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Author Topic: RF Amp Meter system for open wire lines  (Read 2488 times)
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aa5wg
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« on: July 02, 2011, 09:13:25 AM »

Has anyone seen this? 

Does anyone have an idea how the schematic would look like for this device?

http://www.deltaelectronics.com/data/dtcaamm.htm

http://trade.indiamart.com/details.mp?offer=1964783662

73,
Chuck
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« Reply #1 on: July 02, 2011, 10:43:01 AM »

Those are quite popular in AM BC stations.  The pickup is a shielded toroid that the RF sample passes thru, and the meter is a rectified display, either analog or digital.

Pricey, better than $2K for a standard model.  The Delta item is useful on 75M and lower freqs, I have seen the same tech used up to 10M by other manufacturers.  They are very reliable and repeatable.  They are not so fragile in terms of lightning damage, but a direct strike calls all bets off.

One would need two of them for good data on balanced wire lines.

73DG
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aa5wg
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« Reply #2 on: July 02, 2011, 11:40:42 AM »

DG:
Thank you.
Anyone else have a source for the schematic for these items?
Chuck
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« Reply #3 on: July 02, 2011, 11:44:58 AM »

Here is a bit more:

http://amfone.net/Amforum/index.php?topic=25021.5;wap2

73DG
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k4kyv
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« Reply #4 on: July 02, 2011, 12:21:49 PM »

"Modulation Independent", a distinct advantage for measuring AM carrier power output. It is a rectifying instrument, therefore an average voltage→current reading instrument.  With AM, the average rf voltage and current remain steady with modulation, because the positive peak is exactly averaged out to zero by the negative peak.  This is why the DC plate current meter to a an amplitude modulated final is not supposed to move up or down with modulation (excepting, of course, controlled carrier type AM). Carrier shift, however will show up in the reading.

A thermocouple RF ammeter, OTOH, is a true RMS current reading instrument, and kicks up with modulation.  With 100% sine-wave modulation, the rf current should kick up 22.5%. This can be a nuisance when trying to get antenna current readings for the log at an AM broadcast station, but the up-kick can reveal other information about the signal, such as carrier shift, parasitics, etc. The thermocouple meter is sluggish enough that it is only slightly affected by normal voice modulation, but a prolonged tone from such program material as instrumental music or sound effects can make it difficult to read.

Average power is the product of RMS voltage X RMS current, not average voltage X average current. With a steady un(amplitude)modulated carrier, such as the case of FM, RTTY or CW with the key held down, the average current/voltage reading instrument will accurately calculate total output power, but with any kind of amplitude modulation, including both AM and SSB, the reading cannot accurately calculate total output power. With AM, it calculates carrier power as described above.  With SSB (or DSB suppressed carrier), the reading is meaningless, and will indicate much less than the actual output power, with the reading dependent on the waveform of the signal.
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« Reply #5 on: July 02, 2011, 12:28:19 PM »

No schematic for that exact unit. Otherwise it is like any other current transformer and RF voltmeter except for the core. I think the problem with HF as opposed to their BC band product is frequency response but with the right core it may not be a huge issue.

In some military gear, one of the tuning meters or part of an auto tune system senses a proper match by comparing current and voltage on the line.

Those schematics always have a load of some kind on the secondary of the current transformer just like would/must be done for current transformers on 60Hz power.

Looking at T1 in the diagram, it is plain to see the current step-down of 40:1, and the 39 Ohm load resistor. This is from a 400W PEP transmitter and the Z on that line is 50 Ohms, but the principle is the same. It is only necessary to measure the voltage across the load R3 as they do with a rectifier in order to find the average current. Hope this helps.


* tune and load discriminator.png (110.05 KB, 1295x821 - viewed 161 times.)
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« Reply #6 on: July 02, 2011, 01:57:09 PM »

I know this wasn't asked about but I thought I'd just toss out the fact that the thing MFJ sells for measuring current and checking balance on parallel wire line is no gud for AM.  HEAT and Z change big time.
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« Reply #7 on: July 02, 2011, 01:59:57 PM »

As others have pointed out, the basic concept is used in every directional coupler. Modern, high permeability materials make this easy to do, because: if the permeability of the core material is sufficiently high, then (to a very good approximation) the ratio of the current in the N-turn secondary to the current flowing in the wire passing through the core is 1:N.  

http://www.w8ji.com/building_a_current_meter.htm

http://www.w1tag.com/RFA.htm

http://w5jgv.com/rfa-1/rfa-1.htm

I use these types of transformers to make RF pickups (to connect to a scope, without rectifying the rf output), and also as sensors for peak detectors (see the attachment below).

For the circuit shown in the attachment, the current in the 16 turn secondary will be one 16th of the current in the primary wire passing through the core. The voltage across the 50 ohm non inductive secondary load resistor will be (50 ohms/16) x the current in the primary wire passing through the core. If the rf current in the primary wire is 6.4A peak (corresponding to approximately 1kW of rf power flowing into a 50 ohm antennna load), then the peak voltage across the secondary 50 ohm resistor will be 20V.

The difficult part, if it is important to you, is making the pickup very accurate over a range of frequencies and a wide range of current levels. This involves various details having to do with parasitic capacitances and saturation of the magnetic core material. For use in HF applications... particularly below 7.3 MHz, parasitic capacitances should not be a problem. If you use a large core, and a low resistance load on the secondary side (e.g. something less than or equal to 50 ohms), saturation effects should not be a problem either.

Stu


* Envelope Monitor Floating.jpg (63.65 KB, 960x720 - viewed 185 times.)
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k4kyv
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« Reply #8 on: July 02, 2011, 02:09:31 PM »

I am interested in building a variation of the old balanced version of the "Micromatch" that appeared in QST back in the late 40s.  The one described was supposed to be good up to only about 300Ω, but I think it would be easy to make it work for 450Ω.
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aa5wg
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« Reply #9 on: July 03, 2011, 08:18:06 AM »

Hi DG, Don, Stu, Opcom, K5UJ and all:

Thank you for your inputs.  I want to build a light or analog meter to indicate relative output for open wire feeders combined with the link antenna coupler.  But, I would like it to work for CW, SSB, Digital modes, AM and FM. 

How should I go about this? 

Chuck
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AB2EZ
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« Reply #10 on: July 03, 2011, 09:38:55 AM »

Chuck

See the attached

1. Use two separate ferrite toroidal cores (SB-1020-43)

http://www.cwsbytemark.com/index.php?main_page=index&cPath=206_219_234

2. Wind a 16-turn secondary (e.g. #20 or #22 insulated hookup wire) on each toroidal core

3. The DPDT switch will either add the voltages produced across the 50 ohm non-inductive resistor in each secondary winding or subtract the voltages produced across the 50 ohm non-inductive resistor in each secondary winding.

Therefore, the voltage produced across the output coaxial cable will either be proportional to the sum of the currents in the two antenna conductors (which should add to zero if things are balanced), or the difference of the currents in the two antenna conductors. When the DPDT switch is in the difference position... this should produce a response at the output of the coaxial cable, as measured by a scope with a 50 ohm input impedance of:

V= [A x 1/16 x 50 ohms] x 2 x (50/150); where A is the current flowing in one (1) of the two (2) antenna feeder conductors.

Note that the two 50 ohm non-inductive resistors are permanently connected to their respective secondary windings (not switched). This is important, because you don't want to have either secondary winding un-terminated when current is flowing in the antenna feeder. The arrangement shown allows you to safely throw the DPDT switch when current is flowing in the antenna feeder.

Stu


* Envelope Monitor Floating Balanced.jpg (54.92 KB, 960x720 - viewed 182 times.)
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k4kyv
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« Reply #11 on: July 03, 2011, 02:00:45 PM »

Hi DG, Don, Stu, Opcom, K5UJ and all:

Thank you for your inputs.  I want to build a light or analog meter to indicate relative output for open wire feeders combined with the link antenna coupler.  But, I would like it to work for CW, SSB, Digital modes, AM and FM. 

How should I go about this? 

Chuck

I would  look for a matched pair of thermocouple RF ammeters, and place one in series with each feeder.

The line current could vary widely from band to band, particularly if you are using series tuning on some bands and parallel tuning on others, and of course would be directly affected by the power you are running. So ideally, you should have several different ranges available, and employ the ones with the most useful range. They can still be found at hamfests, although they are not as abundant as they once were.

A rectifier type meter would substantially under-indicate for SSB, except when running a steady test tone for modulation.
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aa5wg
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« Reply #12 on: July 03, 2011, 04:20:04 PM »

Stu, Don and all:

I appreciate the information very much.  Thank you.  Stu, I will read up on the circuit(s) you attached. 

Do you think it is possible to use the same circuit with light bulbs and analog meters, i.e 100uA meters or other ranges?

I would like to use this on AM and on the other modes too, i.e. SSB, FM, CW, digital etc.

Chuck
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« Reply #13 on: July 03, 2011, 05:33:16 PM »

Chuck

If you want to use the circuit with an analog meter, I suggest that you do the following:

1. Plug the output into a diode peak detector, followed by a 0-100 micro-ammeter.

See the attachment

You should check for balance by comparing the meter reading when the DPDT switch is in the position where the two secondary voltages are added (ideally this reading should be zero if the antenna feeder wire currents are equal and opposite)... with the meter reading when the DPDT switch is in the position where the two secondary voltages are substracted (ideally proportional to twice either antenna feeder wire's current).

The 100k potentiometer will allow you to adjust the meter reading to be full scale when the DPDT switch is in the subtracting position.

If the transmitted power level is too low to produce a full scale reading... then reduce the value of the 10k ohm resistor that is in series with the potentiometer.

Stu


* Envelope Monitor Meter.jpg (65.2 KB, 960x720 - viewed 147 times.)
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John K5PRO
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« Reply #14 on: July 04, 2011, 06:53:18 PM »

The Delta Electronics TCA series of ammeters are a diode-rectifying circuit, with extra diodes added to compensate the device for temperature changes and to deal with offsets. The late Charlie Wright got a patent for it in the 70s and FCC has approved them as alternatives to thermocouple ammeters.

Their TCT series is just the toroidal current transformer without the rectifying circuit and readout. These are used to provide phase/amplitude information to an antenna monitor for directional medium wave stations. They are used in lieu of sampling loops on vertical radiators.

They used to hand calibrate TCAs to work with a mirrored-scale Beede meter.
I worked for Charlie at Delta Electronics in northern Virginia, for several years. Attached is the front page of his patent, you get the idea. Simple, clever, functional = his style. He and Steve Kershner also developed the OIB operating impedance bridges. They did a lot of consulting and curtain antenna work for Radio Free Europe among others, as they were well connected in the high powered SW broadcast industry.

* Delta TCA patent page.pdf (43.09 KB - downloaded 83 times.)
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« Reply #15 on: July 05, 2011, 06:29:27 PM »

A current transformer should work fine with open wire line as long as there is plenty of insulation between the open wire line and transformer winding.
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aa5wg
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« Reply #16 on: July 11, 2011, 08:25:36 PM »

Don, John, Stu and all:
I am looking at these circuits and taking notes.
Chuck
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