Film-Tech Forum ARCHIVE: Antenna for wireless microphones

This is topic Antenna for wireless microphones in forum Film-Yak at Film-Tech Forum ARCHIVE.

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Posted by Randy Stankey (Member # 64) on 10-21-2003, 06:06 PM:

We just bought a new wireless mic system. We have AKG model # SR-81. Product Spec. page HERE

We have three of these receivers. (Yes, we have the transmitters to go with them! [Wink]

)

We also have an active antenna splitter. AKG model # PS-81 Spec page HERE)

Because we have three venues to manage, all the equipment is installed into a portable "road case". The idea is that I can carry the case to the site, plug it into the sound system and be ready to roll in minutes. I have a pair of 25 foot antenna cables coming out the back of the rack leading to an external antenna. At this point I have just taken the antennas that come with the unit(s) and mounted them on a microphone stand. Thus...

A close-up of the arrangement at the end of the boom stand...

Basically, it's just a wood block and some cable ties to hold the antennas at the end of the boom. They are approximately 1 antenna-length apart. I run the feed cables down the mic stand across the floor and into the back of the antenna splitter.

We're thinking about installing some antennas permenantly in the venues. I'd like to get a factory made antenna but at the prices they charge that's not going to happen. I have three of the arrangements pictured above. (Two antennas came with each receiver.) However, I'd like to see if I can build an antenna.

The system has an adjustable freq. range. They can be set to any one of 75 frequencies between 710 MHz and 745 MHz.

Median freq: 727.5 MHz.
Mean freq: 728 MHz.

One wavelength: 0.4119 Meters. (16.21")

Could this be as simple as cutting a piece of coat hanger wire 16 inches long and connecting them up? It'd be great if I could cut a few hunks of wire the proper length and install them.

Any comments or ideas, please?

[ 10-22-2003, 12:45 AM: Message edited by: Randy Stankey ]

Posted by Paul G. Thompson (Member # 655) on 10-21-2003, 11:08 PM:

Randy, we have some wireless microphones at the station...and they are indeed wireless. The receiver has a couple of 1-foot rabbit ears. The range of the microphones are very great!

Posted by Randy Stankey (Member # 64) on 10-21-2003, 11:36 PM:

This new system we have is going to be a real help! Up till now we've been renting them. The savings in rental will pay for the new gear in a short time.

With the rental gear we get, sometimes they don't work very well. The A/V company only rents out their cheaper stuff. It's often pretty beat up from use/abuse. We have a fair number of events where the things didn't work because of cheap, beat-up gear.

Now that we have our own good quality stuff with an antenna splitter/distro we're hoping a permanently installed antenna above the stage will clear up any reception problems we have had in the past.

From the test runs we did today, things are looking good. We have over 100 foot range. It's just about 100 ft. from the stage to the back wall of the audience... More than enough range for what we'll do.

I just want to know what's the best way to mount the antennas to head off any problems in the future. I WAS thinking that I'd make a set of antennas to mount but I think I'll keep the rig(s) like in my picture. It's cheaper, easier and more fool proof, I think.

Posted by Josh Jones (Member # 447) on 10-22-2003, 12:40 AM:

At our radio station, all the remote vehicles have marti units and senheiser(sp?) wireless mics. We have magnetic mount antennas that you can order for different frequencies. They work quite well, and the marti's are good for about 5 miles a 440 Mhz. I think the senheiser's are good to about 150' at 750Mhz or so.

Josh

Posted by Phil Hill (Member # 371) on 10-22-2003, 02:04 AM:

Randy, that is a really great combo. The extra $ spent to get a diversity receiver is well worth it. Many of the "Saturday Night Specials" are not even a dual diversity receiver and they suck. Good choice!

>>> Phil

Posted by Randy Stankey (Member # 64) on 10-22-2003, 02:20 PM:

If we had the money I was going to try to get the boss to approve a dual diversity AND frequency-agile model but the $$$ was so high!

These babies pack a lot of bang for the buck! They are, as you said, dual diversity. (Two receivers in one package. The system picks whicever receiver is getting the best signal.) They also have 15 selectable frequencies in the 710-745 MHz. That's in between TV channels 54 and 60 if I remember right. (Our nearest channels are 54 and 66.) They have tone code squelch and auto-mute. If the mic drops out, you'll have less noise as the receiver tries to find the transmitter again. The tone code signal also carries a battery level indication which is displayed on the front panel of the receiver. You never have to worry about whether or not there are fresh batteries. Just look at the display.

Right now, we're really happy with them. They work really well and they sound just as good as a hard-wired mic. As soon as we get this antenna question settled we'll be ready to rock!

Posted by Paul Mayer (Member # 355) on 10-22-2003, 02:55 PM:

One thing to consider for your antenna placement is orientation. Most of the sound guys I've worked with here like to set up the antennas at different angles, i.e. one horizontal and one vertical or some such. Gives you diversity for polarization as well as location. Combined with diversity receivers you'll have very good signal coverage.

The last big stage show I worked on used 32 RF mics (Starlight Express at the Las Vegas Hilton). The RF guys on that show had four or five diversity antennas (HME's I think) hung from various parts of the huge truss that was the bulk of that show's set. Nary a drop out heard in almost four years of daily operation.

Posted by Daryl C. W. O'Shea (Member # 1303) on 10-22-2003, 02:56 PM:

Randy, I dug out some Shure manuals... they recommend that you mount your diversity antennas a full wavelength apart... so you might want to separate them further than they are if attached directly to receiver.

They also recommend using 1/2 wavelength antennas if the antennas are "located at a distance" from the receiver. So if you get no joy with your 1/4 wavelength antennas its something to think about.

In case you do end up making your own antennas, Shure's are made from 300 ohm twin lead wire cut to a length that matches the frequency (half wavelength). Connected to the twin lead wire is a short length of RG-174/U which is terminated in a female UHF coaxial connector.

Posted by Randy Stankey (Member # 64) on 10-22-2003, 09:37 PM:

They are mounted about 1 antenna length apart. (Not counting the connector.) Since I'm using just wood blocks and plastic cable ties I can experiment. If I need to I can remount them a different distance apart.