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JetPilot

Optimal Antenna Length for RC Receiver ?

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The antenna on my 50 MHZ RC receiver came in the 1/4 wave length for 72 MHZ, so its obviously not optimized for frequency, the factory just used the 72 MHZ antennas they had. I would like to go bigger than 1/4 wave, I know the 1/2 wave end fed antenna is not good, so should I use a 3/4 wave antenna, or would a 1 wave antenna do better ?

I will also be removing the matching coil in the receiver, as can be seen removed in this picture. I think this coil is to compensate for a shorter than optimum antenna, and I might be better off without it. Any thoughts on this ?

Thanks,

Mike

post-2644-1240788371_thumb.jpg

Edited by JetPilot

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Personally, I think you'd be better leaving it be. AFAIK, they are matched to the input of the Rx. If you add a 1/4 wave antenna, you're still having to match it. Same goes for a half wave. A half wave antenns is around 10' long at that freq.

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A few years back I had a go at trying 3/4 wave RX aerials and played with most lengths only to find what Ron said. The 3/4 and any above 1/4 wave was the same or there abouts to the 1/4 wave as long as they were mached. The added bonus of matching was also less interference but I now just use a 1/4 wave and spend my effort on good insallation. If the RX has a high Z input then it will be very unfussy and matching is not needed.

I always loose video long before R/C.

Terry

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There was a pretty good test done recently by someone that found extending the length of a receiver antenna on a 72 MHZ could make a huge improvement in RC reciever performance. I forgot who did the test and I cant find the article again, but I will search for the article and put a link to it here if I can find it. Also guys in Spain are having really good results with extending antennas on their RC receivers on 35 MHZ. I know the frequency is quite differnt, but 35 MHZ is not that much lower than 50 MHZ. Given the reports, there is a possiblity that something went wrong in some of your tests, and that this may well be worth revisiting.

With lawmate video transmitter on 2.4 GHZ, video range is easily 8 miles using a 1000 mW video transmitter with its stock 3 dBI vertical antenna and a 14 dbi patch on a standard Lawmate reciever. The average RC range on 72 MHZ is very consistantly being reported around 2 miles or less. A lot of us could benefit from better RC range, and if we can find a way to do it with something as easy as bypassing a coil, and extending an antenna, it would help a lot of people. I think 3/4 wave will be the next antenna length I try, all my range testing is done in the air by flying the plane to the point of going into failsafe, so the results should not be corrupted by ground, objects, etc.. I will let you guys know what I find.

Mike

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You may be right about my tests as I only tested one RX on 35Mhz but I for sure did not get a worthwhile increase in range with longer aerials. I gave up on 35Mhz and use 459Mhz now so dont have to worry. If I wanted to increase the range of a 72Mhz RX I think I would try making a simple tuned preamp, it would not add much weight to your big planes :)8 miles is very good too, I only use 100mW and get 1.5 miles of good video.

Terry

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Hi,

Some months ago, on another forum ... same question went up ...

a Dozen Guys measured their antennas lenght for "classical" receivers ( 9 known brands - 24 models);

Funny ! from 26 Mhz to 72 Mhz ... same 1 meter length @ +/- 2 cm ....

No comment.

Now consider amplifying the RX incoming signal will add noise more than anything else ...

Alain

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Yes noise would be even more of a problem than it is now but I don't see any other way of increasing range of a standard system and stay within the law.

Terry

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Hi, Terry

The best way is to have a perfectly tuned Tx/Rx couple ...

I've tuned myself all my receivers to match my Transmitter HF Heads, and the result is my eyes ( pretty good sight ... ) are the only limit.

I also tried flying gliders with a pair of Binoculars ... Radio is still working fine.

so ... what else to ask for ??? ...

Alain

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Hi Alain, before I swapped over to 459Mhz I used 35Mhz and I found the main killer for long range was electrical noise. Even now if I want max range I run my RX off its own tiny lipo and it is opto coupled to the decoder that is powered from the speed controller BEC. Then the aerial exits direct to the outside and is well over a foot from the video TX or motors. I agree good tuning is important, I used to make CB walkie talkies when I was young and peaking all the coils made a huge difference. I think signal to noise is the key though.

Terry

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Never mind.. Where's the delete post button?

Edited by W3FJW-Ron

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The antenna on my 50 MHZ RC receiver came in the 1/4 wave length for 72 MHZ, so its obviously not optimized for frequency, the factory just used the 72 MHZ antennas they had. I would like to go bigger than 1/4 wave, I know the 1/2 wave end fed antenna is not good, so should I use a 3/4 wave antenna, or would a 1 wave antenna do better ?

I will also be removing the matching coil in the receiver, as can be seen removed in this picture. I think this coil is to compensate for a shorter than optimum antenna, and I might be better off without it. Any thoughts on this ?

Thanks,

Mike

Mike,

I think the best way to find the optimal antenna length is to find the RSSI pin of your Rx and measure with a DVM. Use your Tx with a 50 ohm dummy load and measure the RSSI pin with varying lengths of antenna wire and ground counterpoise to get in the ball park. I would do this with the Rx actually in the plane because all the other electronics can have an effect on the tuning. Finally, for fine tuning fly your plane and use your Gecko OSD that has RSSI measurement capability and see what the the voltage is at measured distances from your Tx. If you'd like another set of hands, lets plan a day together, I'd be happy to assist!

-dave

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Hi all, the spanish guys that were mentioned here have some keys that have been mentioned here in this post, there are some general rules.

They don´t uninstall the receiver´s coil at all, they just multply by 2x or 4x the original receiver´s lenght. For a nice tunning and best performance I think a nice way to do this without having to deal with complex and expensive equipment is to run this MR.RC-CAM´s procedure:

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

Done the antenna improvement they are quite careful with the electronic placement inside the plane. Video TX, R/C receiver and servos goes as far away from each other as possible.

Some people also do the optocoupling solution for the OSD channel that goes to the receiver, some other desolder the bec cable that goes to the receiver and replace it with a twisted one and install a ferrite core on it.

Also the guy who made the 18 km trip with 35mhz uses a CF Band (27mhz) on the TX tunned properly to 35mhz and his car as ground plane. He is a very methodic guy, usually prepare a plane without fpv gear, test the behavior and keep on adding stuff 1 by 1, first video tx and cam, 2nd, pan servo, 3rd tilt servo, 4th osd, 5th gps, and so on so if something goes bad he knows exactly that is due to the last component installed.

An example of one of his planes here:

http://www.aeromodelismovirtual.com/showth...=269&page=4

From post # 117 he starts his 2nd easy glider.

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