Because of the need for a very precise indication, also useable in combination with ordinary rotators such as the Channel Master, I tried to find out how I could construct such a useful application.

Qualifications

Indication within 0.5 or even 0.1 degree This must be possible without further changes in the system. Only, the voltage control regulator must be exchanged, be replaced by another. Or the configuration around the voltage control regulator must be modified. Or both. See Indication Unit.

Low Cost Project only needs a few components. The only ( simple but correct ) solution is : mount the potentiometer axis direct ( without plastic wheels etcetera ) on the MAINaxis* of the rotator. Of course the potentiometer must be a hiQ ( high quality ).

*Remark MAINaxis : this is the only part of the rotator which has a tolerance of zero, 0 % ! If the assembly in a correct manner takes place. Also the indication ( LCD or LED ) unit must work accurate ! The tolerance of the electronic circuit must be at least : 0.1 % / 3.6 = 0.027 %.

Solution 1

The Rotator ( a Channel Master ). It is very easy to drill a hole into the main axis of this rotator. After this you are able to draw a 'WIRETAP'. Because of the required low tolerance I had to use a hiQ flexible cross coupler ( steel / teflon ). More in former days this one was often used in HF receivers. Via this component the potentiometer axis is connected to the main axis of the Channel Master ( or another ) rotator.

Because of the use of the wiretap it is easy to fix ( use a piece wire end ). The combination ( potentiometer and flex cross coupler ) functioned very well. High accuracy, low tolerance. You could see on the display the backlash of the rotator. It showed a true value, 0.8 degree in this case ( this Channel Master rotator had not often been used ). A lot of HAM rotators have possibilities to replace the potentiometer systems. The ( here used ) potentiometer is a 10 turn type of high quality. Pay attention well to the linearity ! Many potentiometers do not reach 0.1 degree ( 0.027 % ). But for instance possible is 1.0 degree ( 0.27 % ). See now the pictures : on the potentiometer house have been soldered two 'stug' thick wires in such a way that these can not move in a wrong direction. Use a potentiometer which needs only small turning forces. If that is impossible : mount the potentiometer in a special frame, then you have a much better solution.

Solution 2

Indication Unit ( LED or LCD ). Accidentally I saw an article in Electron ( VERON ) november 2001 from PA0PLY on page 454, 455 and 456. If it is well ( it is again long time ago ) you find also these informations here : http://www.qsl.net/pa0ply

I found the article interesting. However, because of the ( here ) required low tolerances, some little modifications became essential. I replaced the 7805 by a 3.3 VOLT type. Later on I used a special ( high precision ) voltage regulator LT 1086 CT 0.015 %, low drop. And adjustable. Between this regulator and ground I placed another, potentiometer, now no longer using the diode on that place ( see schedule article ). Because of this modification it is no longer necessary to adjust the voltage on the rotator potentiometer ( the wiper, central contact ). The voltage divider on that location is no longer necessary and now it is possible to eliminate* the non linear voltages.
*Remark or to decrease, if you use a DVM with a lower input resistance.

I removed the original trimpotentiometer with the two resistors 4k7 and 82k ( see schedule article ) or formulated better, I did not use this configuration. In front of the now used voltage regulator I placed a cheap 12 VOLT device. The system is more accurate now. The voltage drop or rise ( see Addition, below ) is no longer there. The modification causes less or no non linear voltage drop or rise. Of course you can add a DC opamp but here it is not necessary.

Addition ( PS ) concerning the voltage drop or rise. The voltage drop or rise exists because of the rotator turnings. The potentiometer value is increasing or decreasing. The resistance adaptation ( 'match' ) between potentiometer and voltage divider changes. This creates a non linear curve. The voltage to be measured differs more and more from the desired, original voltage. Of course, this is unwanted now. Before modification the existing maximal error is circa 0.25 % ( depends on the position of the trimpotentiometer ) at the 360 degree point ( if calibrated at 0 degree, the best to do is always at 180 degree ). This value of 0.25 % results in a circa 0.9 degree error. If the antenna rotator direction point differs more from the calibration point the error value becomes larger. After modification the voltage drop or rise has been eliminated, no error anymore. The DVM ( with a high resistance ) measures the voltage on the rotator potentiometer ( the wiper, central contact ). Now the problem has been solved.

If the cable between rotator potentiometer and indication unit is not too long it is possible to use a very cheap LCD or LED unit or a multimeter without zero adjust possibility, in most cases the reached tolerance is between 0.5 % and 1.0 % ( not bad ).

However, if you need a 'zero adjust' ( for instance when the used potentiometer does not reach zero ohm or the cables are very long ), you add a resistor and trimpotentiometer to the circuit. You can adjust now the wanted voltage ( offset ! ) in the system. It works very easy : on the ( multi ) meter min, neg ( - ) input we add a small adjustable voltage. This voltage comes from the voltage divider ( which is compiled by resistor and trimpotentiometer ). The input of the divider is connected to a point with a fixed, stable voltage.

Advantages

1 - It is no longer necessary that the potentiometer value ( depends on the rotator position ! ) starts at zero ( 0 ). Other values are now possible.

2 - The cable to the rotator can be very long, as long as you want.

Added Dutch Text

Stel, elke ader in de kabel heeft bij een bepaalde lengte een weerstand van 100 OHM ( extreem voorbeeld ), vanaf de plus aansluiting ziet men dan R1, kabel ader naar 'boven' ( 100 OHM ), P ( de rotor potentiometer, onder te verdelen in P1 en P2 ) en R3, kabel ader naar 'beneden' ( 100 OHM ), R2, de loper ader ( 100 OHM ), wordt in de berekeningen verwaarloosd omdat deze GEEN kritisch onderdeel is in een spanningsdeler, de meter impedantie is hoog. Nu geldt :

000 graden stand : R3/(R1+P1+P2+R3).U=R3/(R1+P+R3).U
180 graden stand : (P2+R3)/(R1+P1+P2+R3).U=(P2+R3)/(R1+P+R3).U
360 graden stand : (P1+P2+R3)/(R1+P1+P2+R3).U=(P+R3)/(R1+P+R3).U

Voor een rotor potentiometer van 500 OHM geldt nu dat na enige berekeningen zal blijken dat in plaats van 3.6 VOLT ( U ) juist circa 5.05 VOLT benodigd is, je zou dus hier in dit theoretische voorbeeld ook de LT 1086 CT 5 VOLT 0.015 % spanningsregelaar ( zie verder ) met een kleine instel potentiometer kunnen toepassen. Op de R2 aansluiting staat nu :

bij 000 graden : circa 0.72 VOLT
bij 180 graden : circa 2.52 VOLT
bij 360 graden : circa 4.32 VOLT

De benodigde zero adjust spanning ( toe te voegen aan het systeem ) bedraagt nu circa 0.72 Volt. Uiteraard BLIJFT het systeem lineair functioneren, dat ziet men ook in de results : tussen N1 en Z is delta U 1.80 VOLT, tussen Z en N2 is delta U ook 1.80 VOLT. Indien je de berekeningen uitvoert met niet afgeronde waarden dan zul je zien dat het exact goed is.

Overigens een van de aardigheidjes bij het toevoegen van een hulp spanning is dat je met een correctie ( offset ! ) kunt werken. Staan je antennes bijvoorbeeld 15 graden 'ernaast' dan kan je gemakkelijk deze 15 graden bij of weg regelen. Natuurlijk moet een en ander in mechanische zin dan wel in een zodanige toestand verkeren dat bij enige wind niet meteen opnieuw een verdraaing optreedt, anders blijf je aan de gang.

Ook kan je 'rond het noorden' aan een kant ( bijvoorbeeld linksom ) een negatieve aanduiding op het display verkrijgen ( bijvoorbeeld - 3.0 graden indien je rotor in totaal 366 graden kan draaien, aan de andere kant, rechtsom, vind je dan 363 graden, je hebt aan beide kanten een overlap van 3 graden ), een sat tracker met een interface is nu niet nodig ter verkrijging van de gewenste waarden op het display. Indien er problemen zijn met de electronische eind stop ( zie fodtrack en yaesu modificaties ) kan je deze met de hierboven beschreven hulpspanning gemakkelijk elimineren.

Calibration

First adjust the dividerpotentiometer, until the LCD or LED meter display shows zero ( 0 ) VOLT ( this concerns a rotatorpointing of zero degree ). The possibility exists that your rotator can twist further as 360 degree. For example 375 degrees. Now you have the possibility of inserting an area with negative values ( - 15 degree or for example - 7.5 degree* ) in the range. And now it is also possible to use an offset adjust ( after storm or bad weather ).
*Remark now the rotator can turn further around the North ( 7.5 degree ). It does not stop at 0 degree. The rotator N1 direction is : 360 - 7.5 = 352.5 degree. Left of the North. The rotator N2 direction is : 367.5 degree. Beyond the North.

If you would use a 2.0 or 3.3 VOLT regulator ( see also further below ) : Adjust the 10 turn potentiometer to get a '180' indication ( corresponding with 1.8 or 0.18 VOLT ). Or a '360' indication ( corresponding with 3.6 or 0.36 VOLT ). This concerns a rotatorpointing of 180 degree, or 360 degree, prefer 180 degree. Thus a very cheap but also a very accurate solution. Of course you understand that the values 1.8 and 3.6 VOLT belong at the use of a 0.5 K potentiometer. And 0.18 and 0.36 VOLT at the use of a 5K type.

When it is necessary to measure very accurate the antenna pointings, I use, in stead of the DVO, a high precision DVM.

Now this combination is really able to measure with a 0.1 degree accuracy. The required electronic tolerance is 0.027 %, but if you use the LT 1086 CT regulator ( 0.015 % ) it gives a better result. Now you can be sure becoming a 0.1 degree accuracy result.

Everything depend on the used voltage regulators. For example, if you use a LD 2517V33 ( 3.3 VOLT ) in combination with an trimpotentiometer, delivers this circa 0.5 to 1 % tolerance. If it concerns a very low tolerance type you better use in the combination a ( 10 turn ) high precision type trimpotentiometer.

If you use an ordinary trimpotentiometer, you could calculate some extra resistors 'around' this potentiometer. This creates a certain voltage area. Now the adjustings will be more exact and easy. Use stabil metalfilmresistors. In case of a certain situation you can use the voltageregulator LT 1086-5 CT 5 VOLT 0.015 %.

However, in most cases no trimpotentiometer can be placed between regulator and ground. Because of the fact that we can not lower the voltage, without making concessions to the resistance, which would become higher ! Unless the used voltage U > 5 VOLT, see also above ( DUTCH TEXT ). If the voltage U = 5 VOLT or U< 5 VOLT, it is strongly recommended to make an adjust possibility in the LED or LCD meter. A better solution can be : use the high precision adjustable ( low drop ) voltage regulator LT 1086 CT 0.015 %. Use now a precision ( 10 turn ) trimpotentiometer between the regulator and ground.

When distance between shack and rotator is very long ( see also above ) and you do not use shielded cable it can be necessary you place some extra capacitors. Then the indication unit will work without errors.
The End ( hope not in a bad way ..... )

We know it is impossible to reach a pointing tolerance of 0.1 degree ( with a Channel Master ), mechanically !

However, the indication is very accurate, if necessary you can construct it very easy, and it is guaranteed that you will reach a good tolerance with this indication unit. Costs : because of the absence of a DVO ( no use of a DVO ) we found a cheap solution. You only pay for the ( 10 turn ) potentiometers and the special voltage regulator. PS : The use of very low tolerances gives advantages if working satellites. With accurate antenna systems ( good pointings, very low SWR and perfect tuned antennae etcetera ) you will reach very good results ! No longer important is the +/- 3 dB ( usual ) but now is the +/- 0.5 dB beamwidth. Also see http://cebik.com, W4RNL wrote very interesting things concerning beamwidth ( and a lot more ) !

Addition ( PS ) I made some tests via a satellite ( AO-27, ideal because of the constant output signal ) and used 50 mW into a ( fixed ! ) antenna, pointed very accurate to the satellite. When the pointing became worse ( within approximately 4 degree ), while the satellite was 'moving further', it was IMPOSSIBLE to continue with the used actual low power ..... The transmitting antenna was not a very 'sharp' type. Of course I made several of these tests ( during AOS situations ) to exclude possible small polarization differences.

55 ( if necessary ! ) 73 leon ((-,O)) PA3GJE