Audio Technica - Antenna Distribution BNC Connector Questions

soundtech193746

Active Member
Hello all!
Our school is purchasing 6 wireless mics and using the AudioTechnica ATW-A49 UHF LPDA shark fin antennas. Do we need to get specific BNC connectors? or does BNC-BNC work? If not, link to one in the reply section. Let me know if you have any questions.

Thanks!
 
Are you asking about the BNC connectors themselves or the cable that connects the antennae to the receivers/antenna distribution?
 
It should be 50 ohm cable and connectors for antenna (e.g. RG58 cable), correct?
 
It should be 50 ohm cable and connectors for antenna (e.g. RG58 cable), correct?
There is some question as to whether using short runs of 75 ohm makes any difference, with most saying it dosen't. Longer runs would accumulate some attenuation. Mixed runs may introduce problems. Since the receivers are technically 50 ohm, running 50 ohm cable will cause less second guessing if a problem pops up. I've tried swapping 75 and 50 on 10 foot loops for fun and not seen any difference in RF strength, although the resolution on the receivers I use is in 5 Db steps, so it may just not show.
 
RG58 for short runs, RG8 for longer runs.
Yes, With RG58 being 68pf per foot and RG8 being 29pf per foot, the capacitive loading is more than halved, so you can run twice as long a run with less of a loss. Ultimately, it is the capacitor effect that kills the signal.
RG59 comes in at 22pf per foot, so the transmission mismatch (75 ohm) is somewhat offset by the lower loading factor on long runs. (RG6 comes in at 17pf/foot)
 
I haven't come up with a good argument that impedance matching makes any difference with receivers. Chances are that wireless mic receiver inputs are some random impedance anyway, and the manufacturers won't tell us what it really is.

For coaxial cables that are approximately the same diameter, the 75 ohm cable will have less loss than a 50 ohm cable. At 450 MHz, RG-58 is 10.6 dB per 100', while RG-59 is 7.6 dB per 100'. It doesn't matter for jumpers, but it adds up with long cables.

My rule of thumb is match impedance you can, but don't worry about if you can't. If there's a spool of RG-6 or RG-11 on the shelf for a long run, instead of RG-8, the former is going to get used.

For transmitting antennas, such as for IEM systems, impedance matching is essential. That's a different ball game.

If the cables aren't factory made, more important is matching good quality connectors to the cable type, and terminating them properly. That's not trivial because it also means having the right strippers, crimpers, and dies. Generic connectors rarely fit good enough and I avoid screw-ons like the plague. For most folks here, I recommend factory made cables, because high quality tools and materials are expensive.
 
You could be right, but I've messed with a fair number 50 and 75 ohm BNCs over the years, and I've never been able to discern any difference in center pins. IEC standards ensure compatibility.

More likely, the pin wasn't the right length either due to a poor quality connector, or bad assembly. On a good connector, the pin clicks into place. With cheap connectors, the pin does not click in, the length of the pin is determined only by how accurately the cable was stripped, and it can get pushed in.
 
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Hummmm....
BNC_50_75_Ohm.jpg

Image from https://en.wikipedia.org/wiki/BNC_connector For some reason, it would not link in the usual fashion.
 
Hello all!
Our school is purchasing 6 wireless mics and using the AudioTechnica ATW-A49 UHF LPDA shark fin antennas. Do we need to get specific BNC connectors? or does BNC-BNC work? If not, link to one in the reply section. Let me know if you have any questions.

Thanks!
Yep BNC = BNC And if you're using two shark fin receiver antennas be careful to use one horizontal and the other vertical. At UHF polarization matters and we had a lot of drop outs until we noticed both had been installed horizontally and corrected it.
 
And if you're using two shark fin receiver antennas be careful to use one horizontal and the other vertical.
Yes, one never knows what direction the antenna on a body pack or handheld will be facing! 90 degree rotation on the receiving antennas provides the diversity to pick up the signal at any angle. That's why even when using the stick antennas you want them at pretty good opposing angles.
+45 and -45 does the trick, again giving a 90 degree separation.
 
From a SynAudCon presentation:

First of all, a 50/75 ohm mismatch has a resultant VSWR of 1.5:1. That translates into 96% forward transmission with only 4% reflected power. Less than 1dB.

Even using 50 ohm coax exclusively presents mismatches if its length isn't exactly tuned to the frequency - and only one frequency - it's carrying. Since it's always carrying multiple frequencies, it's always a mismatch, though again by an insignificant amount.

Secondly, the antenna you're deploying is varying by as much as 65 ohms (about 37.5 to 100ohms depend on frequency(ies) and near field obstructions); you'd be lucky to see a system VSWR better than 2.0:1 - about 89% forward power.

Lastly, the front end of mic receivers, and I don't care what make/model, including the venerable ole' Sennheiser 1046, is not exactly 50 ohms; it's somewhere between 50 and 75.

Reality is, your fully "50 ohm" system is perpetually mismatched.

_____

Henry Cohen
Production Radio Rentals
 
Without using an antenna distro system, you’ll do better getting the antennas and receivers separated a lot more than the distance your harness allows. Example - we improved performance significantly for a school with a pile of Shure PGXs by creating a mounting solution to space them 1 meter apart across the front of the (fortunately wide) booth. Esp. True for lower price point receivers with not super-great shielding... the intermodulation distortion potential decreases approx with square law /2 as spacing increases. Similar applies to individual antennae ... too close and they interfere. Also beware reuse of longer leftover antennae from older decommissioned receivers that operated in different bands. Found a church system full of those recently.


I built this the other day to get the antenna's out of the rack and into a line-of-sight position.



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Without using an antenna distro system, you’ll do better getting the antennas and receivers separated a lot more than the distance your harness allows. Example - we improved performance significantly for a school with a pile of Shure PGXs by creating a mounting solution to space them 1 meter apart across the front of the (fortunately wide) booth. Esp. True for lower price point receivers with not super-great shielding... the intermodulation distortion potential decreases approx with square law /2 as spacing increases. Similar applies to individual antennae ... too close and they interfere. Also beware reuse of longer leftover antennae from older decommissioned receivers that operated in different bands. Found a church system full of those recently.
True more for transmitting than receiving, thus the "Antenna Farm." I have one rack with 12 antennas sticking out the back. They don't really seem to care. Every metal object is a receiving antenna when you think about it. The signal doesn't know it is not being monitored ;) In other words, the rack, rails, audio cables, power cables all have the same effect as additional antennas.
I tried putting a transmitter at a far distance once and removed all but the receiving antenna, then I added the rest back in. No difference in RF strength. It is true that you can end up with reflections, at least in theory, but haven't seen a problem, at least in the 500Mhz area.
 
It happens with receivers too. The IF stages can leak signal out one antenna and into another. A distribution system with a little isolation can prevent problems. The FCC limits IF leakage, but it isn't zero.
 

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