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whip antenna, can it be rolled like spring or streight?

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arhi

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I got some simple AM transmit/receive modules (RT5, RR30) on 433.92MHz and I'm playing with them.... I'm currently using a piece of wire as antenna on both modules (433.92MHz gives 17.27cm whip antenna - 1/4wl)... and it works ok, but I'm testing with "few cm distance" :) so it would be strange not to work... what I would like to know is this "whip antenna" that is 1/4WL .. does it have to be "streight" wire or I can roll the wire on a stick or something (to get spring like shape to shorten the antenna size keeping the wire length same ..)?
 

Sceadwian

Banned
As long as they're very lower power transmitters it may be safe to do this, but when you coil the antenna you limit it's range significantly and also cause heating in the output stage because it imbalances the antennas impedance at the driven frequency. Suffice to say, for testing this should be safe, but if you make the antennas too small you may not get even the few cm's you need for range.

If your transmitter puts out more than a 100-200mw's of power altering the antenna on the transmitter could damage the output power amp from overheating.
 

arhi

Member
it's one of these http://www.telecontrolli.com/pdf/Wireless15_sf.pdf (rr30 and rt5) .. they use them in keychains to activate car alarms and stuff .. I do not expect some range from it ...

but non the less, the answer is clear, calculation of 1/4WL is for a streight piece of wire :)

I know there are some "coil antenna's" ... where you wind the wire around the housing .. but, could not find any doc on how to home made those :)

thanks a bunch
 

Sceadwian

Banned
Those are called base loaded antennas, look that up you may have better luck, but they still sacrifice range, and if I'm not mistaken have some nasty dead zones in reception. This is one of the reasons why the 2.4Ghz spectrum (in the US at least) is popular, antennas can be made VERY small. There are even chip antennas which basically take up 0 space, that while losing power aren't that bad.
 

arhi

Member
thanks I will look them up...

as for the 2.4G :) yeah, I know :D ... I'm waiting for 2 xbee modules to arrive and 2 MRF24J40MA boards (these ones are with pcb antenna) ... but I ordered some 20x2 lcd's few days ago and guy throw in pair of these as those were his "last" and he is not getting them more as noone use them any more :) so I just tried them out
 

Sceadwian

Banned
Keep in mind, you can tweak the range of an Xbee module a LOT of you use a custom antenna on both ends. This tends to invalidate their FCC (or equivilant) certification though as RF modules are typically certified with the antenna.
 

arhi

Member
Keep in mind, you can tweak the range of an Xbee module a LOT of you use a custom antenna on both ends. This tends to invalidate their FCC (or equivilant) certification though as RF modules are typically certified with the antenna.
I ordered the ones with whip antenna (the xbee's) as I do not need big range and took them for test only .. I was asking in another thread about chip ws whip antenna and got the 3db difference in power (or it was 6 .. never mind - not small difference) so I ordered the whip ones ... the local store have them with "connector" instead of antenna and they also have whole bunch of 2.4GHz antennas :) passive and active .. but for now, I just want to try it (to see if I can make the zigbee net stack work from software etc)
 

Sceadwian

Banned
I thought the chip vs whip was only like 1.5DB? At any rate what you got should work great, and I think this little whips are cute =)
 
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