event DV August 2007 "Wireless Mics - tale of the tape&

[HankCastello]

event DV August 2007 "Wireless Mics - tale of the tape" -

On page 28, in discussing the Sennheiser G2 100, author Anthony Burokas states:

"Truth be told, diversity receivers with two closely spaced parallel antennas are not much better than a single good antenna."

My problem with this is that the purpose of a diversity receiver is to avoid dropouts.  Dropouts are caused when bouncing radio signals come in a way to cancel each other, (phase cancellation) causing temporary loss of signal - "drop outs".

Since radio waves are tiny, it would seem that any separation between antennas, even just a couple of inches, would dramatically reduce drop outs.

I don't see how adding the adjective "good" to a single antenna can be used to suggest that this grants it some sort of protection from drop outs relative to having two antennas (diversity).

I haven't yet read the entire article.  My wife read portions of it to me last night while I attempted to watch some UFC events.  (Her way of protesting the tv content no doubt, but I pretended to be oblivious).

My hat's off to Tony - we need more articles that compare gear and help us to make better choices.  I don't think I could write three or four technical pages without some sort of "issue" cropping up either.  Still, I don't think we should leave wedding videographers with the impression that one "good" antenna can do as much or more to prevent dropouts as two.

[On a Roll]

Wow, I found a subject that I don't completely agree with you on!

I worked for a radio station for a while, and during that time, I learned a bit (too much) about that glorious source of light and energy known as the electromagnetic spectrum.

It's actually a misconception that radio waves are small. In reality, the photons (or the "waves" if you prefer) that make up waves can be quite large. In fact, the waves required to transmit an AM station can be as big as 400 meters!

A big problem with AM Radio (Yeah, I worked at an AM radio station) is that because the waves are 100-400 meters long, you can have HUGE dropout areas. If you looked at an accurate map of the dropout on a single tower AM radio station, you'd see that when dropout does occur, it can affect entire neighborhoods. The worst part of that is that there's no solution to the dropout, because the only way to do so would be to place another transmitting tower far enough away that the waves wouldn't overlap, which can be a stretch as far as 400 meters, or just a shave under a quarter-mile away.

Fortunately, the waves are much smaller in higher frequencies, like VHF & UHF. In VHF application, you need about a meter (just over 3 feet) separation between two receivers for decent diversity, and in UHF, the distance is even less, as little as 30-40 centimeters (1 foot to 16 inches).

Those lengths of measure above are the size of the dropout holes on stage. For example, if you're using a non-diversity wireless microphone, any holes on stage will be as wide as the EM wave. This is why UHF works so much better than VHF; VHF leaves three foot wide holes on your set, whereas UHF leaves a hole half that wide or better. Either way, those dropout spots are the width of the wave being transmitted.

That's where the importance of diversity comes in. By placing two antennas at least the width of your waveform apart, each antenna is guaranteed to be picking up a different EM wave. And that's why two antennas close together is bad.

The closer your antennas are, the better your odds of both of them picking up that canceled out wave, and even though you have a "diversity" receiver, you might still get dropout.

Most rackmount wireless systems eliminate this by placing the antennas on opposite sides of the rack. Since a studio rack is about the width of a UHF wave, this works perfectly. Of course, on a camcorder, your antennas are only a few inches apart, which means you have a far better chance of dropouts.

This is where I put my advice section. Obviously we can't wheel around a 16" wide receiver everywhere we go. But we can make it a little harder for  your mics to drop out. Space is three dimensional (or more, if you buy into quantum physics). If you point your antennas away from one another, you change their angles relative to each other by about 90 degrees. That can sometimes be enough to "catch" a different wave, thus increasing your chances of eliminating drop out.

Sorry Hank, but he kind of got it right. Two antennas very close to each other and parallel isn't doing you much more good than a non-diversity unit. It does some good, and if you point those antennas apart a ways, it'll help even more.

Just my two cents. Peace! :-)

[HankCastello]

Good argument!

While I don't pretend to be a radio expert, I did read quite a bit about this, regarding wireless units for video work.

My problem with the article was that the writer suggested that a single antennae can be better than two.  Any space between the antennaes will help to avoid dropouts, although no one could argue that larger spacing is better.

Better still is "true-diversity" which is two receivers, each with its own antenna.

This is why UHF works so much better than VHF

Actually, that isn't true, according to my sources.  It seems that UHF is almost a line-of-sight deal while VHF does a better job of penetrating walls, etc.  If it penetrates better, then it does not bounce as much and hence causes fewer dropouts.  UHF may be better due to frequency issues, but even that issue has recently been clouded and it may be awhile before we learn the outcome.  Almost surely, all our wireless sets will need to have frequency changes or be replaced altogether like what happened years back with the RC hobby guys.

[On a Roll]

Good point on VHF vs. UHF on penetration. It is true, as the EM waves narrow, they do have more and more trouble penetrating surfaces. Light's a perfect example of that, since light is little more than a much narrower frequency of the same EM waves that produce radio. (Of course, eventually you get so narrow that the waves can slip through the space between atoms,and creating radiation as a side-effect, but that's a conversation for another day.)

I personally prefer UHF for the sake of clarity. Audio sounds clearer transferred over UHF bandwaves than it does VHF, the same way that the much higher band FM sounds far superior to the super low band AM. If you get a higher power unit, the penetration issue isn't a problem (and honestly, who'd going to have their receiver down the hall in a brick-lined room?), and by having a diversity (or as you pointed out, a much nicer true-diversity) unit, you can all but eliminate dropout.

Honestly though, you are right. With more and more RF bandwidth being gobbled up by stuff like HDTV, HD Radio, and all the piles of goodies out there, it probably won't be long before we're all logging in a B&H, looking for new Mics.

In one last word of defense for UHF though, the other nice thing about it is that, at least for now, there's not as much stuff in those frequencies. I had an old cheap VHF microphone that could sometimes pick up FM radio (not surprising, since the FM band is smack dab in the middle of the VHF range), and if anyone used a garage door opener within about 100 yards, it went wild. But that's what you get for trying to save a buck by buying cheap crap from radio shack. Lesson learned.