other sites More sharing options...
Adrian Bassett Posted April 3, 2008 Share Posted April 3, 2008 It would be ok for short range(indoor), but best left alone. You may want to use it in another model. Quote Link to comment Share on other sites More sharing options...
Jonathan Brooks Posted April 4, 2008 Author Share Posted April 4, 2008 Hi Cheers for that. I also asked my local model emporium. They pointed out that HiTec even supply a little plastic thing to wrap the aerial around. The consensus of opinion from the old lags in the shop was that you can coil about a third and that that is better than running the aerial back along the fus. I think i will try coiling it - test the range. if that is not satisfactory i wil start trimming it and testing the rangefailing that i will crash it anyway and buy a proper sized glow model Quote Link to comment Share on other sites More sharing options...
Malc. Nicklin Posted May 6, 2008 Share Posted May 6, 2008 Hi I have been making radios for 50 years now both Ham and RC and instructed in radio and electronics. The antenna is like a tunning folk it has to resonate at the operating frequency to pass the maximum amount of energy to the receiver at that frequency. If this set up is for 35Mhz the true antenna would be 7 feet long , not very practical. The electronics compensate for the short fall in length by tricking the antenna that it is resonant at that frequency. If you fold the antenna back on it's self or cut it the frequency of operation will be higher and you will loose signal strength at the front end of your receiver. What ever you do don't fold or cut the antenna try putting it down the length of one of the wings or just let it blow in the wind behind the model.The antenna is the most important part of the receiver DON'T MESS with it. Quote Link to comment Share on other sites More sharing options...
Former Member Posted May 6, 2008 Share Posted May 6, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
flytilbroke Posted May 6, 2008 Share Posted May 6, 2008 Hi Jonathan, is it posible to zig-zag the aerial down the fuselage, leaving the zig-zags spaced apart? this may solve your problem with minimum lost signal. They can curve to follow the fuse shape too. Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted May 7, 2008 Share Posted May 7, 2008 The point should also be made that shortening the antenna will also make the radio more liable to interference as the receiver will be less able to reject spurious out-of-band signals. Coiling the antenna will have a similar effect as shortening. Even an air-spaced coil will have a de-tuning inductance.The best way to shorten an antenna is to go 2.4Ghz Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 24, 2009 Share Posted March 24, 2009 Rather than start a new thread, I hope it's OK to wake this one up again. I have never been able to obtain a clear answer on this one - specifically, my limited understanding of aerial theory won't answer whether it's the physical length of the wire that affects resonance or the simple distance between the receiver and wherever the end of the aerial happens to be as I've often been told. Obviously the ideal situation is an arrow straight line from receiver to aerial tip but how much would it affect an aerial to have. for example, a 90 degree bend half way along its length? Would the electrical length remain essentially the same (I assume inductance effects could be ignored) or does the view that the straight line length should relate to the wavelength hold any water? I can visualise that might be relevant if it was a full wavelength aerial but I can't see it relating to an 1/8th wavelength one. Assuming a bend is not a significant factor, is there an advantage to be had in having parts of the aerial at different orientations to avoid a total end on presentation? I'd appreciate it if answers to this one came from people with in-depth knowledge of the subject e.g. radio hams, radio/electronics instructors, designers etc. rather than those like me who know the stock answers and maybe have just a little knowledge of the subject. Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted March 24, 2009 Share Posted March 24, 2009 Do you require a CV? The receiver sees the complete antenna not the end. Routing the antenna in an L shape will have little effect its performance. At 35Mhz the antenna is not critically directional (other than perhaps direct end-on). If the antenna is routed in a square or is folded there is a possibility the opposite sides may affect each other, reducing efficiency. If the antenna/model is then operated in an electrical noisy environment the wanted signal may then be below the noise. An antenna with several zigzag bends can be lengthened slightly to compensate for any resonance loss. Most receiver antenna are 1metre(39inches) in length, this is less than the resonant frequency (42inches), the receivers circuitry loading determines the final optimum resonant length but any I have measured have been all 1metre in length (probably cut to cost). Unfortunately the only optimum environment for 35Mhz(or for that matter 2.4Ghz) is free space. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 24, 2009 Share Posted March 24, 2009 Thanks Brian - no CV necessary but I didn't want a stream of people telling me not to roll up/shorten the aerial instead of those who understood the question! I've lost count of the number of times I've heard that you "shouldn't bend the aerial, it won't be a resonant length any more" and similar quotes. I've always felt that the electrical length of the aerial is the critical factor rather than the distance (due to its routing) in a straight line from end to end. I think you've agreed with this if I'm not mistaken? ...But I'm not sure why your zig zag example needs the extra length - is there some inductance to take into account? I don't think electrons take much notice of slowing down for corners - or do they? They do have (a little) mass I suppose... My understanding was that the practical length is reduced by a factor from the theoretical length to take some other factors into account which might explain the shorter than theoretical distance you mentioned. As I've said, I have little RF knowledge so please don't take my questions as any argument with your information - I'm just trying to understand the subject a little further. Edited By Martin Harris on 24/03/2009 21:49:15 Quote Link to comment Share on other sites More sharing options...
flytilbroke Posted March 24, 2009 Share Posted March 24, 2009 Malc, Eric, Myself, and Brian have given good answers/suggestions. At least enough to give a good idea of what is required and what will be most effective. Enjoy Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 24, 2009 Share Posted March 24, 2009 Flytilbroke, I hope you don't think I'm implying any criticism of the previous answers - I was hoping for replies from some of these knowledgeable previous contributors on a closely related but slightly different question. I think you'll see I was actually describing most of the named people in my request! Edited By Martin Harris on 24/03/2009 22:03:07 Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted March 25, 2009 Share Posted March 25, 2009 A zigzag antenna is effectively a pulled out folded antenna. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 25, 2009 Share Posted March 25, 2009 Brian, Sorry if I'm being a bit slow but at what point do the folds start to be significant? Am I right in thinking that once you have coils or folds lying parallel, inductance effects start to become meaningful? In the bottom case, will resonance be more or less acheived due to the electrical length being the same as the straight aerial with only slight inductance effects or is it the fact that you have 4 short aerials which interract against each other? If so, as the angles straighten, perhaps it's the case that the overall length starts to resemble the tuned length and the opposing sections have proportionately less adverse effects? At that point is it the fact that the electrons can start whizzing up and down at the speed of light without too much hindrance - the time and speed equating to the wavelength? Hopefully the ramblings above might make some sense and perhaps you could help me understand the mysteries of aerial theory! Edited By Martin Harris on 25/03/2009 20:26:59 Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted March 26, 2009 Share Posted March 26, 2009 Because of the low voltages involved, the folds/bend/coils will need to be close together to have a meaningful effect, known as mutual inductance. A changing current in one coil will induce a voltage in the other. A theoretical resonant antenna will be a different length to a practical resonant antenna. Theoretical calculations assume free space (ie. NOT influenced by the Earth). A practical antenna thus needs correction. Velocity of propagation in a wire is different to that in free space, also the diameter of the wire needs to be considered (for us at 8th wavelength the difference will be small). Nearby objects will have a greater effect. If a stationary model did have a resonant antenna then as soon as it leaves the ground the antenna will cease to be resonant, ie. True resonance is achievable for a single location and the antenna will need to be brought up to resonance for different locations, impossible for a fast moving model. As the gain increases with height (Double height=+6dB=double the range) this is not an issue. As to the zigzag antenna, the interaction between the legs is dependant on signal strength, proximity and leg length, as the angles straighten mutual inductance ceases to be relevant but the apparent length (as seen by the transmitter) will however remain relevant. The relative signal strength can be measured for various angles of zigzag and maximum strength also determined with a sensitive voltmeter at the receivers demodulator IC (if you have the IC pinout data). Then of course there is impedance matching, best not to go there, just keep the antenna as straight as possible or go 2.4Ghz. Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 27, 2009 Share Posted March 27, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted March 28, 2009 Share Posted March 28, 2009 A word of caution. Base loading is usually employed to bring an antenna to resonance, not to artificially shorten an existing antenna. It will certainly reduce the antenna gain. Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 28, 2009 Share Posted March 28, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted March 29, 2009 Share Posted March 29, 2009 Eric, I think you have misinterpreted my somewhat brief note, or more likely I have not explained myself clearly. I was trying to caution (in a non technical way) that Base loading is integral to the resonant antenna. My aim was to warn, the non-technical, against unilateral tampering. I had no intention of contradicting the textbooks or your good self. Apologies if I gave that impression. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 29, 2009 Share Posted March 29, 2009 Thanks gents, I think I may have clarified a few points in my head with your assistance. Could you confirm that I'm correct in thinking (in simple terms) that if you have a resonant length (or thereabouts) of aerial wire, it makes very little (if any) difference if, say, you had a 40 degree bend half way along its length (the resonance being due to the physical length of the wire) and possibly a slight improvement end on. The apparent length in this case (measured in a straight line from rx to aerial tip) will only have an effect on the maximum amount of RF energy captured (when at 90 degrees to the tx aerial) and no effect on resonance? Quote Link to comment Share on other sites More sharing options...
Former Member Posted March 30, 2009 Share Posted March 30, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Chris Channon Posted April 9, 2009 Share Posted April 9, 2009 Hi all, just out of interest, Why is there no set length to a TX aerial? Just looked at an old Sanwa tx aerial and it is 9" longer than a Futaba aerial, so if these thingies are matched to work, how come a Sanwa RX will work with a Futaba Tx, As i say, just wondering ! Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 9, 2009 Share Posted April 9, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Chris Channon Posted April 11, 2009 Share Posted April 11, 2009 Cheers Phil, Regards Chris Quote Link to comment Share on other sites More sharing options...
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