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minimum distance with LDPA antennas


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I am doing my "homeworks" about antennas ;-) 
 
It is adviced to have a minimum distance of 10 feet between receiver and transmiter.
It seems to me that it's correct with omnis antennas.
What's about LDPA antennas? As they give a gain, the minimum distance, in the axis, should be longer ? 50 feet ?
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I am doing my "homeworks" about antennas ;-) 
 
It is adviced to have a minimum distance of 10 feet between receiver and transmiter.
It seems to me that it's correct with omnis antennas.
What's about LDPA antennas? As they give a gain, the minimum distance, in the axis, should be longer ? 50 feet ?

Who advises that? Seems odd.

Best,

Larry F

Lectro

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My feeling is that the 'advice' about 10 feet minimum distance is to prevent overloading the front end of the RX. With 50 or 100mW I suspect you could get closer than this with a good quality receiver, but I guess it's a rule of thumb.

 

If you have several channels of talent who are 30 feet away say and another really close who puts the receiver into overload aka gain compression which will cause non-linear response, thus reduce the level of the distant signals by the amount of compression, and, much worse, cause massive intermods.

 

Assuming you take a figure of 10 feet as the minimum for a dipole then a typical LPDA, say the Lectro LP500, has 4dBd gain (dB relative to a dipole). This equates to achieving the same received level at a distance of 16 feet. Hence if you accept the figure of 10 feet as being a minimum, then keep on-axis transmitters 16 feet away if using an LPDA.

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I made a habit of setting my multiple talent mics away from the recievers even before i was using an RF amp and LPDAs.

I always make sure talent does not stand by my cart or a bag when he/she walks away from a scene .

Also when I retrieve a talent TX I have to shut it down before I put it in my bag.

Are you are familiar with block separation when using RX and TX next to each other? it is the same basic principle.

 

As far as distance..... I have not found that I need to put the txs further awau from RX when using an amp and LPDAs.

 
 
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If you are unsure of the actual minimum range of your setup, a rough and ready way to test it would be to take two transmitters, TX1 and TX2, tune your receiver to TX1 and set TX2 to a nearby frequency (say TX1 + 500kHz). Place TX1 at a long distance, say 30 feet, from the RX antenna and observe the received signal strength on the RX meter, now turn on TX2 and move TX2 closer and closer to the RX antenna  until you notice even the slightest reduction in received signal on TX1 frequency (you will probably need an assistant for this! Also you will need good resolution on the RX meter). This is the point where compression begins. If you then double that distance to allow for resolution issues on the RX meter this gives you a rule of thumb safe minimum distance for your setup. This method will work best if you are using a dipole and you are approaching from a different direction (90 degrees probably best) to TX1 so you do not cause too much fluctuation in the TX1 received signal due to your presence in the path of the signal! You can turn TX2 on and off to see if it is the TX or your presence causing the change in received signal. I just tried this with an SRA with AMM whips and 2 x SMDB (50mW) and the distance I got was about 1.5 feet, so 3 or 4 feet seems safe for this setup.

 

I hope this makes sense!

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Thanks Marc, it's one of the better document I have read ! There's also some usefull infos on Shure website with many pdf. 

  Who advises that?

 

As far as I can remember I think it's written in one of Shure's documents. I'll try to find exactly in which document.
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