Den Moran Posted December 22, 2014 Share Posted December 22, 2014 Hi! Can anyone tell me how long a 2.4 aerial should be please? What cable is best to use, and does it matter if its 'too long', or is this critical? Thanks! Den, Abertawe Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted December 22, 2014 Share Posted December 22, 2014 Should be in the order of around 29 - 32 mm but the best answer is to measure one from the same manufacturer as there can be small differences due to design factors. If you can tell us the make and model I'm sure someone can measure one for you... Why do you ask? Have you broken one off? If the tip has been shortened, it it usually just a matter of cutting back the coax outer to leave the inner at the correct length. Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted December 22, 2014 Share Posted December 22, 2014 Yes it matters if its too short or too long or your range will be impacted, the aerial is tuned to its targetted frequency by being a specific fraction of the signal wavelength, for 2.4Gig the wavelength is 12.491 cm, therefore 1/4 Wavelength is 3.12cm. so as Martin says around 30mm is required Quote Link to comment Share on other sites More sharing options...
Paul Jefferies Posted December 22, 2014 Share Posted December 22, 2014 I have just asked the same question on the single channel forum because I am converting an old 27Mhz Skyleader to 2.4 Ghz.. The Single Channel "Guru" is Phil_G who also posts on this forum so I hope he will excuse me for quoteing his reply.......... "The mathematical 31.25mm neglects the velocity factor, and since we dont know the exact value its usual to estimate about 0.9 so the quarter-wave becomes 28mm, but its not that critical in an R/C transmitter which is very much a compromise anyway" However all those I have measured are between 32mm - 33mm so I suggest making it around 32 mm and then range it on the ground and if the range does not seem as much as you would expect, shorten it by 1mm at a time and re-check it. Paul Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted December 22, 2014 Share Posted December 22, 2014 The question is "where do you measure it from" - if its one of the aerials that is on the end of a length of co-oax then it would be from then end of the coax sheathing, but if its the type terminated directly onto the PCB (a la Orange) where does it start? from the PCB or where it exits the casing? It may only be 1 or 2 mm but in a 32mm aerial thats 5%! Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted December 22, 2014 Share Posted December 22, 2014 Doesn't work that way. FM rx's were just a length of wire, but a 2.4 system uses a Co-ax cable, and if it's cut, then that aerial won't work anymore and needs to be replaced. As Dave H pointed out as well, it's a Co-ax cable and when replacing the aerial, one end goes to the GND of the board and the other on the receiving side. You can fit longer aerials - ideal for carbon fuselages, but just soldering some more wire onto the cable won't do anything - probably cause problems. Quote Link to comment Share on other sites More sharing options...
Paul Jefferies Posted December 22, 2014 Share Posted December 22, 2014 Den, You don't say what you are trying to do...... Is it a Transmitter or Receiver antenna you are asking about? Paul Quote Link to comment Share on other sites More sharing options...
Den Moran Posted December 23, 2014 Author Share Posted December 23, 2014 I have several damaged aerial leads and wish to replace them. Apparently it's a design weakness with Turnigy receivers that causes them to break/defray at the point of exit from the case. I know they are co-axial, and the 'FPV' video on youtube shows how to solder it in, but the length he is stating is 55mm for the whole thing. Does anyone know how I can replace them? It's not a question of cost, the new ones are reasonably priced, but I want to fit a better receiver aerial that won't snap off. Thanks everyone, Happy Christmas! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted December 23, 2014 Share Posted December 23, 2014 Posted by Paul Marsh on 22/12/2014 20:39:24: Doesn't work that way. FM rx's were just a length of wire, but a 2.4 system uses a Co-ax cable, and if it's cut, then that aerial won't work anymore and needs to be replaced. As Dave H pointed out as well, it's a Co-ax cable and when replacing the aerial, one end goes to the GND of the board and the other on the receiving side. You can fit longer aerials - ideal for carbon fuselages, but just soldering some more wire onto the cable won't do anything - probably cause problems. I've certainly always understood that the overall co-ax length isn't important and the active (critical length) part of the aerial is the exposed length of coax inner. If the Turnigy receivers use simple wires soldered direct to the board then you'll just need to replicate the length of the originals - perhaps using stranded cable for more resilience if the originals are solid? Perhaps best if we wait for confirmation from one of the hams or RF experts on the forum. Edited By Martin Harris on 23/12/2014 17:41:40 Quote Link to comment Share on other sites More sharing options...
Masher Posted December 23, 2014 Share Posted December 23, 2014 I am not an expert or a ham but I do have 40 years experience as an RF engineer. I wouldn't advise people to mess with antennas - too much potential for error unless you know what you are doing and are expert at fine soldering. A range check may detect errors but you could have a dry joint which fails later. Can't you just replace rx (you say they are cheap) then do what I do on Spektrum rx's - put a small length of small bore heat shrink over the plastic boss and antenna where it exits the case. Then carefully heat and shrink to leave a better protected antenna. If there is not enough plastic to attach the heatshrink tubing to, what about glueing a small length of plastic tube onto the side of the case surrounding the antenna cable - Eg a bit of snake outer. Use something like UHU POR or even silcone sealant to secure (NOT cyano) and again you have added some protection. Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted December 23, 2014 Share Posted December 23, 2014 I reckon the length of antenna is tuned to the rx board. Maybe some makes allow different lengths, but without manu tech specs, then you are stabbing in the dark and best stick with what the item had before. I know, if you ran the 2.4 without the aerial, it would burn out the modulator, due to unable to dissipate energy as in a transmission, overloading the componants and damaging them. Also, the GND of CO-AX cable must be soldered to the -gnd of the board, with the middle part of the cable on the rf side, a short between the two will probably ground the board out destroying it and at the very least affecting your range considerably. Quote Link to comment Share on other sites More sharing options...
Phil Green Posted December 23, 2014 Share Posted December 23, 2014 Posted by Paul Marsh on 23/12/2014 17:54:16: I know, if you ran the 2.4 without the aerial, it would burn out the modulator Quote Link to comment Share on other sites More sharing options...
Masher Posted December 23, 2014 Share Posted December 23, 2014 You are unlikely to destroy a rx by shorting out the antenna - it will be ac coupled and is not at the output of a power device as in a Tx. Main issues are (a) you don't know what the design requires and (b) you are adding unreliability Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted December 23, 2014 Share Posted December 23, 2014 Think about your TV aerial - or dish - Paul. There's no calculation required to work out how long the bit of co-ax running from the chimney to the TV needs to be - it simply conducts the signal picked up by the aerial (or LNB) to the equipment. In our case, the aerial is a simple length of wire which just happens to be the carefully measured unshielded inner part of the co-ax cable located at the end of it. Saves making an unnecessary joint... Edited By Martin Harris on 23/12/2014 18:05:09 Quote Link to comment Share on other sites More sharing options...
Masher Posted December 23, 2014 Share Posted December 23, 2014 In your analogy Martin, the coax is in a 75Ohm system so length is unimportant. In our rx, this is not the case. You could probably get away with any old coax, approximate length gobbed on but why risk models and worse? Quote Link to comment Share on other sites More sharing options...
Dickw Posted December 23, 2014 Share Posted December 23, 2014 In our radios the aerials seem to be a 50 Ohm system and I have not found the length of the co-ax to be critical - just the bare "active" tip. I have used UFL to UFL cable assemblies cut to make longer aerials for some of my Rx. No soldering, no effect on range, and proved reliable over several years. Dick Quote Link to comment Share on other sites More sharing options...
gangster Posted December 23, 2014 Share Posted December 23, 2014 My advice is unless you are going for very short range ie indoor. Don't even think about it. The aerial length will be very critical for correct operation. It varies from make to make for a number of reasons including the matching circuits in the receiver, how the coax is terminated and indeed the characteristic of the coax. Yes it possible to damage a receiver by having a wrongly matched aerial. The damage occurs when the model goes out of range and crashes not only the receiver can suffer but the model, other people property etc. Those Turnigy receivers are so cheap that it really isn't worth risking a bodged repair far cheaper to replace the receiver. Quote Link to comment Share on other sites More sharing options...
Masher Posted December 23, 2014 Share Posted December 23, 2014 Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted December 23, 2014 Share Posted December 23, 2014 Same here: QED. Quote Link to comment Share on other sites More sharing options...
Den Moran Posted December 27, 2014 Author Share Posted December 27, 2014 OK! I'll take the path of adding strength. Thanks for a brilliant response, everyone, Den Quote Link to comment Share on other sites More sharing options...
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