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signal reflection

 
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Bobby Digital
Antenna-Theory.com Newbie


Joined: 14 Jan 2011
Posts: 8

PostPosted: Sat Jan 29, 2011 1:16 am    Post subject: signal reflection Reply with quote

On the Small Loop Antenna section, the author says that:

As a result, these antennas are most often used as receive antennas, where impedance mismatch loss can be tolerated.

Since when is receive reflection loss better than transmit reflection loss?
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Sat Jan 29, 2011 4:42 am    Post subject: Reply with quote

Hi Bobby,

Excellent question. And may I just add that your third album made your second album look like your first album.

In terms of absolute efficiency, the mismatch loss in the receive-antenna system works both ways - the loss is equal in both directions.

However, in terms of transmit power, radios can be damaged when the VSWR is too high. That is, all the reflected power returns to the source, which significantly lowers its life (and drains the battery).

For receivers, mismatch loss just weakens the signal further, but no damage results.

On an unrelated topic, do you feel the san francisco giants are overrated?
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Sat Jan 29, 2011 4:52 am    Post subject: EVM - Error Vector Magnitude Reply with quote

Oh yeah, and one more thing.

If you're working with digital communications, the transmitter is trying to send symbols that represent the ones and zeros. A common digital metric is EVM (Error Vector Magnitude), which represents the "distance" from the sent symbol relative to the symbol you were trying to send. EVM significantly increases with mismatch loss, and shows up as artificial noise that degrades the SNR and hence the bit error rate.

As the VSWR creeps up, the EVM eventually dominates and you can't send any data, no matter how much power is sent. I don't know you've ever seen an antenna trying to transmit digitally with a very high VSWR, but it ain't pretty.......
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