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Microphone Rx and IEM Tx antennas in each others' nodes?
Russell Ault:
Hi all!
I suppose this is another in my series of "too many antennas in too little space" questions (you can see the other one about circulators and diplexers in the Lounge).
Correct me if I'm wrong, but from what I've been reading it looks like a 1/2 wave dipole antenna has a radiation/pickup pattern that produces nodes along its axis, so that a vertically-oriented antenna emits/receives basically no radiation directly above or below it. If an IEM Tx antenna were mounted directly above or below a microphone Rx antenna and on-axis, would these nodes prevent the IEM transmitters from overloading or desensing the microphone receivers? In practice, would this allow someone to co-locate an Rx and Tx antenna (thereby eliminating antenna separation concerns) without the need for extensive filtering/diplexing?
My gut tells me that I'm missing something, but on paper it seems to work, so what am I missing?
Thanks!
-Russ
brian maddox:
--- Quote from: Russell Ault on October 11, 2015, 01:46:54 PM ---Hi all!
I suppose this is another in my series of "too many antennas in too little space" questions (you can see the other one about circulators and diplexers in the Lounge http://forums.prosoundweb.com/index.php/topic,155230.html).
Correct me if I'm wrong, but from what I've been reading it looks like a 1/2 wave dipole antenna has a radiation/pickup pattern that produces nodes along its axis, so that a vertically-oriented antenna emits/receives basically no radiation directly above or below it. If an IEM Tx antenna were mounted directly above or below a microphone Rx antenna and on-axis, would these nodes prevent the IEM transmitters from overloading or desensing the microphone receivers? In practice, would this allow someone to co-locate an Rx and Tx antenna (thereby eliminating antenna separation concerns) without the need for extensive filtering/diplexing?
My gut tells me that I'm missing something, but on paper it seems to work, so what am I missing?
Thanks!
-Russ
--- End quote ---
This is a great question. I'm very curious what our resident experts say....
John Rutirasiri:
Russ, this has nothing to do with the antenna orientation. Assuming you are not on overlapping channels between the IEMs and wireless mics, the phase detector in the PLL of each receiver would not allow RF from neighboring transmitters to be picked up or overload the RF front end. There is also a guard band on each side of the allowable channel. It's why you can have a rack of wireless mics (of the same freq band) operating together.
Best regards,
John R.
Henry Cohen:
--- Quote from: Russell Ault on October 11, 2015, 01:46:54 PM ---Correct me if I'm wrong, but from what I've been reading it looks like a 1/2 wave dipole antenna has a radiation/pickup pattern that produces nodes along its axis, so that a vertically-oriented antenna emits/receives basically no radiation directly above or below it. If an IEM Tx antenna were mounted directly above or below a microphone Rx antenna and on-axis, would these nodes prevent the IEM transmitters from overloading or desensing the microphone receivers? In practice, would this allow someone to co-locate an Rx and Tx antenna (thereby eliminating antenna separation concerns) without the need for extensive filtering/diplexing?
My gut tells me that I'm missing something, but on paper it seems to work, so what am I missing?
--- End quote ---
You're very close, but to say ". . . so that a vertically-oriented antenna emits/receives basically no radiation directly above or below it" is not entirely correct. Whereas the radiation pattern approaches 0dB (and note that it is relative, not an absolute value - 0dBm) at 0° (directly above the radiating element if right side up), it is not in fact 0dB of energy radiated or received. Even the highest Q speaker in an anechoic chamber will deliver some measurable energy in its node, and conversely, a cardiod mic in the chamber will pick up some measurable sound in its node, if you stand close enough. And therein lies the key.
This very practice of putting antennas in each others' node has been used for decades by the LMR and cellular industries, but only as an adjunct to appropriate filtering. Placing the antennas in each others' nodes means the antennas can be placed closer together (a very big deal on towers where the cost of a square foot at higher points on the tower can make the cost of a Manhattan apartment look reasonable) and lower filter insertion losses. But those antennas are still far enough apart to achieve at least 20dB path loss relative to each other at frequency. (With some high powered LMR systems, the RF energy from the TX antenna must effectively be down 170dB at the input to the co-located receiver.)
Coming back to our low power world of wireless mics, IEM's and coms (and IFB's for you TV folk in the audience), placing antennas in the other's node is just as valid as the physics and math remains the same: It's a trade off between spatial and frequency separations, receiver selectivity - aided by filtering if necessary - and RF power levels (both those leaving the TX antenna and those reaching the RX antenna from the intended transmitter). In the end, experimentation with those variables, based on what you need to achieve, is the only way to see what will and won't work.
Henry Cohen:
--- Quote from: John Rutirasiri on October 11, 2015, 02:02:50 PM ---Russ, this has nothing to do with the antenna orientation.
--- End quote ---
Antenna orientation has a tremendous amount to do with the concept of utilizing opposing antennas' nodes to reduce desensing of the co-located receiver. It is however only one of several variables.
--- Quote ---Assuming you are not on overlapping channels between the IEMs and wireless mics, the phase detector in the PLL of each receiver would not allow RF from neighboring transmitters to be picked up or overload the RF front end. There is also a guard band on each side of the allowable channel. It's why you can have a rack of wireless mics (of the same freq band) operating together.
--- End quote ---
Whereas frequency separation is certainly important, it's not the only variable or means to achieve appropriate receiver performance. Spacial separation, receiver performance and relative power levels are just as important. It's a balancing act. I can place an antenna for a transmitter near your receiver antenna that is 100's of MHz away from your receiver's tuned frequency and completely desense it (if not outright fry it :o)
Think about those times when a mic has to be very close to a speaker, and the decision becomes to use a cardoid mic and a higher Q speaker; you still have to roll off the low mid and/or higher frequencies and can't get too loud.
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