Baluns and UnUns

This month's discussion is about BalUns. These passive devices do a whole host of things for the improvement of your antenna system. Two types are covered, the Voltage BalUn and the Current BalUn. The two work pretty much the same at the feed point.
Core saturation may be a problem with a Voltage BalUn. Care should be taken not to over-drive the device.
Transmission line radiation can be inhibited/controlled with the Current BalUn.

The URL below will show the complete article with the accompanying references.

http://dl.dropbox.com/u/33621962/Baluns.pdf

Enjoy the article, it was fun putting it together.
73
de Fred N1TF

It's Winter – What Next . . .

This month I wrote a basic discussion covering the Off Center-fed Dipole, how it differs from a Windom and giving some references for your reading pleasure. I think the OCD is a very practical antenna for multi-band use and performs well when placed against others such as G5RV or a center-fed dipole with open wire feed-line.
The URL below will show the complete article with the accompanying references.

http://dl.dropbox.com/u/33621962/OCD-blog.pdf

Enjoy the article and get ready to weed out the ineffective sky-warmers from your antenna farm.
73
de Fred N1TF

ALC – Automatic Level Control

Class A, AB or B amplifiers are circuits that yield gain while maintaining the characteristics of the input waveform. The amplifier configuration, be it grounded grid/base, grounded cathode, etc will determine the inversion or non inversion of the signal but suffice it to say that the output of an amplifier of good design will leave all the characteristics of the input waveform in tact. The design of the amplifier is determined by a set of parameters set down by the designer to meet the requirements of the application and will have a list of items such as size, plate/anode voltage, ventilation requirements, drive requirements, load, input and output impedance, etc, etc.

Lets look at only one of the design parameters; drive requirements. The amplifier needs a signal to process. The signal must be linear to start with and of a character such that the power level does not damage the amplifier input and does not induce distortion through a mismatch or excessive amplitude. All of these potential problems can be accommodated by the use of limiters, filters and/or attenuators.

There is a circuit in your transceiver that controls the final output. The “Automatic Level Control”, commonly called ALC, circuit monitors the amplifier output to provide negative feedback to reduce the input signal in an attempt to maintain the linearity of the signal being transmitted. Within your transceiver there are several monitoring points along the signal path which combine to create this feedback. Check the ALC level on the meter when transmitting and adjust the Mic Gain control to reduce the limiting. The meter gives an indication of the amount of feedback being generated and should not be full scale all the time.

When an external Linear Amplifier is attached, the ALC signal from it should be connected to the transceiver ALC connector so as to include the external amplifier's signal into the equation.

NOTE: Follow the setup procedures for the external amplifier to adjust the internal ALC circuitry before connecting it to the transceiver.

Additional information can be found in “Section 17.8.3” of the 2011 ARRL Handbook.

Distortion of the transmitted wave causes poor audio quality, out-of-band emissions and interference to other services or in the worst case a salutation from the “OO” or the FCC.

73,

de N1TF - Fred

Basic ALC Block Diagram by N1TF

“Reprinted with permission, 2011 ARRL Handbook; copyright ARRL.”

Limiters; What and Why

An FM receiver has several stages to process the signal and decode it for use by the operator. The first stage would normally be a preamp followed by an RF amplifier feeding a mixer. The output of the mixer goes to an IF stage or two that, in turn, feed the limiters. The output of the limiter is fed to a detector to decode the intelligence from the signal. The detector, or demodulator or decoder, is looking for a signal devoid of any amplitude variation and consisting only of a frequency or phase shift that contains the information, be it voice, data, music or whatever.

Note that the Limiters are in the IF stages prior to the detector and are for the express purpose of removing any amplitude component from the received signal prior to detection.

Most signals on the repeater are what we lovingly call Full Quieting. This means that the limiters are functioning because the strength of the incoming signal is such that the threshold of the limiting circuits has been met and the amplitude component is being stripped from the IF signal. Therefore, the amplitude component does not reach the detector and has become a non-issue.

Conversely, if you hear a hiss or crackling on the signal it is most likely caused by a signal that is not strong enough to make the limiters go to work and the amplitude component is reaching the detector. Generally, this component is decoded as a hiss, static or whatever the detector thinks it is. The end result is a less intelligible signal; or, in the worst case, the signal isn't strong enough to open the squelch and nothing gets through.

Remember that when evaluating a signal through a repeater, there are two receivers involved. The repeaters and yours. You have limiters too; and, for the same reason,they yield the same results.

73, de N1TF-Fred

What's Different?

"The budget should be balanced, the Treasury should be refilled, public debt should be reduced, the arrogance of officialdom should be tempered and controlled, and the assistance to foreign land should be curtailed, lest Rome become bankrupt. People must again learn to work instead of living on public assistance."

 - Cicero , 55 BC

So, evidently we've learned nothing in the past 2,066 years.

Equipment Repair

I had a little problem with my FT-900 at the beginning of the summer. Display was flashing and no audio All other functions appeared to be OK. I did some investigation and located the problem to the V+ to the VCOs. That was as far as I could go because I couldn't make out the markings for the components.
I checked around and finally sent the unit to Burghardt in South Dakota for service. I would recommend them for service if you need it. The turn-around was 3 weeks from ship to receive. The unit was clean and fully functional. I am pleased with the service and the functionality of the FT-900. 5 Stars to Burghardt.
de N1TF.

Field Day 2011

The last full weekend in June is set aside for the ARRL Field Day. This 24 hour 2 day exercise is for the purpose of training for the operators who participate. Equipment must be set up and tested and the operation attempts to make as many contacts as possible in the given time period. The serious side is the exercising of the equipment and the setup and tear down. The fun part is the operation and the camaraderie which will last long after the contest is over.
This year the weather was almost perfect and the Falmouth Club set up at the Barnstable County Fairgrounds on Cape Cod. I got a chance to operate 10 meters for a while. I sat down and the band opened. I logged about 20 contacts in less than 10 minutes and the band closed.
Dave, my grandson, was there too. He got to operate the GOTA (Get On The Air) Station and eat. It was good to have him with us.
If you are interested in some more information on Amateur Radio Activities Visit the web sites for the American Radio Relay League, The Falmouth Amateur Radio Association and The Southeastern Mass Amateur Radio Association.
Enjoy your summer!