ARTICLE FROM JUNE 1996
This month most of my focus has been on my 444.350 repeater located at Winfield. This repeater serves as the hub for our entire link system so its operation is very important. Upon my return from a business trip to Europe in May, I discovered that the hub repeater was off the air due to transmitter failure.
As soon as possible WB0VHB (the hub is located at a commercial site he has at the Winfield High School) and myself got to work on the problem. The hub repeater is a old 100W Motorola Micor radio that I converted to a repeater about three years ago. The hundreds of hours of continuous operation since than had caused some heat stress failures in the Power Amplifier section. At first glance it looks like a voltage regulator transistor and a RF driver transistor are goners.
Fortunately I have two other junk Micor chassis to steal parts and RF modules from for the repairs. As soon as time permits I will rebuild the old repeater. In the mean time I put the new repeater RF deck on the air that I had recently built to be the core of a new hub repeater. My long term plan is to have TWO complete repeaters at Winfield. The new one being the main repeater, the old one to be a hot spare.
Completion of that project is on hold till the R&D work is finished on the new ICN fiber optic linking system and the Hyper PL system. I want the new repeater to incorporate the Hyper PL from the start to make our change over to the that system simpler when the time comes.
Now this brings we to a real problem that Randy and I have had. It seems that whenever we are working on the repeater system and REALLY NEED test signals and reports from across the system, most of you turn your radios off! I know part of the reason is that the level and equalization setting tones we sometimes transmit are annoying. Please tolerate them for a few minutes and keep listening.
Our last two work sessions at the hub involved making some critical audio adjustments. The problem being that the old controller was not designed to work with the new RF deck. All the levels and EQ are different. When the time came for on the air testing we could not get a signal from 145.37 or 146.76 (or almost anywhere else for that matter)! When you hear any of the sysops working on the system, please stay on the air and be prepared to give assistance with test signals. All is needed is some conversation, it is very important to us!
Progress is still being made on the ICN project. I have the first iteration of the analog board laid out and KD0YU has all but finished all of the digital design portion. Dave has done a huge job on this and deserves a lot more support on this project than he has gotten. We hope to begin hardware testing of the HPL system for the fiber link within 30 days. So far everything looks good.
I have been working on a grant proposal for funding of the development of the fiber linking equipment. So far all of the prototype work has been funded by the hams working on the project. The cost is up into several hundred dollars at this point and may go over a thousand. Not to worry, SEITS will not get tapped for this cost! Instead it will be covered by a grant by the State of Iowa Disaster Services. The sale of the linking hardware will be future fund raiser for SEITS. There will be more discussion of this subject at the next meeting.
Mark N0RXD informs me that the Iowa City club has purchased a new UHF antenna for the link radio for our Iowa City 145.47 repeater. This part of the link system is critical since this link transmitter carries the signals of Cedar Rapids and Waterloo as well as Iowa City to the Winfield hub.
The current antenna has never worked well due to matching section problems. The new antenna is on hand and will be installed by commercial antenna riggers within a week. This change will ensure noise free signals from that branch of the system this summer. Thanks to Harvey N0LBG, Mark N0RXD and Johnson County Disaster services for their support on this!
KE0BX asked a excellent question: When tuning a FM radio, the standard way of measuring sensitivity is the amount of RF signal in microvolts that gives 20 dB of quieting. When all you have is a digital AC voltmeter and there is no decibel scale on the meter, how can you tell when you have exactly 20dB of drop in noise (quieting)?
This is really very simple. If you work out the calculations, a signal 20dB down is one tenth the voltage. That is, if you start with a reference signal of 1 volt on your meter, when it drops to .1 volts, you have had a 20dB reduction. Note that a 20dB increase would be from a refernce of 1 volt to 10 volts.
To measure the sensitivity of a FM radio connect your digital volt meter across the radios speaker terminals. With no signal injected into the receiver open the squelch and set the volume control to give you a steady reading of the noise voltage. The exact value is not important! All you need is just enough noise coming from the speaker to give a steady reading on the meter.
Connect your signal generator and increase its RF output (with no modulation). As you increase the signal, the noise from the speaker will decrease (quiet) and the reading on the voltmeter will drop. When it has dropped to one tenth of the reference reading, you have measured the point where 20 dB of quieting has occurred. Record the reading of the output of your signal generator. This is the sensitivity of your radio.
Many of us in the club have copies of an old share ware program called "RFS" that was written for broadcast engineers. It has a excellent electronics calculations section that is simple to use and covers many of the basic equations. If you need a copy let me know. It lets you calculate decibels both as voltage and power.
We also have a few copies of the ISIS Cadpack schematic drawing and PC board layout software available. This is a excellent professional program. All of the work for the ICN project is being done on ISIS. If you are interested in buying a copy, let us know. We will have them at the next meeting.
If you purchased a copy of ISIS at the last meeting, I would love to see what results you have had with it. Bring the printouts of any schematics of boards you might have done. If you have questions on using this program, the meeting would be a great time to ask them. I will bring my notebook computer for a demonstration.
The ARRL is now publishing back issues on CD-ROM. The first disk they have published is a complete set of 1995 issues of QST, QEX and the National Contest Journal. Everything including all of the illustrations and photos (most in color) is on the disk. A search engine is also included. The price is $19.95 plus $4.00 shipping for ARRL members.
If you have a CD-ROM drive and run Windows, this disk is a great idea. It eliminates keeping a huge pile of old magazines around for reference. Anything that reduces the volume of "stuff" in my shack is very welcome indeed.
Fair Radio Sales (phone: 419-223-2196) has just come out with a new catalog. Fair Radio is the oldest and best surplus dealer in the U.S. Most is real military electronics and communications gear. Some stuff from the second world war is still available! This catalog is fun reading for any radio nut. Most of it is completely useless. Some items are extremely interesting. For example, you can buy a Collins 100 watt antenna tuner for only $70. I bet those sell out fast! If you like surplus and build amplifiers, tuners or UHF gear, you will want this catalog.
May QEX (the ARRL experimenters journal) has a very interesting article by ARRL Zack Lau KH6CP/1 on the design of a no tune two meter transverter! This design uses two identical PC boards for the receiving and transmitting converter sections and "brick" RF amplifier modual for the 10 watt output stage. A simple way to convert your 28Mhz HF rig to 2 meters with no adjustments other than the local oscillator.
This would be a great club project if we only had the time! I guess we had better concentrate on finishing all of the other projects we have started.
David Metz, WA0AUQ
Back
to SEITS Presidential Notes
Back
to SEITS Homepage