Circuit wanted: Antenna Current lead or lagging Antenna Voltage?

[MikeMl]

I need a circuit which will produce a DC level proportional to the jX term at the feedpoint of an antenna. The frequency range is 1 to 20MHz. The circuit should work with RF levels from ~10W to 1000W. The antenna impedance range: 5<R<500, -500<jX<500 Ohms.

I am building an Arduino-based automatic antenna tuner, and this will be the transducer.

 

[ronsimpson]

How is this different than a reflected power mete?

 

[MikeMl]

How is this different than a reflected power mete?

The detector in a standard SWR bridge knows nothing about the sign of the reactive part of the antenna impedance.

 

[Tony Stewart]

Mike since this is a wide range in frequency and power level, it would be useful to know the source impedance.

A current sensing phase detector is possible using a limiter to make it independent of power level if the voltage range and impedance range is estimated.

It is also possible to have a stepper controlled LC impedance matching tuner with PLL (<$1k) for communications

https://digitalcommons.calpoly.edu/...e.com/&httpsredir=1&article=1191&context=eesp

 

[MikeMl]

Mike since this is a wide range in frequency and power level, it would be useful to know the source impedance.
...

Thanks for the link. The system will autotune a screwdriver antenna. During tuning, the radio output could be constrained to ~10W, but it will have stand 1kW without being switched out of circuit.

It will be for a 50 Ohm system.

 

[rjenkinsgb]

It could be rather simpler and cheaper to use conventional SWR type circuitry and emulate a person tuning - try fractional changes either direction and see which causes an SWR reduction.

As you are using a microcontroller to provide the automation it's a fairly trivial addition to the code in that.
That's the big advantage of a computer based control over a "dumb" servo system that must have directional info to work.

 

[MikeMl]

It could be rather simpler and cheaper to use conventional SWR type circuitry and emulate a person tuning - try fractional changes either direction and see which causes an SWR reduction.
...

One application is a screwdriver antenna. The SWR is maxed out for most of the tuning range. The SWR dip is very narrow, so you have to scan the entire tuning range to find it. There is no "slope" in 90% of the tuning range that is useful in figuring out which initial direction to tune...

 

[unclejed613]

the MFJ-259 schematic has the info you need. the cluster of diodes, caps and resistors around the antenna connector (upper center of schematic) has the various measurement outputs. the op amps (upper far right) provide gain and adjustment for the signals coming out of the measurement cluster.
the MFJ-259 calibration procedure identifies the measurement signals (and also has a short description of how the analyzer works) , so you should be able to find what you are looking for.

 

[MikeMl]

the MFJ-259 schematic has the info you need. ....

I still have one of those on the self that has been replaced with a RigExpert AA-600. Not very useful for my project because I plan to use the transmitter as the signal source, so the "Phase discriminator" should be like a directional coupler in that it passes anywhere from a few W up to ~1kW with low insertion loss. The MFJ analyzer is based on a bridge circuit, which cannot pass RF power, and would have to be switched in and out of the circuit.

So far, the best read on this topic I have found is here.

 

[unclejed613]

AN/GRC-106 antenna tuning indicator description

i've actually got a couple of years experience using the 106. the meters are center-zero meters. the antenna tuner in the 106 comes pretty darn close to the ideal of "able to tune up a doorknob at any frequency". it looks as if the meter circuits are straightforward and simple.

 

[MikeMl]

unclejed613
Yes, I had found that tuning indicator in that webpage I linked. They are still available mil surplus. I have one on order and will play with it.

 

[unclejed613]

if i remember correctly, that assembly is made up of the components on small pcbs inside a stack of machined aluminum blocks, not easily taken apart, and difficult to reassemble too... it's hard to find one of those radios in one piece, especially with the meters still present. either they are removed to resell the meters separately, or the meters are removed because the needles have radium paint on them.