Extending your 35 mhz antenna

[Coyote]

One mod thats very popular with FPV flying is extending the length of the receiver antenna. A rx antenna is not a tuned antenna but the length can have a great effect on its performance.

This is done to extend the range of the reception. Its very effective. But its not as easy as doubling the length, it should be though but when the units are mass produced they are not cut accuratly in the first place.

Most rx antennas are supplied with around 1 meter long antenna length, this is practical and very useable. This length is called a 1/8 wavelength antenna. Its 1/8 the size of the 35mhz wavelength.

If you double this then you get a 1/4 wavelength antenna, more usefull and still fairly practical.

If you use a 1/2 wave antenna your getting pretty good performance but your antenna is getting very long at this stage, over 4 meters in length. Most stop at this point. When using a 1/4 wave antenna be carefull not to be standing on the wire when launching. Or it will end up being a shorter no wave length in particular antenna and bad for flying lol :)

Below are the lengths for each frequency

Channel        Frequency    1/4 wave            1/2 wave

channel 55    34.950        214.59cm            429.18cm
channel 56    34.960        214.53cm            429.06cm
channel 57    34.970        214.47cm            428.94cm
channel 58    34.980        214.40cm            428.82cm
channel 59    34.990        214.35cm            428.69cm
channel 60    35.000        214.28cm            428.57cm
channel 61    35.010        214.22cm            428.45cm
channel 62    35.020        214.16cm            428.33cm
channel 63    35.030        214.10cm            428.20cm
channel 64    35.040        214.04cm            428.08cm
channel 65    35.050        213.98cm            427.96cm
channel 66    35.060        213.92cm            427.84cm
channel 67    35.070        213.86cm            427.71cm
channel 68    35.080        213.79cm            427.59cm
channel 69    35.090        213.74cm            427.47cm
channel 70    35.100        213.67cm            427.35cm
channel 71    35.110        213.61cm            427.23cm
channel 72    35.120        213.55cm            427.11cm
channel 73    35.130        213.49cm            426.98cm
channel 74    35.140        213.43cm            426.86cm
channel 75    35.150        213.37cm            426.74cm
channel 76    35.160        213.31cm            426.62cm
channel 77    35.170        213.25cm            426.50cm
channel 78    35.180        213.19cm            426.38cm
channel 79    35.190        213.13cm            426.26cm
channel 80    35.200        213.07cm            426.14cm
channel 81    35.210        213.01cm            426.01cm
channel 82    35.220        212.95cm            425.89cm
channel 83    35.230        212.89cm            425.77cm
channel 84    35.240        212.83cm            425.65cm
channel 85    35.250        212.76cm            425.53cm
channel 86    35.260        212.70cm            425.41cm
channel 87    35.270        212.64cm            425.29cm
channel 88    35.280        212.58cm            425.17cm
channel 89    35.290        212.52cm            425.05cm
channel 90    35.300        212.46cm            424.93cm 

As you can see there is only 2.13cm between the start and the end of the channels we use if making a 1/4 wavelength antenna. Because you can or have to swap crystals at meets then cutting to 212.46cm will cover all the wavelengths available.

Because of the nature of Uk flying a 1/2 wavelength antenna is a bit over kill for the ranges we fly. But the info it there, choose to use at your own discretion. 

So making your antenna 211.40cm would be ideal for general use. Or 422.80cm for a 1/2 wavelength. If you want to tune it to the channel you most use the data is there for you.

[Jay28]

Nice post Ian,

I modded my Hitec 08RD receiver with a 1/4 wave extension a long time ago. I certainly receive a much better signal with it. The antenna goes through a piece of push rod tubing which sticks out of the back of my easystar by about 10", the rest of the antenna just hangs freely. The tubing keeps the antenna from getting caught up in the rudder and elevator. I have another mod on this receiver, a JST plug come out of it which is internally connected to RSSI & ground. I am actually removing this receiver tomorrow to make way for bigger and better things.  :D

Jay.

[Coyote]

Thanks Jay, good to see you`ve been there and done this yourself. Yes the incresase in performance is well worth new length antenna and 5 mins work.

Great idea with the tubing, works a treat. I did it on my wing. Stright off the edge off the wing just free. The speed keeps it nice and straight.

Wint my Axn the antenna started at the bottom of the fuse and i made a slit to push  it into for protection down to the rudder and again just hanging from the rear.

Well worth doing :)

[Rick]

Is there any info on whether the antenna needs to lay a certain way or not?  Does it all have to lie in one line?  can you have the receiver in the middle of the two meters (have I seen something about the other half coming off the board of the receiver itself?)
Would be nice for a 2 or 4m aerial if your receiver was in the fuse, and the aerial went up and out of each wing and then trailed from there if you see what I mean.

What sort of improvment can one expect to get distance wise?

thanks for the info
Rick

[Coyote]

A few people have tried a few different ways, coiling the wire round etc but all have the same effect reduce overall range.

The best way to have it is trailing in a straight line behind the plane so that the antenna is at its full length to match with the wavelength its trying to receive :

[QuarK]

Just a thought; if youre going to extend the length of your receiver antenna do you:

(a) extend the existing length by soldering more wire to it or
(b) solder a new wire to the receiver of the desired length (if so, what sort of wire needs to be used?)

I assume that the antenna joint is easy to get to in the RP8D1...

Thanks 'n Cheers

[Coyote]

Yes ive dont it to a RP8D1 myself the connection is right on the end of the board.

You can do either Quark. Make a new wire or solder an extension

Personally i prefer to replace the whole length. piece of mind and strength is not comprimised at all.

Either way make cable legth too long first, then make connection to the board or exsisting antenna and once your happy then measure the whole length and trim to suit :)

[QuarK]

Thanks Coyote, will any similar wire do?

Just noticed that your not far from where I work, currently working in Darlington, but live in Oxford...

???

[Coyote]

As longs as its as thick as original and stranded wire, solid will break inside insulation and wreck performance.

Yeah just down the road from there, well 30 min drive, use to work there plenty before :)

[QuarK]

Thanks Coyote, appreciate the help...
;)

[Coyote]

Ok just to show you every days a learning day i`ve been loking into rx circuits a little more and how to use antennas for better efficiancy.

Turns out the standard receiver unit uses a coil in the antenna circuit which has a direct effect on the antenna length. So really it is tuned.

Because of the coil in the circuit you have to cahnge your calculations to match.

The new calculations for 35 mhz in general are quite easy to remember

101cm or 39" for standard length

So a 1/4 wave antenna will be 202cm in length and a 1/2 404cm

This is a better option say up to 1/4 wavelength to get an increase efficiancy for your system, although going longer will increase the sensativity because of the coils influence in the circuit the antenna then becomes less efficiant.

Bypassing the coil will solve this issue but not for everyone to try. Then the range for that length antenna gets to really really impressive distances. In which case the 202 / 303 / 404cm antenna would then be scrapped in favour for the 422cm length instead. 

You can still use a 2m+ antenna but its range will not increase as much as normally thought.

Food for thought, it seems all the manufactorers work off the 101cm standard length too. As they are trying to get the most from their systems this would seem to be the best choice of lengths to go with.

[Rick]

So you suggest just stick with standard? Thanks for researching and sharing by the way!

[Coyote]

No doubling the 101cm antenna to 202cm will work fine and get you a lot more if not double your range. Everything after that will increase range but then due to coil wont have such a great effect unless the coil is removed from the reciever. :)

[electrotor]

You might find this article of interest. It has been around for many years and keeps popping up. Whilst the tests were conducted in a reasonably well thought out manner you have to bear in mind that they were not done under strict lab conditions. In addition the lengths quoted are against a 72MHz system. However the general findings are quite relevant to this thread. Like so much else in radio, there's theory and there's practice.

http://www.rc-cam.com/ant_exp.htm

[Coyote]

Yes that article would seem to throw a spanner in the works. But on 35mhz is been proven to work very well with 8km+ ranges.

The proof is there even with 72mhz tests that as the length is increased the performance increases to a point then tails off. This i presume is the effect of the coil and the antenna matching. So what i`ve concluded so far seems from the 72mhz results to hold true. Once you get so long it has little effect unless you change the configuration of the receiver itself

I think im going to do some of my own tests on a 35mhz receiver using 101cm multiples and see what results we actually achieve

Thanks for that Electrotor