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Antenna design questions...

pseudolobster

pseudolobster

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I decided to hack apart an old wireless keyboard I had laying around in order to make a small remote control to change the volume on my computer from a distance.

I've hooked everything up okay, and it works fine, though I found out the antenna they used in the original design is a four foot piece of wire wrapped around the entire inside of the keyboard! I'm trying to build a handheld device here, so that just isn't going to work... I've tried cutting the wire down to exactly 1/4 its original length, though the range drops down to about three feet... I tried wrapping that wire around a nail, though the range dropped... I really don't know much about antennas, but I swear there has to be a way I can wrap a few turns of wire around a ferrous core and still get some decent range without having to hold a 4 foot loop of wire above my head.

The transmitter runs at 27mhz... Does anyone have any suggestions how I could cheaply build a small antenna for this thing?
 





Loaded antennas are a bitch to figure out without an antenna meter. Perhaps you could find an old CB handheld with a "rubber duck" antenna. It is usually a coil without core about 12" long. 1/4 wavelength at 27 MHz is LOOOONG. Some RC toys running on 27 MHz have short antennas with base loading coils. You nail trick may have been too much inductance and acted as a trap. Try removing the nail.

Mike
 
The thing is that it depends on the amount of "turns" you do around a metallic core and the length of the antenna. As Mike said, try using nothing as the core and let us know how it performs. There's no trick to it, if you need 3 meters you'll have yo use 3 meters of wire. But there are some nice ideas out there like newer car antennas which have a shortened length with more turns (not like the 1 straight wire old car antennas). You should look up for one of those and ask if you can modify it to Rx or Tx at 27MHz.
 
Thanks for the input guys... I've tried wrapping a full 42" piece of wire around a ballpoint pen tube (I figure that's non-ferrous enough.. I guess this wire is stranded, so it doesn't hold a coil shape on its own), though my range is still only about four feet or so... If I uncoil the wire and hold it out as a big loop I get at least 15-20 feet.

Maybe it's my imagination, but I seem to get better range if I unsolder the other end of the loop, the side that's connected to the gnd side of the circuit... Any input on that?

I think I'm going to check out some thrift stores this weekend and see if I can find an old CB or an RC car or something I can salvage an antenna from... I don't have an oscilloscope or an antenna meter or a frequency generator, or frankly the patience to fiddle with this much further... I just wish this thing ran at 2.4ghz or 900mhz or something, I've got a whole lot of those antennas laying around...
 
1/4 wave at 27mHz is roughly 102". If you take a 102" piece of wire and coil it up you would have a loaded 1/4 wave vertical antenna. Two 102" lengths, one on each side of the transmission line is a 1/2 wave dipole. I believe you can load loops as well, but I can't remember if it complicated things or not. Fir a more manageable amount or wire, you could use 1/8 wave at 51". The length of wire is crucial to having good propagation, since any length other than exactly 1/8, 1/4, 1/2, 5/8, and full wave will have a standing wave ratio (SWR) that is too poor, which in turn causes some of the output power to be reflected back along the transmission line into the final amplifier. In high powered systems, a poor SWR can damage the transmitter. In low powered systems like yours, it just blocks a great deal of the power from leaving the antenna.
 
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Some of my old R/C radios have a 27Mhz receiver with an antenna
that is about 35-45" long... and if we don't use the full length...
the max range drops...

Jerry
 
So, I measured the stock antenna in this thing and it's 49.5 inches long... Not sure what to make of that... I think I may try the 52 inches Electrofreak suggested..

I've been getting a huge drop in range if I coil it... The only way I can get full range out of it is to hold the loop up in the air, without it crossing over itself at any point... It also works well if I unsolder the other end of the loop and hold it straight out... Unfortunately I'm beginning to get the impression I'll need a four foot long stick attached to this thing if I want to use it from more than three feet away, in which case I'd be better off using the four foot stick itself to change the volume.
 
Well, I finished up my remote today... I replaced the big ugly pushbuttons with one of those "ninja remotes" from DX, and wired it up to the stop, play, fast fwd, rew, volume up and down and mute. I also replaced the AA batteries with a CR2032 button cell because honestly, they're cheaper than AA's and I have a million of them. I included pretty pictures if anyone's interested...

Anyways, I still haven't found an old RC car or CB radio, so the hunt is still on for an antenna shorter than this 49.5" piece of wire...
 

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Thanks Jerry, I'll keep those in mind for future projects... For this one though, a wireless keyboard was a better solution IMO, no need for drivers or anything, windows just detects it as a generic multimedia keyboard, it works in all programs, it even works in linux, no extra messing around... For something that I'm not hooking up to a computer though, those tx/rx pairs look great, and the price is really decent. I'll definitely bookmark those.
 
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