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the ATU in front of a series fed AM antenna

The first job of the ATU is to match the tower to the transmitter load of 50 ohms. Secondarily, it provides the proper bandwidth ideally for equal response +/- 15 kHz from the center carrier. Sometimes this can be a simple network, sometimes it requires some complexity, especially with odd height radiators. There's not a lot that can go wrong, other than being hit by lightning and melting some coils, or a capacitor opening (and sometimes shorting). Other issues are if a critter gets inside and cooks itself, and I've seen water get into an ATU and form a path to ground. The ATU is extremely critical to a well operating station with good bandwidth.
 
The ATU catches lightning from the tower. Sometimes it gives it to your transmitter.

It normally has a name, most of them are redundantly named Harris, a few are named Kintronics.

It sits in a field hiding behind grass and sometimes trees, an occasional fence, and rusts. Sometimes there are twins (1430 Indianapolis), triplets(formerly WSVX), quads, even sextuplets (WIBC).

It has a single eye. Most of the time the eye has stopped working. The eye is still visible through the one cloudy clear ocular component.

Upon death the autopsy finds : A capacitor, two tuning coils, rf ammeter, clips between the input and output to various points on the components.
 
It sits in a field hiding behind grass and sometimes trees, an occasional fence, and rusts. Sometimes there are twins (1430 Indianapolis), triplets(formerly WSVX), quads, even sextuplets (WIBC).

It has a single eye. Most of the time the eye has stopped working. The eye is still visible through the one cloudy clear ocular component.

Upon death the autopsy finds : A capacitor, two tuning coils, rf ammeter, clips between the input and output to various points on the components.
Click to expand...

The ATU also serves a vital environmental function as a preferred home for wasps, bees and field mice.
 
There's a fairly involved explaination at Wikipedia. With that being said, others have highlighted the basics in earlier posts. The thing I found myself needing to do is rebuild a horribly maintained AM's ATU recently. There's several basic configurations on how to properly match the 50 ohm transmitter to the tower. I'll search for the little DOS program I was able to run to get me answers tonight. The main thing you really need is some way to be able adjust the input and output impedences to match properly. The "design" this site was using was far from being matched as there was fixed caps and only one coil. It was a mess. I opted to split a inductor (into two electrically) by using three tap points on it and using fixed caps to make a proper match. A minimum cost was involved in doing this. You really, really do need a Operating Impedence Bridge or at least a basic RF impedence bridge + generator to be able to #1 measure the tower impedence and #2 tune things up. Luckily I have one so I was able to get this poor little station back to a decent match. The previous engineer (died many years ago) appearently just put something in that would sort of match and cranked the fine-match on their Harris Gates One until it worked. The bandwidth sucked. An old-timer told me a little trick with AM stations that has proved pretty useful for me. If you tune across a station with a decent analog radio and the station doesn't seem very 'fat' but real narrow on receiver tuning, you're ATU bandwidth isn't good. The before on this station was very tight. After the ATU change, it was nice and broad with better fidelity. :)
 
As well stated by other posters, ATU provide the function of making the antenna complex impedance (resistance and reactance) complementary with the characteristic impedance of the transmission line, now days mostly 50 ohms. "The most efficient transfer of power occurs when the impedances are matched." Now this can get dicey if there is a excessive amount of reactance and/or resistance and if the ratio of the resistance and reactance is high meaning a high “Q” factor, Q is basically reactance divided by resistance. In its simplest and most prevalent forms, an ATU will consist and arrangement of inductors and capacitors described as either a “T” or “L” network. Depending on the measured impedance of the antenna, the designer has several options to consider including the desired phase-shift and linearity necessary to present the best wide-band load to the transmitter. Depending on the application the ATU can be designed to also provide some degree of filtering as either a “low-pass” or “high-pass” filter, current leads or lags the voltage vectorialy speaking. All-in-all, very important considerations that must be taken into account. A through treatise of this subject is beyond the scope of this forum but completely searchable on the internet with many articles, white papers and computer design programs available on the topic. I think the highest failure prone part would be the capacitors, in some cases hard to see if they are damaged where inductor malfunctions are completely visual; component discoloration cracks, and heating all basic clues in locating problems. Hit-and-miss construction and adjustment of these systems is almost impossible due to almost infinite combinations and inductor mutual coupling. In any case, the complete familiarity, use and operational knowledge of a high-quality impedance measuring device (bridge or vector network analyzer) is imperative.
Best regards and good luck!
w/
 
With that being said, having a VNC makes the job MUCH easier. I've had the privilege of helping someone tune one up that has a vnc. It was a cakewalk to get his am right at a nearly perfect match opposed to messing around with a OIB.
 
Here is another tool I acknowledge using. The MFJ 259B was designed for amateur radio antenna measurements. It works well on FM and should do the job at an ATU looking to the antenna.

http://www.universal-radio.com/catalog/meters/1611.html

It works most frequencies in the broadcast spectrum. It also has an option for 440 mhz and grid dip adapters.

In the past I have used watt meters and transmitters to determine of an antenna was resonant. Problem with this is the inability to put a transmitter at an ATU, the questionable reliability of jumpers in MW applications and just the total mess you see. Most equipment is really expensive. This is small, battery operated, cheap to buy, priceless to have.

Tuning Broadcast antennas is so much easier as this can be plugged in to the antenna on the ground to check resonant frequency for FM.

My only problem is the rf susceptibility. To much rf is smoke filled adventure. This will excite any antenna system enough to determine impedance, and resistance. Using this at an ATU would allow you to see more in one minute than you could get with 50 trips from the ATU to the transmitter and back.

Even though it's output level is low it is enough to drive a frequency determined antenna through 500 feet of cable.

It is nice to plug in the unit and make changes and see it display the resonant frequency, as well as impedance. If you don't have one, buy one.
 
OKCRadioGuy said:
Cool. Am I right on the frequency format? The input box has no explaination of what input format it wants (khz, Mhz, etc.)
Click to expand...
Input as powers of ten, as an example 1 e9 = 1000 kc, you may also use direct cps, 1000000. You can play with the Q value but you want to keep it low.

Best regards,

w/
 
ChiefEngineer said:
Here is another tool I  acknowledge using.  The MFJ 259B was designed for amateur radio antenna measurements. It works well on FM and should do the job at an ATU looking to the antenna.

http://www.universal-radio.com/catalog/meters/1611.html

It works most frequencies in the broadcast spectrum.  It also has an option for 440 mhz and grid dip adapters.

In the past I have used watt meters and transmitters to determine of an antenna was resonant. Problem with this is the inability to put a transmitter at an ATU, the questionable reliability of jumpers in MW applications and just the total mess you see.  Most equipment is really expensive.  This is small, battery operated, cheap to buy, priceless to have.

Tuning Broadcast antennas is so much easier as this can be plugged in to the antenna on the ground to check resonant frequency for FM.

My only problem is the rf susceptibility.  To much rf is smoke filled adventure. This will excite any antenna system enough to determine impedance, and resistance. Using this at an ATU would allow you to see more in one minute than you could get with 50 trips from the ATU to the transmitter and back.

Even though it's output level is low it is enough to drive a frequency determined antenna through 500 feet of cable.

It is nice to plug in the unit and make changes and see it display the resonant frequency, as well as impedance.  If you don't have one, buy one.
Click to expand...

The Mississippi's Finest instrument may be adequate for measuring, determining and setting component values not connected to an antenna. Its lowest frequency is published as 1.7 mHz. The output is very low on these amateur instruments. Now I will say that I have never attempted personally to use one of these MF(J) in MF but am aware of attempts by others. I now use an instrument manufactured by Array Solutions Power AIM-120; however, I still retain my OIB-3 and GR-1606 because I am "old school" and set in my ways to a large degree!  ;D

http://www.arraysolutions.com/Products/poweraim_120.htm

Best regards,
w/
 
The MFJ unit I have goes to .4. The nice thing about this is the way you can see changes at the ATU. So many ATU's are so messed up. A lot of the old Harris ATU's...corrected...they have the Gates chrome and red logo...have non roller tuning coils with those little "clips" to change the settings. THese do come loose. Unlike the tunable coils looking for the last true setting is awful. I tend to use a permanent marker to mark the clip locations.
 
Irronically, that's what I just rebuilt and tuned about 3 or 4 months ago. It's an early 60s Gates unit. I ended up going with a lagging 90 degree ATU because I was able to re-use the coil by tapping it in 3 places with one end going to the TX, the middle going to the fixed cap, and the top headed to the antenna system. It kept the cost wayyy down which is what the client needed. :)
 
OKC, are you referring to those old Gates ATU's with one big coil, a cap and numerous taps? I've found them harder than hell to adjust. I usually just counted the turns to find the approximate inductance on the left and right of the coil, then used the bridge to find the middle spot for the correct impedance. They would always eventually tune up, but due to the mutual coupling across the big coil, they were always a trial and error project. I can't imagine that they were ever used in a DA.
 
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