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Antenna tips for better wireless performance


glenn
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Tips to getting the best distance from your wireless transmission 
 
Never use amplified antennas. They can easily create intermodulation interference that the best receiver front ends can not filter out. Even if you use one with a RF pad set to Zero it is a problem if the amp is still in line.
 
Always use passive directional antennas with low loss coax. Use LMR-400 coax for longer runs, it has a loss of 3dB per 100 foot. This is more than acceptable as to not affect range and to get the best performance from the receivers front end filtering. Digital wireless needs a linear transmission path. Amplified antennas are very non linear when overloaded and sometimes without overload. If running 10 FT of coax you can use high loss coax as the loss is 1/10 of the 100 foot run. 10dB loss per 100 feet is fine on a 10 FT run.
 
Using directional antennas is the best way to get the greatest distance and to minimize interference. You can double the distance your wireless will work and minimize interference from interfering walkies, cell phones and television stations if they are coming in from different directions like from the back or the sides of the antenna.
If you use 2 shark fins for receiving, separating them more than about a foot is unnecessary. Mount them both in vertical orientation and parallel to each other. If using a fin for IFB transmission, mount it below the 2 receiving antennas by a few feet (2-3) for isolation from the receiving antennas. Do not mount it at the same height as the receiving antennas as it will couple more noise into the receiver than it needs to. This trick can lower RF receiver noise by 9dB.
 
If you have a choice, stay away from dipoles. "Shark fins" Log periodic antennas are much better for digital modulations like Zaxcom Wireless and are directional. Log periodics tend to minimize the nulls that can happen as mics move through free space as their multiple elements even out the signal. Do not use a dipole with a log periodic antenna for antenna diversity. Using a good antenna with a bad antenna is not diversity, its a problem.
 
Get your antennas up in the air as high as possible. This will help to get obstacles like people out of the way and guarantees you that someone near the sound cart can not get closer to the antenna than the height of the antenna helping to prevent overload and intermodulation. 10 feet or higher is best.
With Zaxcom wireless use ZHD96 modulation indoors and in reflective outdoor spaces. Use XR in Iowa corn fields where there are no reflections.
 
Don't keep transmitters on at your cart that are inside your receiver's filter bandpass. This can have the effect of desensitizing the receiver and make it easier to create intermodulation if there are other high power transmitters nearby. We call this near/far effect. It is much easier to go 500 feet if you do not have an actor with a live transmitter looking over your shoulder at the cart.
 
Avoid antenna distribution amplifiers if possible. They, like amplified antennas, can cause intermodulation that can really mess up a receiver. If you must use one make sure it has a band pass filter in front of it or built onto it. Our Micplexer II has the Zaxcom flux capacitor filter that limits the bandpass to 35MHz and also has overload lights to tell you someone is looking over your shoulder with a live transmitter.
 
Keep transmitter antennas away from direct skin contact. The body does a great job of sucking all the RF out of the air and de-tuning the transmitter antenna. Use a 1/2" diameter or better a 1" diameter foam tube to hold the antenna away from the skin if placed under clothing like on a leg or arm.
 
If a transmitter is being used very close to the receiver like 10 feet or less use the lowest power transmitter setting. This is just good practice and can minimize interference to receivers that might pick up signals if they do not have a transmitter to listen to on their own frequencies. Our receivers can decode a signal with only 7 dB of signal to noise at a level of -100 dB noise floor. You will never be too close to a Zaxcom receiver at any power level to cause it to not decode.
 
I have been wanting to put this out for a while. This is by no means a complete list but I hope it might prevent some bad situations as more sound mixers move from analog to digital wireless.
 
For bag use, Keep UHF IFB transmitters at least 50 MHz away from the frequencies used for body pack receivers. If possible use a filter on the transmitter to eliminate any wide band noise that might fall into the receivers pass band.  No receiver front end can eliminate the wide band noise from a transmitter located within a foot of the wireless receivers in a sound bag. This is because the transmitter noise is in the receivers pass band and can not be filtered out by the receivers front end filter.  Any claim that a receiver by it self can solve this problem is sales promotion nonsense.
 
Glenn
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A 1" diameter foam tube on a thigh rig?!!!!!  That could easily be mistaken for...

 

 

 


 

 

 

 

 

 

 

something else!

 

I use PW helicals and hardly ever have problems, no amps.  A conversation I had with Lectro Gordon years ago gave me two great tips:

 

- Have the two receiving antennas on different planes; have one a foot or so higher than the other.

 

- Use the hottest gain possible because stronger signals travel better.

 

I had and have no reason to doubt these two tips, and I'll add yours to them!  Except the foam tube thing, I just can't see that flying on any costume other than say a Friar's robe's backside.

 

Thanks Glenn!

 

Dan Izen

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Awesome information, Glenn! It's nice to have it all consolidated in one location. I'd like to add a perspective from the live sound world that may be helpful as well.

 

When deploying your LPDA style receiving antennas, polarization is an important consideration. In my field we use handheld RF mics very regularly and as you might imagine, the angle of the Tx varies quite a bit during a performance. As a result we need to be able to receive waves that can be in any orientation. The solution that is often employed is to place the two antenna 90º to each other and 45º to the ground plane. In other words they look like a V in the air. This allows either antenna the best chance at receiving waves in just about any orientation. The majority of waves aren't going to be coming in all fully horizontal or fully vertical. That being said, if most of your Tx's in the field are placed in the vertical position most of the time, then having your antennas placed vertically makes sense. However, if you have a mixed bag of vertically placed and horizontally placed Tx's in the field then I would encourage the V style setup if you're having issues with the fully vertical placement of the LPDA's. 

 

Another consideration for receiving antenna is to make sure they are not too close to flat, reflective surfaces like walls and especially corners in buildings. Radio waves are just light waves we cannot see and they will scatter just like any other light source on reflective surfaces. Placing an antenna array in a corner will create all kinds of standing waves, hot spots, and cancellations that will make for a bad time. 

 

I hope some of that is useful to someone!

 

Jeremy Grodhaus

 

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