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Old Apr 26, 2011, 05:21 AM
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does the length of Coaxial Cable of 2.4 GHZ Antenna affect the performance of receive

does the length of Coaxial Cable of 2.4 GHZ Antenna affect the performance of receiver?

I just crash my Copter-X helicopter, and have damaged one of the two antenna of Futaba 2.4 GHZ Fasst receiver. There are 2 antenna, lets call it Antenna-A and Antenna-B.

Part of the Helicopter have hit the middle part of Coaxial Cable, its almost cut in. I can see parts of the shielding wire.

have performed a range check, and have verify that the Antenna that was damage was not performing any more.

I was hoping I could revived the damage antenna by shortening the Coaxial cable, I just cut the Antenna, and stripped the Shield Part, I make sure the Antenna (expose wire) is exactly the same length. but the Shield Part is shorten to 20 mm, the working Antenna is 100 mm.

after doing the above modification, and performed a Range Check. The Performance of the repaired antenna is not as Good as the Original.

by the way, I have 2 Identical Receiver. So I know how good the Antenna performed.........I just use the Palm of my Hand just to Cover up the Antenna and see the LED light change from Green to Red.

look like I really to purchase the Antenna and replace it.

or we need to cut on a specific lenght, a multiple of a Wave Length of 2.4 GHZ.
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Old Apr 26, 2011, 06:56 AM
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United States, IL
Joined Jul 2009
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My recommendation would be to just replace the antennas. They use a standardized type of connector and antenna for 2.4GHz. Your Futaba distributor or dealer might even carry them or be able to order them for you. If not, you should be able to find some on-line. The length of the antenna leads could have some effect, but it's hard to say with any certainty how much of one. There are also extended lead antennas that you can get. And though I haven't personally tested them, I hear they work well.

From Hobbico, the standard antenna replacement part number is: HSP79493
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Old Apr 26, 2011, 02:16 PM
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Sao Paulo, Brasil
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Since the unifilar 3.1mm antenna is does not have the right impedance (50 ohms), the cable must match the impedance between antenna and RF circuito, acting as a impedance transformer.

If you simply cut it to a smaller length you will probably not match the impedance and the performance can decrease dramatically.

The better is to replace the whole antenna by a new one. If it is not possible, try to cut exactly one wavelength (~82mm - speed of light / 2425 * 0.66) if possible, so impedance must match, but PLEASE do a good range test before flying.
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Old Apr 27, 2011, 01:20 AM
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Ok......Thanks for all your suggestion.

Have open up my CORONA 2.4 GHZ Receiver, unfortunately, the antenna was Solderly directly to the board.

I was hoping, I could find a Junked 2,4 GHZ Wireless Router

The Length of Unmodified Coaxial cable is 100 mm.
The Length of modified Coaxial cable is 20 mm.
Both Measurement taken from the exit point to the Base of Antenna.

look like my next solution is just remove the Coaxial Cable, just need to toltally Stripped the Shield, and just leave 31 mm of antenna only, Just like Spektrum AR500 Receiver, I guess, this might work.

anyway, I could easily compare its performance thru a Range Check.
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Old Apr 27, 2011, 01:37 AM
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Quote:
Originally Posted by alexcmag View Post
Since the unifilar 3.1mm antenna is does not have the right impedance (50 ohms), the cable must match the impedance between antenna and RF circuito, acting as a impedance transformer.

If you simply cut it to a smaller length you will probably not match the impedance and the performance can decrease dramatically.

The better is to replace the whole antenna by a new one. If it is not possible, try to cut exactly one wavelength (130mm) if possible, so impedance must match, but PLEASE do a good range test before flying.
This explanation did surprise me, but it seems to be logical.
At the first sight, coaxial part of antenna is just a void length extender which should not electrically affect operation (except some attenuation), however, as the active 31mm part is probbably balanced (dipole) and coax line is unbalanced, it may create reflections and thus the length of coax may get important..

How long is the original antenna coaxial part? The speed of light in coax cable is reduced to cca 70 percent, so the wavelength is not 135mm inside it, but only approximately 95mm - does it seem to match this?
Perhaps one half of wavelength (internal, so count cca 47mm) is added to deal with mirrored polarity of reflected signal.

But if receiver is not inside carbon fuse, there is no need to use coaxial length extension at all - just the 31mm long simple wire, directly at the receiver connector, stripped off the shield. It is the same antenna as the one at the end of long coax cable..
Perhaps he may make one such direct antenna on connector, from damaged one, and use it temporary - until he will get complete new replacement.
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Old Apr 27, 2011, 02:49 AM
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Yes, I am also surprise with my Range check experiment......I can not put the Transmitter to a Power Down for a very long time, so I just put it inside a non-working refrigerator inside our home, then walk outside our house until the Receiver goes into fail safe mode.

Light travel at 300,000,000 meter per second,
so the wavelength = 300,000,000 meter / 2,400,000,000 Hz = 0.125 meter = 125 mm

signal travel slower by 10% in the copper wire, so if you multiply 125 mm x 90% = 112.5 mm
the result which is very near the actual lenght of the Coaxial cable, 100 mm (rubber gommet to base) + 10 mm (rubber gommet to connector) = 110 mm

this is how I remember during my High School Physics.

I have cut the coaxial shield to 20 mm, this is about 1/5 of the wavelenght.....this might be the reason its performace drop.

maybe if I cut it in a multiple to 1/4 wavelength. I might get a better performance.
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Old Apr 27, 2011, 03:02 AM
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I recall that it was slower than 0.9, and testing by making 1/4 wave shorted stub (as out of band noise filter and protection against ESD) I have got around 0.7 in real. But it was only amateur attempt so I am not any sure, even if it works well..
Here are som numbers even much lower http://en.wikipedia.org/wiki/Velocity_of_propagation

1/4 and 1/2 wave are a magic numbers that may change things dramaticaly. Better use full wavelength.

Be carefull when touching the antenna by metal, and/or soldering, to avoid dramatic range decrease by damage to receiver circuits - an internal noise may develop that will make rx useless.
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Old Apr 27, 2011, 02:33 PM
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Quote:
Originally Posted by alexcmag View Post
Since the unifilar 3.1mm antenna is does not have the right impedance (50 ohms), the cable must match the impedance between antenna and RF circuito, acting as a impedance transformer.

If you simply cut it to a smaller length you will probably not match the impedance and the performance can decrease dramatically.

The better is to replace the whole antenna by a new one. If it is not possible, try to cut exactly one wavelength (130mm) if possible, so impedance must match, but PLEASE do a good range test before flying.
I am sorry, but this information is not correct. The lenght of the shielded part of the antenna is not relevant. (As long as it is shorter than 50cm, so that no noticable attenuation occurs, that is.)

Thus, it is perfectly ok to cut off the damaged part of the antenna, and then cut back the shielding so that the center conductor at the end has the original length of 30mm.

The shielded part has in no way anything to do with any wavelength.

It is technically not possible that an antenna modified in this way has a worse reception, provided that no invisible damage has occurred before, and that the physical location of the antenna tip still allows good reception, meaning that it is still at a distance to conductive materials, although being shorter than before.

Here are two examples of changing antenna lengths:

http://christian-hanke.blogspot.com/...a-antenna.html

http://www.socal-rc.com/forum/showthread.php?t=7178
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Old Apr 27, 2011, 03:53 PM
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Sorry for the 130mmm, I put "speed of light / 2450 / 0.90" on Google and rounded the number, but it is actually "speed of light / 2425 * 0.66" = 81.6mm on a shielded cable. But it is better to confirm this number also.

As I know, if antenna (dipole or monopole) are balanced with circuit, the coax cable is simply an extension and can have any length, but must avoid multiples of wavelength due to reflection.

If the antenna is not balanced, the cable can be used to balance the impedance, in this case the coax length is very important.
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Old Apr 27, 2011, 04:32 PM
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Julez, at the first sight, it did seem to me so..
.. but I am afraid that there is much more to unveil down deep inside short wave signal propagation, than just a simple attenuation of the feed line.
E.g. http://www.microwaves101.com/encyclo...cfm#properties
If there is a balun at the end of coax line, perfectly matched, and dipole is perfectly matched as well, then coax feed line length mean nothing.
Ok.. However, I am pretty sure that our antennas are far from such case (even if balun is there, but especially if it is not there and it is just the stripped end of core...).
SWR is getting important, and thus, the length of feed line may get important as well.
How can "in no way anything to do with any wavelength" result in this rule?
"two identical mismatches can be made to cancel each other by locating them approximately one-quarter (or perhaps three-quarters) wavelength apart."
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Old Apr 27, 2011, 09:22 PM
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Maybe my Futaba Receiver was damaged due to crash, thats the reason why the damage Antenna give a poor performance after I cut it short.......anyway, will try to swap the 2 antenna and check its performance.
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Old Apr 28, 2011, 11:51 AM
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The only active part of our antennas is the unshielded center conductor at the end. This must have the correct length, 30mm.
The length of the shielded part is not important.
Have you ever wondered, why all MHz RXes had more or less the same antenna length, but the coax antennas of different manufacturers all have pretty random lenghts?

If someone really believes that a specific lenght of the shielded part has some advantage, please apply this lenght. It will work out for this person; the placebo effect is well proven.

If anyone thinks, that specific lenghts have an advantage, please explain why all manufacturers, who should know best, ignore this.
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Old Apr 28, 2011, 03:46 PM
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Canada, AB, Calgary
Joined Dec 2004
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Here is a question I send some time ago to Futaba

in some of my F3F glider I need to extend the antenna of my HS6008HS receiver so they come out the fuselage at a different place.

Can I replace the stock antenna by the HSP79766 one ?

Do I have to replace both antenna's or can I just replace one and keep the second one as is ?

HSP79766 ANT EXT 400MM
HSP79493 2.4 Receiver antenna original

And here is the answer I received...

Yes, you can use that antenna with that receiver. You will need to replace both antennas. You only get one antenna with that part number. So you will need to order two of them....
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Old Apr 28, 2011, 05:28 PM
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United Kingdom, Oxford
Joined Feb 2003
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All GHz antennas have precisely trimmed lengths unlike MHz receiver antennas which were more or less non-resonant or if they are then made so by base loading.
I have to agree with alexcmag's assessment of the situation, a stripped length of coax does not represent a balanced antenna, therefore the outer conductor of the cable becomes part of the antenna and the length all the more critical being only a wavelength or less from the RX.

A.
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Old Apr 28, 2011, 05:43 PM
most exalted one
United States, CA, Hemet
Joined Aug 2002
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186000*5280*12*25.4 = 29937984000 / 2400000000 = 124.72.. 1/4 = 31mm
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