Film-Tech Forum ARCHIVE: How to build a Yagi antenna ?

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Posted by Randy Stankey (Member # 64) on 03-10-2004, 05:15 PM:

I'm thinking about building a couple of Yagi antennas for my wireless microphone system. It's UHF. Frequency range of 710 MHz to 745 MHz.

I have some programs to calculate the lengths and spacings of the elements. I think I can make the antennas OK but I'm looking for some tips on how to do it in order to get the best performance, etc. out of the system.

If the antenna is correctly designed for a specific fequency you don't need an impedance matching transformer. Right? Well, what if the antenna is designed for a range of frequencies? Do you design for the center of your bandwidth and just let it hang out? Or, do you use a balun?

Also, I assume antena design isn't quite as critical for receivers as it is for transmitters. If you get it wrong for a receiver it just won't work right but it you mess up with a transmitter you can cause things to fry. Right?

I figure I'll make the antenna out of 12 AWG solid copper wire (2.05mm dia.) and hot glue the elements down to a piece of 1/4" Luann plywood.

Got any ideas? Or am I just barking out my ass again? [Wink]

Posted by Daryl C. W. O'Shea (Member # 1303) on 03-10-2004, 05:22 PM:

I'd just aim for the centre freqency. That's the way AKG and Shure design theirs.

With such a lower power output, I wouldn't worry about frying anything. I'd only bother with the receiver end anyway... just double the gain on the receiver and get the transmitter for free.

Your performers would kind of look funny with a yagi strapped to their head anyway.

If you're still having problems with drop outs you might want to consider placing the receivers in the wings somewhere and just bringing them up in the snake if you've got the room.

Posted by Greg Mueller (Member # 17) on 03-10-2004, 05:31 PM:

Back when CBs were popular I had a book called "The Truth About CB Antennas" by William Orr. Had all sorts of helpful info in there. I built a 4 element Cubical Quad once out of dowels and copper wire. What a lunker. I hook some rope to it so I didn't have to climb up on the roof to turn it.
If you want to build a Cubical antenna try using aluminum tube with the tip part of a fiberglass fishing rod stuck in it, for the spreaders. You can get rod parts at sporting good stores. Try the library for the book.
They also had a big section on building Yagis

Posted by Paul Mayer (Member # 355) on 03-10-2004, 05:35 PM:

Sounds like a fun little project. But I have to ask, why a yagi? Are you having such signal strength issues at your venue that you need the gain? Normally a couple of simple dipoles should work just fine, especially if you mount them at different angles to give you some polarization diversity.

For any given range of RF mic frequencies at UHF you shouldn't have to worry much about antenna bandwidth--it just isn't that critical. If you're really concerned about it, you might try thicker antenna elements to lower the Q... [Smile]

You're right, antenna design for receivers isn't quite as critical as it is for transmitters. With low-power solid-state transmitters feeding mis-matched loads (like RF mics without their antennas), the reactance currents wont get high enough to do much other than maybe generate a little heat. In higher-power set-ups it is quite possible to fry the finals, so care must be taken with the load tuning there (the old "Tune for minimum smoke" thing [Wink]

Posted by Randy Stankey (Member # 64) on 03-10-2004, 05:56 PM:

We're getting pretty good performance out of the antennas that come with the system but there were one or two times when we had drop outs. It takes about a 1/2 second for the receiver to mute itself so there's a burst of static when they drop out.

The location of the antennas isn't ideal. I simply affixed the two antennas that came with the system to the end of a wooden pole (in the shape of a letter "T") and mounted them up high on the wall backstage. I ran coax cable from there to the back of the receiver. The receiver is backstage and the audio output(s) go into the microphone ports on the wall backstage. The mic lines go to the mixing console at FOH. Because there's lots of brick and steel backstage there are some locations onstage that are "in the shadows". They work well about 95% of the time.

Another thing about the way I have the antennas mounted is that there isn't enough seperation between them to take advantage of the receiver's diversity. Since they are both in the same place, if the transmitter goes into a "shadow" area where its signal can't be heard by the receiver, there is no way for the other antenna to pick up the signal and allow the diversity to switch. If I can build some nice remote antennas with good range/gain, I'd like to mount them up high, over the audience (in the catwalks) and point them toward the stage and seperate them far enough apart to allow them to work with diversity.

We have three recievers connected to an antenna distro. The whole setup is mounted in a portable rack. Each receiver came with its own set of antennas. Since we have three venues where these things are used, I mounted two antennas in each auditorium. Now, we're getting requests to use the system in other locations. I'll need at least one more set of antennas for "portable" use. I don't feel like paying the manufacturer's prices for their antennas. ($100 each)

I've seen the external antennas that Shure sells for their systems. They are just small Yagis etched into a pice of heavy duty PC board. I see no reason why I shoundn't be able to make my own.

Once I get the fourth set of antennas working well I can make more of them and replace the ones on the stages. If I can do this right, I hope to be able to make the system go from 95% to virtually bulletproof.

Posted by Daryl C. W. O'Shea (Member # 1303) on 03-10-2004, 06:02 PM:

At the risk of sounding like an ass, have you adjusted the squelch in both the receiver and especially the transmitters? Usually they come new turned way down.

Posted by Paul Mayer (Member # 355) on 03-10-2004, 06:44 PM:

Yeah, location can be tough on stage. On Starlight Express at the Las Vegas Hiltion the sound guys had four antenna locations up on the overhead truss (DSL, DSR, USL, USR IIRC). They were just HME dipoles (three of them mounted at different angles) at each location summed into either RG-8 or RG-11 running down to the backstage receiver rack. That system provided very good coverage for the 32 RF mics on stage--I don't remember ever hearing a dropout in the three years I was a followspot operator on that show.

Your idea of putting a gain antenna up somewhere in the antipro is a good one though. We've done that several times in the showrooms of Las Vegas, usually just a UHF TV antenna pointed towards the stage. Crude yes, but it worked for us.

Posted by Randy Stankey (Member # 64) on 03-10-2004, 07:37 PM:

The receivers have automatic tone code squelch. There is an output volume control on the back of the receiver. There are no other external adjustment on them.

There is a gain control on each transmitter. They are adjusted to approximately 70%. Going any higher than that doesn't really seem to provide any better signal strength but it seems to chew batteries faster.

Posted by Bruce Hansen (Member # 281) on 03-11-2004, 11:54 AM:

Some dumb questions:

Are your receive antennas oriented the same way as the transmitter antennas. Far too often I have seen a receiver rubber ducky type antenna "pointed" at the transmitter, instead of oriented vertically, like the antenna on the transmitter. I have also seen the Sure "etched" antennas oriented so that they pick up off the SIDE, instead of the front.

Long coax runs at 700+Mhz are a no-no. You can lose lots of signal through a long coax run. At UHF freqs. you may be better off using short coax runs, and having a longer RF-through-the-air path.

Posted by Randy Stankey (Member # 64) on 03-11-2004, 12:09 PM:

The antennas are almost perpendicular to the ground. They are tipped by a few degrees to make their pattern intersect the floor better, rather than be parallel to it.

The receivers are connected to an antenna distribution amplifier. All three receivers are connected into the distro. The two antennas connect to it, not the receivers themselves.

The distro. has built-in, switchable cable attenuation/compensation. I'm looking at several different places to mount the antennas. (Over the proscenium, In the box booms, Upstage wall, pointing downstage, etc.) One consideration is to keep the cable runs as short as possible.

Right now, there is no cable run longer than 25 feet. I hope to keep it down to 50 feet or less.