Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Henry Cohen]

Impedance mismatches are to be avoided at (almost) any cost. Every time a impedance-mismatch occurs standing waves will occur, therefore degrading your signal. So: All cable, connectors and antenna either 50 ohm or 75 ohm.

NO COMBINATIONS OF THE TWO!

This is a common misconception as it pertains to real world performance for low power wireless microphones, coms, IEMs and IFB's.

First of all, the mismatch between 50Ω and 75Ω is 1.5:1 - This is a return loss of 14dB, equivalent to a power loss of .177dB (96% transmitted power). This is better performance than the typical wideband antenna (LPDA, omni, circular polarized) sold for use with wireless mics, coms and IEMs, where a VSWR of 2.0:1 (return loss of -9.5) is considered good.
Here's the formula and online calculator.

Secondly, remember that an antenna is a transducer, which means it's actual impedance varies with frequency (just like a speaker). With wideband antennas marketed to cover in excess of 200MHz, their impedance will vary anywhere from about 37Ω to over 80Ω, being 50Ω roughly only in the center of the band.

Third, as it regards the front end of the wireless mic receiver, it too varies in impedance, somewhere between 50Ω & 75Ω with frequency due to parts choices and tolerances, and circuit design.

In the end, using 75Ω coax to the antenna will be no better or worse than using 50Ω coax. What will matter is the shield construction of the coax: A low loss braid over foil dual shield construction (e.g. LMR series, Belden 9913F7, PWS S9046), having 100% coverage, will exhibit far less attenuation and permeability to RFI than a single shield style such as standard RG58, RG8X. RG8, and RG213 having only 95% - 96% shield coverage.

There can be some merit however to matching impedance when connecting passives (splitter/combiners, filters, isolators, hybbrids, etc) together if they're being being used within their optimal performance range, but again, the mismatch loss is so small it's insignficant in RX applications and only marginally an issue in most TX situations. Now, if you're deploying a large distributed antenna system, with a lot of passive components throughout, the cumulative mismatches could add up enough to impact TX performance (but then your gain structure map would reveal that).

On the flip side, you can toss this entire commentary into the garbage if you're building commercial RF sites for land mobile radio, cellular/PCS/LTE/AWS, satellite earth stations (or the satellites), TV/radio transmission facilities, etc., where every .01dB matters.

[Geert Friedhof]

Debbie wants to use a feedthrough, which, if 75 ohm and used with 50 ohm cables, will (can) have an extra signal loss up to about 10% (0,5 dB), which might be acceptable or not, but easily avoidable. It all adds up. I stand by my previous recommendation: use either all 75 or 50 ohm, especially when splitters, feedthrough's and barrels are in play.

Of course,  it all depends on (very) specific cable lenghts, frequencies and antennae used.

[Debbie Dunkley]

Taking into consideration everything discussed here, I have everything I am going to need on order :  2 x Sennheiser half wave whip antennas, 4 x 50 ohm feed through BNC connectors and 2x 50 ohm 10ft coax with BNC.
I already have a blank 1u rack plate which I will drill out for the BNC feed through's. I also have a mic mount that can be adapted to attach the whips to the boom I am already using to keep my router up high.

I'll report back when it all arrives and I have used it at a show.

Thank you again everyone.

[Debbie Dunkley]

I had a tough job finding BNC feed-throughs at 50ohms. Neutrik is only 75ohms and any others that I recognize are very expensive.
So I took a risk which didn't work out very well. I purchased a 10 pack for cheap and they were - VERY. 
They arrived today.....2 of the locking nuts came already cross threaded and stuck tight, all the others seem to slip so I can't get them tight enough to hold without twisting. Also the BNC fit is different on each one - mainly far to tight to get the BNC cable in and out.  Chinese no name stuff....
Anyway,  Amazon refunded my money so I need some decent ones.
I've seen some Amphenol which I have had experience with - mainly XLR's....
Any other suggestions?

[Mac Kerr]

I had a tough job finding BNC feed-throughs at 50ohms.

Don't use feedthroughs. Connect your remote antenna directly to where the feedthrough would lead.

Mac

[Ike Zimbel]

It is all very strange. The last time we played the same venues as these 2  shows in question was 6 - 8  weeks earlier with no problems using the old rack, same iem units, same mic unit - same everything in exactly the same position and configuration. The rack is the only difference.

The rack material could be affecting this. Over the years I have noticed that plastic can be affected by whatever material is used to color it. Back in the 80's there was a brand of cable with a polymer shield that I used quite a bit (can't recall the name) but I noticed that the jacket would behave differently when stripping it, depending on the color of the cable. Some colors were more liable to split or crack than others.
With the two racks, it's possible that the new one has, for example, more carbon black in it for pigmentation, which could affect it's RF transmission/reflection qualities.

[Debbie Dunkley]

The rack material could be affecting this. Over the years I have noticed that plastic can be affected by whatever material is used to color it. Back in the 80's there was a brand of cable with a polymer shield that I used quite a bit (can't recall the name) but I noticed that the jacket would behave differently when stripping it, depending on the color of the cable. Some colors were more liable to split or crack than others.
With the two racks, it's possible that the new one has, for example, more carbon black in it for pigmentation, which could affect it's RF transmission/reflection qualities.

Interesting theory Ike - thank you.

[Debbie Dunkley]

Don't use feedthroughs. Connect your remote antenna directly to where the feedthrough would lead.

Mac

I'm going to need some kind of coupler - I have female BNC on my cables and female BNC on the antennas.

[Debbie Dunkley]

One other thing.

Each time I try things at home when testing intermodulation/RF drop out etc, I get a result that can easily change when used live. Of course I understand why this is the case - all the wifi traffic, venue appliances etc...but it makes it tough to test anything I do until I am in situ.
With that in mind, today I tested for RF stability with the only change being the aluminum shelf I made last week and I placed it between the top iem unit and my mic receiver unit. Also 1 space gap in between.
I had the iem rack in the garage and I was able to move all the way to the other side of the house - 1 firewall and 3 house walls between us and it worked perfectly.
Last time I tried this with the top iem unit on, and it failed miserably standing right next to it!!
This could be a good sign already so I am hoping for a good result.

[Scott Holtzman]

I'm going to need some kind of coupler - I have female BNC on my cables and female BNC on the antennas.

You sure about that?  Those should be males, the feedthroughs are f/f.  I don't know what Mac's objection to them is.  It will cost you about 1 db.

BTW - BNC stands for Biconic Naval Connector, in case you are ever asked.  Many reference it as a Bayonet-Neill-Concelman connector however I have numerous avionics and military manual substantiating the Naval Connector reference.

[ProSoundWeb Community]