2m Spring VHF Sprint

By Mike April 12th, 2011, under Uncategorized

I went up to Brush Mountain last night (EM97sg) to take part in the first spring sprint this year, on 2m. I used my Kenwood TR-9000 rig along with the Mirage B-108 80W amplifier, and operated out of the back of my Jeep. For an antenna, I got my Cushcraft 13B2 up and running, which seemed to work quite well. All in all, I made 14 QSO’s with the farthest being FN01xt up in northern PA.

Now off to get my 222 Transverter (W7BAS variety) up and running again for next week!

What Local TV Stations Should I Care About?

By Mike February 24th, 2011, under Wireless Mics

When setting up a wireless microphone system, it can be difficult to determine what TV stations have the potential to cause interference, and even harder to know which ones WILL cause interference. The only surefire way to do it is to bring in a real spectrum analyzer, which will cost ten’s of thousands of dollars. However, it is possible to get a pretty good idea of the RF environment with a bit of effort.

To get an idea of what TV stations are in your area, go to TV Fool and use their TV Signal Locater tool. Plug in your venue’s address (or coordinates, if there isn’t a physical address). If you are inside, leave the Antenna Height box blank. If you are outside, enter the height you expect to place your Wireless Mic antennas at. The click “Find Local Channels.”

TV Fool will search its database and tell you what channels you are likely to receive, and how strong they will be. This information is based on topographic information and the FCC’s TV database, and is a pretty good approximation. Scroll down to the table that TV Fool generated.

Each row of this table represents a TV station. The ones at the top are the strongest, and the ones at the bottom are the weakest. Generally speaking, if you are indoors you probably only need to worry about the ones with a light green, yellow, or red background. The green ones are the most likely to cause interference, and the yellow ones may cause interference. The red ones might be a problem, but it’s much less likely.

Make a note of the channel number under the “Real” column of the table for all green and yellow stations, and make a separate list of the red stations. Don’t bother with the gray ones; they won’t be an issue.

Now, you need to convert the TV channel numbers to frequencies. This website has a nice table which you can refer to. Avoid setting your mics to any frequencies within the TV channels you jotted down above.

This should help you get on your way with frequencies which are not occupied by TV stations in your area!

Do Multicouplers (Antenna Distros) Prevent Intermod?

By Mike February 17th, 2011, under Wireless Mics

I recently heard someone mention on a forum that if you do not use an antenna distribution unit (multicoupler) with your wireless microphones, then you would be facing intermodulation problems. This post will hopefully dispel that belief and explain why antenna distros are useful, and what they can and can’t do,

Antenna distro units are composed of two things, typically. First, the antenna distro unit amplifies the signal with a (typically wideband) RF amplifier by about 7 dB. Second, the unit splits it with a (close to) ideal splitter. The net result is that the signal at the output ports is pretty close in power to the signal coming into the input port.

Now, here comes the problem. When you run the signal through an amplifier, the amplifier will generate intermod products. Period. The good news is that if the amplifier is well designed these products will be very weak and hence will not cause much in the way of issues. If your frequencies are coordinated, none of the third-order (most powerful) products (IM3) will fall on any of your operating frequencies anyway. But, if any one signal is hot (a relative term), then any IM3 product involving that guy will also be increased in amplitude. The amplifier also adds noise to the signal, but if you did things right that’s not an issue.

So, the bottom line is that antenna distro systems do nothing to *prevent* intermod. This is not to say they’re useless by any stretch, though. What they do for you is let you share a (good) pair of antennas while keeping the signal at roughly unity gain, and help to reduce local oscillator leakage from one receiver to another*. They also often provide power distribution, which is valuable when you have >12 systems in a rack.

On a side note, when you have a high density of transmitters on stage, and they’re running high power (>10 mW or so), the transmitters themselves can create intermod! Remember that a transmitter is also a receiver, so while the signal from your power amplifier is going out of the antenna (most of it, at least…but some will go back in too because the antenna is imperfect!), signals from all around it are also going *in* to the PA. If the transmitter does not have a way to separate those signals and get rid of them, then they will leak into the PA and mix with the generated signal, creating intermod products. These products are potentially amplified, and transmitted over the air. No amount of receiver filtering will help you at this point.

The solution to this issue is to run the lowest power necessary for a solid link. Another tip is when your mics are lined up waiting for performers, put them all on a metal table and put a coffee can over each mic. This will reduce crosstalk and hence, intermod.

*In a receiver, there are signal generators (local oscillators) that are used to bring the signal down to a frequency that can be demodulated. These signals can leak out of the receiver port and, if an antenna is directly connected, they can actually be transmitted. If a bunch of antennas are located in close proximity, then it’s quite possible that another receiver will see that signal and it’ll get added to the mix and potentially cause direct interference or more intermodulation products.

What’s Really In The TV Bands?

By Mike February 2nd, 2011, under Uncategorized

The TV Bands in the United States get a lot of use, despite what you might think based on the White Spaces proceedings in the past few years. Some of it may be obvious, but other users might be downright surprising. This post is an attempt at a comprehensive list of users of the TV Bands in the United States.

Before we begin, here is a list of channels and what frequencies they correspond to:
2-4: 54-72 MHz
5-6: 76-88 MHz
7-13: 174-216 MHz
14-36: 470-608 MHz
38-51: 614-698 MHz

Read the rest of this entry »

222 MHz, The Homebrew Way

By Mike January 24th, 2011, under Uncategorized

I’ve had an interest in getting on bands above 6m for quite a while, but for various reasons I’ve tended to stall out in the process and never complete the projects. This time, though, I set a hard deadline of January 22nd to have a working 220 MHz SSB station.

The project started when a friend found a transverter board online by W7BAS that uses a 144 MHz IF. This is convenient because I just got a Kenwood TR-9000 2m rig for Christmas this year, and it’s of the variety that can be easily modified for transverter operation. I purchased the board, and got it about two weeks ago.

I have to complement Bruce for his board design. Unlike some boards out there, significant thought was put into the kit design to make it easy to assemble. Not only does the board have labels for all components, but the kit comes with all components in individually labeled containers so that one can keep track of parts really easily. No components were smaller than 0805, which made assembly extremely easy with a SMT solder station. I’d imagine it would be much harder without a fine tip soldering iron and magnifying glass, though.

W7BAS Transverter Board

W7BAS Transverter Board

I had few problems with the board after assembly. I discovered later on that the receive MMIC amplifier was not receiving bias, but that was due to a mis-soldered RF choke.

The board is designed with performance in mind. The receive path has a 222 MHz three-stage helical filter immediately following the mixer to filter out LO and IF leakage (though the third harmonic of the LO is still a potential issue at 234 MHz), and the LO, RX, and TX paths have small filters inline after each amplification stage to keep the signal path clean. I have not done a formal spurious response analysis, but undesired signals appear to be at least 25 dB down from the fundamental at 222 MHz.

After fixing the receive preamp, I found sensitivity down to CW signals (as measured by ear with the TR-9000) to be good to at least -136 dBm, and possibly better though I did not try. Drive from the transverter is also very good with output power better than the specified 10 mW, though I did not record actual CW values. I will do this next time it’s on the bench.

A transverter isn’t much good with drive in the milliWatt range, though, so I purchased an M67712 power module from RFParts. It cost around $85, and can in theory make close to 30W with linear amplification (Class AB, not C). The module is easy to use, with RF in, RF out, bias, and ground terminals. I used a specially designed board with it, but I don’t know that it would be strictly necessary for good performance. Due to drive issues from my Kenwood TR-9000 (read: too hot!), I can only get to about 4W output before I run the risk of damaging the transverter.

I found that the entire system, with the power module, makes about 20 dB gain from IF drive power. That is to say that the 222 MHz output power is 20 dB higher than the 144 MHz IF drive.

To complete the package, I used a G6Y relay with a W1GHZ board for T/R switching, and bundled the thing in a metal box I got from a friend.