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Old 01-21-2017, 04:28 PM   #1
axefly
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Default jr 9xII 35mhz transmitter aerial length ?

hi everyone

i received a new replacement aerial for my jr 9X2 35mhz radio but unfortunately it doesn't fit, and compared to my broken aerial the sections are somewhat shorter, does anyone know what the exact length is tip to tip so i can shoe horn an aerial from the broken one's i have to make one with the correct length.

update:

i had 2 people telling me the lengths, one said it's 1150mm and the other said 1117mm, anyone know why it differs, does every 35mhz radio has it's own sized aerial or are they changed in size according to which country one lives in, i don't want to blow the radio or decrease it's range so i still don't know what length it should be.
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Old 01-21-2017, 05:17 PM   #2
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Simple math really. Radio waves length is directly proportional to frequency.

For full wave length in meters = 300 / Frequency in Megahertz.

Whip Antenna usually are 1/4 wavelength. Don't bust your balls trying to figure it out, just use THIS CALCULATOR and enter your TX frequency. Do not forget to select 1/4 wavelength. You got two different answers because someone changed the frequency they used to calculate. It wil twice as long as you were told. Around 7-feet at 1/4 wavelength.

FWIW a whip antenna is super simple to make. Just a peice of #14 or #12 AWG solid copper wire from any box store cut to length.
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Old 01-21-2017, 05:41 PM   #3
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i appreciate you trying to help but the measurements above was not calculations, they actually have taken out the aerials and posted the sizes to me showing the lenghts in fotos next to measuring tapes, so the calculator must be wrong, it leaves me to think each radio his tuned to a specific lenght and that it is not universal and possible also custom to alength depending on where in the world you are, i'm in south africa, maybe they made a batch arials and tuned the radios to as longer than normal length, it just doesn't make sense that thise 2 above are different lengths, but those arials were taken out and measured tip to tip...
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Old 01-21-2017, 05:47 PM   #4
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Originally Posted by axefly View Post
hi everyone

i received a new replacement aerial for my jr 9X2 35mhz radio but unfortunately it doesn't fit, and compared to my broken aerial the sections are somewhat shorter, does anyone know what the exact length is tip to tip so i can shoe horn an aerial from the broken one's i have to make one with the correct length.

update:

i had 2 people telling me the lengths, one said it's 1150mm and the other said 1117mm, anyone know why it differs, does every 35mhz radio has it's own sized aerial or are they changed in size according to which country one lives in, i don't want to blow the radio or decrease it's range so i still don't know what length it should be.
Which Tx or Rx ?

I have just removed the antenna from my JR Propo 35Mhz radio Tx and it is 1070mm (42.25") long when extended ...

I removed it from the Tx because first section is mostly internal to radio case and I wanted the real extended actual.

That radio is original and no modifications etc.

As to your situation ... the 35Mhz frequency band is quite wide and therefore the antenna length can have a variation and I would be quite happy to use something in region of 1100mm ... but I think 1150 may be at limit.
We do not fly to max limits of our radios - well out of sight - so a little reduction based on length is not a problem.

The RF circuitry really gets upset when it cannot propagate RF power ... such as antenna not extended or missing ...


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Old 01-21-2017, 05:51 PM   #5
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I've got an old 35MHz Futaba Tx here and i just checked the antenna and it's 1050mm from the point it emerges from the case, there is probably 100mm inside the case.

So I'd not worry at all about +/- 50mm or so.
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Old 01-21-2017, 05:53 PM   #6
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Originally Posted by JetPlaneFlyer View Post
I've got an old 35MHz Futaba Tx here and i just checked the antenna and it's 1050mm from the point it emerges from the case, there is probably 25mm inside the case.

So I'd not worry at all about +/- 50mm or so.
Exactly .... given the wide band as well ....

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Old 01-21-2017, 06:25 PM   #7
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Originally Posted by axefly View Post
i appreciate you trying to help but the measurements above was not calculations, they actually have taken out the aerials and posted the sizes to me showing the lenghts in fotos next to measuring tapes, so the calculator must be wrong,
Really? Then every antenna made by pros is wrong.

If in fact the manufacture is using 1.115 meters means it is 1/8 wave length. Use the calculator I gave you, select 1/8 Wavelength, and enter the operating frequency in Mhz.

FWIW 1/8 wavelength is extremely inefficient.
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Old 01-21-2017, 07:45 PM   #8
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Tx, its a jr 9xII 35mhz

To me it's just strange why does no one person's Tx antenna are the same, the original antenna i had was longer than 1000mm, closer to 1117 or 1150 somewhere why do they vary them from tx to tx ? Doesn't that mean if mine is tune for say 1130mm, then using something like a 1000mm antenna it will be out of tune ?, why would JR which is obviously a world class transmitter company make the aerial lenghts greater than 1000mm, i have heard everytime you change the transmitter antenna, you need to re-tune the radio on the inside and i have no idea how to do that and there is no one than can do it here.

i just don't want the radio tuo burn up or be damaged and i don't want to lose range.

if i make it between 1000-1150mm will it be just fine or do i have to retune my radio so it does not overwork or lose range.

Originally Posted by solentlife View Post
Which Tx or Rx ?

I have just removed the antenna from my JR Propo 35Mhz radio Tx and it is 1070mm (42.25") long when extended ...

I removed it from the Tx because first section is mostly internal to radio case and I wanted the real extended actual.

That radio is original and no modifications etc.

As to your situation ... the 35Mhz frequency band is quite wide and therefore the antenna length can have a variation and I would be quite happy to use something in region of 1100mm ... but I think 1150 may be at limit.
We do not fly to max limits of our radios - well out of sight - so a little reduction based on length is not a problem.

The RF circuitry really gets upset when it cannot propagate RF power ... such as antenna not extended or missing ...


Nigel
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Old 01-21-2017, 07:50 PM   #9
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Originally Posted by dereckbc View Post
Really? Then every antenna made by pros is wrong.

If in fact the manufacture is using 1.115 meters means it is 1/8 wave length. Use the calculator I gave you, select 1/8 Wavelength, and enter the operating frequency in Mhz.

FWIW 1/8 wavelength is extremely inefficient.
i see what you say, but why was my original antenna which came from JR antenna length greater than 1100mm ? Surely they which is are a world class radio manufacturer knows their equipment better than anyone else ?
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Old 01-21-2017, 07:52 PM   #10
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Originally Posted by JetPlaneFlyer View Post
I've got an old 35MHz Futaba Tx here and i just checked the antenna and it's 1050mm from the point it emerges from the case, there is probably 100mm inside the case.

So I'd not worry at all about +/- 50mm or so.
Thanks for measuring JPF, starnge why a futaba matches another person's jr x388s radio length of 1150mm see i just don't get it, it all leaves me to wonder, how would i know what length is pefectly tuned to my radio (i mean inside the radio there is something you need to adjust according to aerial length sorry i'm not the expert so don't know the terminoligy
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Old 01-21-2017, 08:02 PM   #11
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Originally Posted by axefly View Post
Thanks for measuring JPF, starnge why a futaba matches another person's jr x388s radio length of 1150mm see i just don't get it, it all leaves me to wonder, how would i know what length is pefectly tuned to my radio (i mean inside the radio there is something you need to adjust according to aerial length sorry i'm not the expert so don't know the terminoligy
35Mhz ... are you UK based ?

Then its worth contacting Mike ... pal of mine who's the ex Service Manager of McGregor RC who are UK importers of JR gear. He left after they had the fire that destroyed all the archives of the older radios. But he has a setup and archive that will sort this so easily.

He's contactable via Mainly Trains and Planes web site ...

I don't want to post his details online ... so please google them and they can put you in touch with him.

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Old 01-21-2017, 10:38 PM   #12
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Antennae can also be electricaly shortened by adding a coil in it's top, centre or base of. A base loaded coil would be inside the Tx case.

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Old 01-22-2017, 01:32 AM   #13
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Originally Posted by axefly View Post
i see what you say, but why was my original antenna which came from JR antenna length greater than 1100mm ? Surely they which is are a world class radio manufacturer knows their equipment better than anyone else ?
RF is RF, makes no difference who the manufacture is.

As frequency goes up, length shortens. Any good antenna you buy is cut longer than needed, or lower frequency than target frequency. Then the user prunes the antenna down to nail resonant frequency. Great example is telescopic antennas because you can adjust the length to match a band of frequencies. Once cut you are locked into a frequency.

The answer to your question is what is the exact Frequency and is simple math formula a 6th grader can master. Example plug in 35 Mhz, then try 35.1 Mhz. You will get two different answers because the frequency changed. 35.1 will be shorter than 35.0. So it is important you use the exact frequency you TX at.

Try these two numbers and see what you get. It will open your eyes.

31.008 Mhz
31.92 Mhz

31 MHz does not equal 35 MHz. Here is the 6th grade math behind it.

1/8 Wave Antenna Length in Meters = 35.66 / Freq in Mhz

FWIW if the antenna were 5/8 wave length long, 5 meters, would have a range approx 10 times that over 1/8
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Old 01-22-2017, 01:50 AM   #14
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Originally Posted by ron_van_sommeren View Post
Antennae can also be electricaly shortened by adding a coil in it's top, centre or base of. A base loaded coil would be inside the Tx case.
Exactly, but at a cost of efficiency (range).

If in fact the operating frequency is around 35 MHz, and the factory whip is 1 meter, it is 1/8 length whip give or take a few mm depending on the exact operation frequency. What the manufacture is doing is selecting the middle frequency of a band of operating frequencies. Example say a band from 35.46 MHz (1.006 meters) to 35.86 MHz (.994 meters). Therefore the antenna is cut at the center frequency of 35.66 MHz or exactly 1 meter.
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Old 01-22-2017, 03:14 AM   #15
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Back in those old days when we used 36 and 27 mHz we would take our planes off with the antenna only pulled out halfway and then pull it right out when we gained height. Never caused a problem.
We used to have coloured ribbons on the antennas and if it was pulled right out then you could be sure the ribbon would get sucked into the prop and that was goodbye antenna.
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Old 01-22-2017, 10:16 AM   #16
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Nigel i am not in the uk i'm in south africa, that's why i'm wondering if for a certain reason aerial lenghts could possibly have been sized and radios tuned to a certain length for us here compared to the rest of the world, maybe there is a good reason ? maybe i'm chasing up a ghost with all this ? i will look up his site and send him a mail thanks.

Derek i can see you know quite a bit about antennas however it still doesn't help, what will help is someone telling me what length i need to make my antenna, for my model make radio, what is it tune to on the inside.

it's a jr 9xII like mentioned 35mhz, i use a synthesized spcm receiver jr rs77s, so the receiver scans for the tx signal (binding obviously everyone knows) but it's not a physical based crystal tx and rx thats why i mention it.
so my module at the back reads band no. 55-90
TS-35JU
then there is two pots on the module and both arrows seem to point at 55, so i guess the receiver binds to 35.055mhz ?

Now again will i have to change the calibration somehow on the inside if i change my antenna lenght to anything between 1000 and 1150mm i don't know you probably need special equipment for that and is it required ?

Or can i use any aerial length between 1000mm and 1150mm without a problem, i just want the aerial to be as long as it should be acoording to the manufacturer, jr america could not help they say they never imported the radio, but will try your link Nigel thanks, in the meantime, if anyone has an answer to the above questions please do continue the discussion, i can only learn and perhaps get a solution.
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Old 01-22-2017, 12:09 PM   #17
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Honest - I think you are over-complicating it.

All the radios have a compromise antenna length depending on their RF section and how much shielding in the case.

All you need worry about really is to obtain the JR 35Mhz antenna that screws into the mount. An antenna of around the 1020mm mark +/-20mm will do the job fine as long as it fits the screw mount.

Problem is that 35Mhz gear is out of production for guys like JR / Futaba etc. Its only the RTF cheap jobs that still supply with 300m range gear on 27 and 35Mhz.

Is there no JR agents in SA ?

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Old 01-22-2017, 05:59 PM   #18
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Thanks Nigel after a lot of digging i found 4 people willing to check on their 9XII radios for me one said his is1150mm the other 1158 and the other 1164mm so looks like indeed my antenna should be between those lenghts, i made mine 1158 and will now be flying like that, thanks for the discussion
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Old 01-25-2017, 04:03 PM   #19
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Originally Posted by dereckbc View Post
Simple math really. Radio waves length is directly proportional to frequency.

For full wave length in meters = 300 / Frequency in Megahertz.

Whip Antenna usually are 1/4 wavelength. Don't bust your balls trying to figure it out, just use THIS CALCULATOR and enter your TX frequency. Do not forget to select 1/4 wavelength. You got two different answers because someone changed the frequency they used to calculate. It wil twice as long as you were told. Around 7-feet at 1/4 wavelength.

FWIW a whip antenna is super simple to make. Just a peice of #14 or #12 AWG solid copper wire from any box store cut to length.
Dereck, thanks for this info.

I dug out my old Airtronics Vanguard VG6DR on channel 34 (72.47 mHz) and measured the antenna. Came out very close, ~42" actual vs ~39.3" calculated, and presume 'close' is OK.

Still fly with it. Have one of the new receivers and crystal from Headsup that is much lighter than the original plus it has the plugs to fit the ESC and current servos. Works well and fits in with a balsa plane from that era.

Regards, Bill

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Old 01-25-2017, 04:43 PM   #20
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Originally Posted by Abuelo View Post
Dereck, thanks for this info.

I dug out my old Airtronics Vanguard VG6DR on channel 34 (72.47 mHz) and measured the antenna. Came out very close, ~42" actual vs ~39.3" calculated, and presume 'close' is OK.

Still fly with it. Have one of the new receivers and crystal from Headsup that is much lighter than the original plus it has the plugs to fit the ESC and current servos. Works well and fits in with a balsa plane from that era.

Regards, Bill
You are welcome Sir. Yep 1 meter exactly @ 72.47 Mhz for 1/4 wave length.

Most calculators will be a bit on the long side allowing the user to prune the antenna for optimum match. Take two identical radios operating on the same frequency and there will be a small difference in length of antenna at optimum performance. Say 1/4 inch. Just the difference inside the radio TX output impedance. With Plane radios you cannot tune the antenna because the manufacture did not give you the means to do so. All ham, professional, and even some CB radios have built in SWR bridges built into them so you can tune the antenna. The other method is using an In-Line SWR meter which again is not an option with Plane radios.

If I were you I would cut it to 40 inches and call it good. FWIW if you want to play with the numbers a bit for 1/4 wave antenna to find the length in inches the formula is:

Length in Inches = 2848 / Freq Mhz.

Now from that equation you can solve for Frequency = 2848 / Length in Inches. So if you take 2848 / 42 inches = 67.81 Mhz.

Now for your cell phone works at 800 and 2200 Mhz in the USA
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