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Antennae Wire Length - Whats the best length to use?

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Postby Tim Morris » Wed Jun 11, 2003 3:51 pm

i am trying to figure out if the length of wire from the reciever to the wtc connector, plus the wire from the wtc .


i was trying to calculate the optimal length of this wire and i got:

i took 1/75000000=x
x times 186300x5280=y

then y times .25 quarter length is 3.28 ft

is that right? when you count the wire from the reciever to the connector then add the external wire after the bulkhead connector to this also? isnt it also true that one cannot cut the external wire unless you re seal the end with something that works good enough to deny seepage to the wire end?

how important to reception is this?

also i am looking for more guidelines to reduce noise as it affects the reception. for example is it better to bundle power wires together as closely as possible to reduce noise?
Best Regards,
Tim Morris
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Postby Robse » Wed Jun 11, 2003 5:10 pm

Hi
I'm about to put my receiver inside the same WTC as the engine etc. as well. I'm planning on cutting the wire from the receiver and replace it with a shielded coax-wire from the reveiver and to the plug, and then put the un-shielded wire I just cut off on the outside of the WTC, and away from the engine etc. This moves the receiving 'antenna' away from the engine and servos, and not least the regulator! (Quite noisy too) Remember to connect the shielding in the coax wire to GND. (GND=ground) To reduce noise, add a pair of small capasitors on your engine poles, 100nF should do, or something like it. Connect the capasitor between the engine's + and -.

U also ask to bundeling: Yep! Put all supply-wires in a bundle of their own, AWAY from signal-carrying wire bundles.

U also ask to seal the end of the antenna wire in the water: Do that. Not doing it allows a tiny (but still) electrical connection between the wire and the ground connected metal structure of the WTC-ends and mechanics, and that might put a damper on your reception. Even if your system GND is sealed from the water, you will still ground your antenna... NOT good for reception.
Second, moist inside a wire is NOT good, as U will not be able to spot a corrotion problem before the wire is nothing but an empty plastic tube... Great for drinking coke, but bad for radio reception. :D

About the length: I'd say that the length that the receiver comes with has been calculated to match the wave-length of the frequency that it works with, so just maintain that length, and U should be fine. It is not VITAL for receivers though. Transmitters, on the other hand, HAVE to have a matched antenna in order to reduce the so-called SWR. (Standing Wave Ratio) A mis-matched antenna on a transmitter REALLY effects the range, and if bad enough, damages the transmitter. For the same reason, NEVER transmit while the antenna is not fully extracted.
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk

"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"
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Postby Bigdave » Thu Jun 12, 2003 9:38 am

Hi Guys, The best length is what was on it when you bought it. It is tuned for that length. It may sound like a dumb :p statement but you should not alter that length. Just as long as the wire in not coiled up you can just leave it inside of your WTC. I would not suggest running it past your motor if possible. One trick I always use is I run my antenna out the end cap of my WTC inside a length of golden rod. If you are not familiar with golden rod it is used in R/C airplanes to connect servos to the control surfaces. It is blue plastic tubing with yellow tubing that runs inside. I use the yellow tube. I just glue the tube into the end of my WCT and run the antenna out into the tube. Then I seal the end with a drop of RTV. You can bend the tube into any shape you want, and can make it any length you want. I usually make mine about 2FT. I have used this for more years than I care to remember and have had no problems or leaks. I have even used the tube to blow into to add pressure to the inside of the WTC to check for leaks. Give it a try. :D
David Welch
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Postby Ramius-II » Thu Jun 12, 2003 3:13 pm

Hi Tim:
The formula for antenna lenght is 11810 / frequency in MHz. time .96 (velocity factor). Thus at 75.590 MHz the antenna lenght is 37.5". The distance is measured from the printed circuit board or the end of the coax. This is a continuous / accumulative lenght so if you pass it through a connector it is all part of the lenght. :;):
EMI/RFI/E-I E-I OH is a bit more tricky and not impossible. If you are running both a power lead and a ground lead to a destination it is typically best to put the wires in an electric drill and twist them together. Twisting will add capacitance as well as provide a bit of shielding. Keeping the receiver battery close to the receiver is also a good practice. Batteries are the best filters. Another technique (in the early stage of construction) is to go to a "single point ground" system. This is where all ground connections go to a single stud.

Bypass capacitors of .01 mfd are very good too. You can not have enough of them. Be sure the cases of all your motors have a nice heavy ground wire well soldered to them.

Voltage regulators like to have a 1 mfd cap on their output along with any other filter caps. It's common practice to put "one of each" that is a 1 mfd, .1 mfd, .01 mfd and .001 mfd! The last, .001 mfd is only necessary if you are working with high frequency stuff like 2.4 GHz.

If all this does not handle the problem, then a ferrite bead or torroid on the receiver power lead! :)

Best, Ed
:D
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Postby 70-1048541373 » Mon Jun 30, 2003 11:01 am

Hello All,

Have a couple of quick questions about some of the suggestions in this thread:

1- When you said attach the shielded coax-wire to the receiver .... would you un-solder the existing wire and solder the center wire of the shielded coax-wire right too the board or leave an inch or so of the existing wire and attach to the end of that?

2- When you suggested "Remember to connect the shielding in the coax wire to GND" ... do you mean a GND "within" the receiver?

Thanks for the help!
Mike
70-1048541373
 

Postby Robse » Tue Jul 01, 2003 9:47 am

Hi, Floats_like_a_Rock.

1) Well, I would leave an inch or so on the circuit board of the receiver, so I wouldn't have to solder on the board it self.
It most likely a multi-layer PCB (Printed Curcuit Board), and the last thing we want is to dammage the board. (Been there, done that.. :( ) I would also make sure to tie the coax cable to something fixed, so that the mechanical stress from the vibrating cable during sailing does not end up in the small, thin wire from the receiver it self, breaking it with time.

2) Yes, Sir. Connecting the shielding to the ground within the receiver ensures low noise penetration, and equal electrical potential. Not connecting the shielding anywhere almost turns it into an antenna, and connecting it to ground (GND) somewhere else poses the risk that you get "ground-loops". Ground-loops causes interference, and are hard to pin-point. Eg. when building HIFI equiptment you should put *ALL* ground connections at ONE point within the 'box', and not just the nearest screw or bolt point. Remembering this praksis everywhere saves you alot of troubles, regardless what U build.

Hope this answers your Q's? If not, please let me know. :)
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk

"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"
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Postby Bob the Builder » Mon Sep 29, 2003 10:39 am

Ramius-II wrote:Hi Tim:
The formula for antenna lenght is 11810 / frequency in MHz. time .96 (velocity factor). Thus at 75.590 MHz the antenna lenght is 37.5". The distance is measured from the printed circuit board or the end of the coax. This is a continuous / accumulative lenght so if you pass it through a connector it is all part of the lenght. :;):

Best, Ed
:D

I have a quick question on a related topic.

I just finished modifying a 16th century galleon model conversion to RC, and (as I was on a strict budget), I utilized the running gear from an old Radio Shack RC car that I had lying around.

The car's antenna says that it's on 27mhz. Somewhere along the way, I misplaced my antenna, and now that I want to run one up one of the masts, I'm not sure of the correct length to use.

The above post's formula makes my antenna length somewhere around 105 inches.??? Does this formula not apply in this case, or do you use a smaller fraction for the antenna length? I'm sure that the original antenna was only about 14" long (approx).

Any help would be appreciated.




Edited By Bob the Builder on 1064846664
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