Lena Posted November 12, 2011 Share Posted November 12, 2011 Hello, Can anyone tell me the formula for working out the length of the ariels on radio mic transmitters please? Is the 1/4 of the wavelength? I believe this has been asked on here before but I can't find it here or anywhere else on the internet. Ta Link to comment Share on other sites More sharing options...
brainwave-generator Posted November 12, 2011 Share Posted November 12, 2011 It is indeed quarter wavelength. But it's usually wise to subtract about 5% to account for the mounting hardware etc. The exact figure you need will, unfortunately, rely on the material used in the antenna. Link to comment Share on other sites More sharing options...
james.sealey Posted November 12, 2011 Share Posted November 12, 2011 Can anyone tell me the formula for working out the length of the ariels on radio mic transmitters please? Is the 1/4 of the wavelength? Most single units will use a 1/4 length, for compactness if nothing else. However, if you get an antenna kit such as this Shure one you'll find it comes with 1/2 length. Are you trying to work out a long a replacement needs to be? James Link to comment Share on other sites More sharing options...
ramdram Posted November 12, 2011 Share Posted November 12, 2011 http://www.csgnetwork.com/antennaedcalc.html ANY Ham site will have more than enough formulae to calculate just about anything in the rf dept. HTH Link to comment Share on other sites More sharing options...
Bobbsy Posted November 13, 2011 Share Posted November 13, 2011 There are tons of sites that will do the calculation for you...for example THIS ONE.. As has been said, performance is actually better if you can go to half wave but this is often too long for practical use. Also, you'll find that the antennae that come with your radio mic transmitters are an arbitrary length for somewhere in the middle of the frequency range it can tune to. Link to comment Share on other sites More sharing options...
paulears Posted November 13, 2011 Share Posted November 13, 2011 A quarter wave aerial is only at it's most efficient when operating against a ground plane - something big and metalic, so the dangly VHF wires we used to have were rarely very efficient. For something up higher, a half wave works, but it's really just a quarter wave working against another quarter wave. With UHF, aerials are smaller, and less efficient due to this smaller radiation area - but quarter waves are not that frequency sensitive related to their length, so although they're not as efficient away fro the centre frequency, the fact they're bunged up against salty sweaty bodies probably contributes more to low efficiency than the actual mm accurate length. The old adage that a coathanger up in the clear may often be better than £300 worth of flashy aerial down low does hold true. Those little helically wound rubber duck type aerials on old VHF kit were pretty inefficient things, but the stiffness meant they didn't get scrunched up as much as the dangly wire versions, so often worked better in practice. Link to comment Share on other sites More sharing options...
mutley Posted November 13, 2011 Share Posted November 13, 2011 When I worked at a UK holiday resort, I replaced lots or aerials on Sony WRT805 beltpacks with a single piece of stranded wire, inside the double sheath of a piece of microphone cable (Musiflex), which was superglued into place. This actually worked just as well as the original aerial with its cheap threaded cast alloy base, but was more flexible and didn't work loose! The formula for wavelength is the same as speed=distance/time. Speed = 3x108 m/s; distance = wavelength shown as lambda (λ); time = frequency in this equation. So for 454MHz: λ = 3 x108 / 4.54 x108λ = 66cm, therefore a quarter wave aerial will be about 16.5cm But as already mentioned, being so close to a salty, sweaty body comprised of 70% water, it's never going to be efficient anyway, so just copy the length of the original aerial and "it'll be reet"! Link to comment Share on other sites More sharing options...
Jivemaster Posted November 13, 2011 Share Posted November 13, 2011 The easy way is to hire or borrow a like mic and measure the aerial. Link to comment Share on other sites More sharing options...
paulears Posted November 13, 2011 Share Posted November 13, 2011 Why would you want to do that when the maths is simple and takes a fraction of the time (and costs nothing!). Also Mutley's calculation can be made a bit easier by some simplification as in 300/frequency in MHz, divided by 4 - which produces the same answer - and as we've said, it doesn't need to be mm accurate. Link to comment Share on other sites More sharing options...
DanHiggott Posted January 29, 2013 Share Posted January 29, 2013 I have built this website which does the calculation for you: RF Toolbox Cheers!Dan Link to comment Share on other sites More sharing options...
noiseman Posted January 29, 2013 Share Posted January 29, 2013 I have built this website which does the calculation for you: RF Toolbox Cheers!DanI've entered the frequency but how do you get the result?Am I being stupid?? Link to comment Share on other sites More sharing options...
paulears Posted January 29, 2013 Share Posted January 29, 2013 I can't make it work either, but 75 divided by the operating frequency gives you the quarter wave length in metres? Just like we said above? EDIT I just looked at the source code for your web site - I have no idea why anyone would write so much when a calculator does it so much more easily - or long division if you fancy it! EDIT 2 I looked at the code again and realised you need to enter 835000 to get the length animation to run rather than 835 - I got confused by the message to remove the decimal point. Link to comment Share on other sites More sharing options...
boatman Posted January 29, 2013 Share Posted January 29, 2013 Here's one that does work (albeit in feet & inches): http://www.csgnetwork.com/antennagenericfreqlencalc.html Link to comment Share on other sites More sharing options...
DanHiggott Posted January 29, 2013 Share Posted January 29, 2013 I can't make it work either, but 75 divided by the operating frequency gives you the quarter wave length in metres? Just like we said above? EDIT I just looked at the source code for your web site - I have no idea why anyone would write so much when a calculator does it so much more easily - or long division if you fancy it! EDIT 2 I looked at the code again and realised you need to enter 835000 to get the length animation to run rather than 835 - I got confused by the message to remove the decimal point. Hello, I built the site more as a way of teaching myself the programming language, rather than to solve the small problem of figuring out how to figure out the length of an aerial... so don't worry too much about my sanity! If you follow the instructions you can see that the calculator only works between certain frequencies, and you don't need to worry about entering a decimal place. You do need to write the frequency using 6 numbers though, i.e. 650000 for 650.000MHz. Dan Link to comment Share on other sites More sharing options...
noiseman Posted January 31, 2013 Share Posted January 31, 2013 I built the site more as a way of teaching myself the programming language, rather than to solve the small problem of figuring out how to figure out the length of an aerial... so don't worry too much about my sanity! If you follow the instructions you can see that the calculator only works between certain frequencies, and you don't need to worry about entering a decimal place. You do need to write the frequency using 6 numbers though, i.e. 650000 for 650.000MHz. DanIt's following the instructions that leads to the confusion. "Enter the six-figure frequency you are using, omitting the decimal place eg 863500" would have done it. ;) Learning the coding is great but the practical detail of making it usable is just as, if not more important surely? Just sayin' like :) Link to comment Share on other sites More sharing options...
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