FlooredCOBRA

I while back I had a crash where my antenna wire got cut off. All I had was about a 2 inch wire left over.

I went to the hobby shop and asked them if they have replacement antenna wire for the JR 549X. He said they do and pulled out a long black wire and said it was the one for it. My question is the length if anyone knows what the correct length is for it. I have replaced receiver ant. wires on ground vehicles when I worked at a hobby shop long time ago and never had a problem. I replaced Futaba ant. wires which we had ordered directly from them for replacements. So I did not have a question about it.

My concern is that it will be for a plane and the range is way more than a car or boat. I just now got done soldering it back on to the circuit board and it looks clean and like it should work.

If anyone has any input on what the proper length is or a web site or something that I can get an idea and compare to one I have would be great. I planned on doing a rang check an all but would like to see if what you guys opinions are on here.

I posted this in the radio servo receiver forum but maybe you guys in here can give me an idea

Flypaper 2

Most are 39 ins from case to end.

FlooredCOBRA

okay thank you. I will check to see what mine is

Rodney

Anything from 39 to 42 inches will work well.

vica6m5

My Feedback: (2)

41 or 42 " I think...

FlooredCOBRA

well I got the antenna soldered back and put the crystal in it. Now I found the receiver wont work. I guess something else in it is messed up also. I may just have to go out and buy another 549X JR receiver.

Homebrewer

My Feedback: (8)

Antenna Replacement:

It is extremely easy to create an unwanted solder bridge when soldering a new antenna. Carefully inspect both sides of the PC board where the antenna terminal is and check for any possible stray solder that could have caused a short.

klhoard

My Feedback: (10)

I highly recommend that you send your receiver to Radio South. His shop is expert at repairing radio equipment and has great turnaround time, especially from the Atlanta area. I have dealt with Tony for years, he will take care of your stuff. . .

I always send in my receivers after a crash, even if they work well and range check. A crash can create small cracks in the circuit board or damage the ceramic filter and it won't show up until later.

www.radiosouthrc.com

n7tb

If you know the frequency of your receiver (not channel) you can apply a standard formula to calculate antenna length. I forget the exact formula but you could go to www.arrl.net, the ham radio website and should be able to search for the formula. Someone else here may have the formula memorized. At the FM frequencies we operate on, the antenna length should be pretty accurate. The only caveat is if the antenna is "loaded" at the receiver. That would shorten the antenna length.

Good luck!

Terry

Rodney

N7TB, not so, in a perfect world with perfect zero loss coils and capacitors, you could calculate the length; but, in our RC receivers the loading coil and capacitors make this calculation difficult and unreliable. The antenna is base loaded so is shorter than your calculations indicate. The only reliable way is to use the proper instruments to measure for peak tuning.

Jemo

My Feedback: (5)

The average 72 mhz antenna length is 39"

234/freq. = length in feet.

EG: for ch 38. / 72.550



234/72.550 = 3.225 ft. x 12 = 38.704 inches

klhoard

My Feedback: (10)

Geez guys,

By the time you figure it out, he could have his receiver back from the shop!!!

Jemo

My Feedback: (5)

nothing to figure out, make it 39 inches and it will be fine.

Bish Wheeler

I'm not sure, BUT, I think these antennas are "long wire" antennas and I think they have no particular specified length. I am a HAM (General class) and all my stuff is on 6 meters (50 Mh +/-) and my antennas from Futaba are the same length as the 72 Mh stuff.

I will check on this and see if they are indeed "base loaded", but I kind of think they are not.

Bish
K7QCP

Steve Lewin

Since the antennas on the 35MHz equipment I use are also exactly the same length I'm not sure any formula based on frequency is going to help you .

Steve

Jemo

My Feedback: (5)

Steve,
All antenna are based upon frequency.
But, there are many variables involved. How the radio manufacturers handle Our equipment as far as wire length is unknown to me.
I do know that it is not nearly as critical as many may think. For instance when you happen to clip a few inches off your receiver antenna, after ripping through that barbed wire fence at the end of the field.....

You will not see any difference as far as reception goes, the next time you fly.

Crash_N_Burn

"For instance when you happen to clip a few inches off your receiver antenna, after ripping through that barbed wire fence at the end of the field.....

You will not see any difference as far as reception goes, the next time you fly"


I think that depends on if you rip off the first few inches, or the last few inches... LOL


Your correct, jemo, I'm just kidding you....it doesn't matter...a short bedspring would probably suffice.

Steve Lewin

Then they must use an interesting formula that comes up with the exact same number for 35, 50 and 72 MHz .

I know what you mean but it would be more accurate to say the antenna length is based on the receiver design. E.g. I own a couple of Rxs which specifically say in the instructions that the antenna may be shortened by anything up to half the length if less range is acceptable. No suggestion that doing so may in any way affect the tuning or frequency response of the Rx.

Steve

N1EDM-RCU

I think your best advice was from klhoard - send the rx to Radio South.

Yes, there is a definite length to an antenna, and it can be calculated, but that's not the end of it. After putting the new antenna wire on, the rx needs to be tweaked so that the antenna is as efficient as it can be.

Shortening an antenna won't cause instant disasterous results. But it's still not a good idea to shorten it.

As an analogy, a properly tuned antenna will let the receiver 'hear' the signals much better, while an improperly tuned antenna will be slightly deaf. When the plane is close to your transmitter, it will hear fine. But as the plane gets further away, the receiver will find it will be harder to hear your tx, compared to a properly tuned rx. Personally, I don't want to take that kind of a risk with my plane and other people's bodies.

It's never a good idea to monkey around with an antenna wire. And, you could have suffered some other damage to your RX and a checkup will uncover this.

I've also sent rx's back to Tony at Radio South and had very good service. It gives peace of mind, and is cheaper than a new RX. Plus, I think that they vibration-check the rx as well.

Just putting in my $.02

Bob

JLS

My Feedback: (19)

What size of wire should I use to replace my antenna? Is there anything special about antenna wire? Where is the best place to get it.
Thanks
JLS

Bish Wheeler

Just use ordinary #16 stranded copper wire, plastic coated of course.

Bish

N1EDM-RCU

Bish,
I think that you will find that #16 wire is much too heavy - that's almost the size of hose wiring. For example, ordinary lampcord is #18AWG, and the #16 is thicker than that.

Antenna wires are somewhere around the #22 or #24AWG size, but it's still best to send the rx back and have it checked out if there is any question in your mind....

Bob

Bish Wheeler

Bob,

You are correct. #22 is just right. Just a case of my fingers not in tune with my mind.

Bish