phil
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Posts: 14
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Post by phil on Aug 19, 2014 22:08:02 GMT -5
I am a relatively experienced user of the AIM 120 and have used is many circumstances at AM stations measuring antenna impedance, adjusting ATUs, determining coax length, etc. However there is one nagging issue that always seems to crop up when measuring antenna systems such as unipole (skirted) towers that have a fairly high inductive (+J) reactance value. An OIB-3 measurement and an AIM 120 measurement never agree. The AIM seems to display a Rs value about 10 ohms higher than the OIB-3 when the X value is over a couple of hundred ohms inductive.
So last night I hooked up a precision 100 ohm non inductive resistor and a large 50 microhenry 15 amp coil in series. When I measured the resistor just by itself the OIB (OIB-3 and Potomac SD-31 generator set to 1 MHz) and the AIM agreed within 1 ohm Rs at 1 MHz. Once I put the coil in series the OIB-3 actually dropped the resistance value by a about 1 ohm and displayed close to 300 ohms inductive reactance which is about right for 50 microhenries at 1 MHz. The AIM 120 on the other hand displayed about 113 ohms Rs and about the same X as the OIB-3. I have not figured out what this discrepancy is. The DC resistance of the coil is very low so that is not a factor. I feel I may be missing something here that is a very simple explanation I have just not stumbled on it yet. My next move is to get out the GR 1606A and see what I get there. Just like the old days.
Anybody out there have a similar experience?
Phil
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phil
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Post by phil on Aug 20, 2014 8:48:51 GMT -5
As sort of an answer to myself on above, AM radio engineers have long inserted a counter reactive component when measuring high reactive loads to increase bridge accuracy. This would mean for above that to get a truly accurate reading of the R, a high quality capacitor of the correct value would need to be inserted in series with the circuit (or antenna) being measured. This eliminates the inaccuracies caused by the high reactance at the bridge terminals. I had thought the AIM 120 would not require this type of treatment and would see the correct R value in a complex reactive load. However it appears to get the actual circuit conditions for R the high reactance at the input terminals of the Power Aim 120 must likewise be compensated close to 0.
Any comments out there? Phil
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Post by Bob on Aug 20, 2014 9:54:48 GMT -5
Hi Phil, This is an interesting experiment.
The AIM itself is accurate but it responds to the impedance that appears at the connector. This impedance can be more complicated than expected due to stray capacitance and/or inductance that is not readily apparent. The word "large" to describe your coil may indicate it has capacitance to ground that affects the reading significantly. There may be stray capacitance in the whole circuit that appears in parallel with the circuit under test.
I modeled an RLC circuit using Speq Math and found that putting even 20 pf in parallel with the 100 ohm Resistor and 50 uH Coil significantly affected the apparent resistance.
For example:
f=1MHz R=100 ohms L=50 uH C1 in parallel with coil = 30 pf
Calculated Z = 100 + j334
The capacitance of the coil itself affects the reactance but not the resistance. ----------------------------------
Now, include stray capacitance in parallel with both the resistor and the coil:
Let C2= 25 pf
Calculated z= 111 + j351
The stray capacitance in the whole circuit now affects the real part of Z.
The stray capacitance is hard to quantify but this example shows there is the possibility of some effect on the bottom line results. If be sure to put the cal loads at the end of the cable that you're using to connect to the R-L components under test. A cable even 1 foot long will have significant capacitance but this will be canceled by the calibration process when the cal loads are placed at the end of the cable. A cal resistor with leads can be use for convenience to make connection with gator clips.
The stray capacitance should be minimized by careful placement of the components and the cables. If possible, operate the AIM on battery power with short leads to the battery and run the computer on battery power. RF chokes in the DC supply leads and on the comm cable to the pc may help too.
--73/Bob
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phil
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Post by phil on Aug 21, 2014 7:37:37 GMT -5
Bob, thank you for the thoughtful response. Strays are bad thing when trying to get some precision. AM engineers have been fighting this for years and you would think that at the comparatively low frequencies in AM broadcast that this would not be so noticeable. Well as you have calculated and by what we are seeing they are a factor. We use heavy 18 inch clip leads from an OIB-3 for our AIM measurements and yes I do have a custom cal for them.
Last night I inserted a vacuum variable capacitor in series with my coil and 100 resistor. I adjusted the combination to 0 reactance (perfect resonance as shown on the AIM). I was still seeing 113 ohms of real R. These are all very high quality parts. So now the real question is when measuring an antenna Z what is the value of real R and imaginary X. These are different from the perspective of the broadcast industry standard OIB-3 bridge and what is obtained from an analyzer. This begs more experimentation in real world dynamic conditions not just static parts trying to simulate.
Best 73, Phil
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Post by Bob on Aug 21, 2014 10:29:27 GMT -5
Hi Phil, The strays can add up and become significant, even at BC frequencies. If the AIM is modeled as a "sphere" it has a capacitance of (very) roughly 10 pf with respect to the earth. Even in outer space, an object has capacitance with respect to the universe. There are some interesting examples on this page: Capacitance. Large components required for high power applications have several pf of stray capacitance at least. Using a balun to reduce the common mode current in the test circuit will help. This should have high common mode impedance at the frequency of interest. The balun itself doesn't have to carry high power while testing, so it can be physically small, which is an advantage to reduce it's own stray capacitance. -73/Bob
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phil
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Post by phil on Aug 21, 2014 12:21:47 GMT -5
Now I see why the ARRL lab test used the smallest parts its could find in their test with known reactance values. They built them right in the connector and thus eliminated most of the strays. Their measurements concluded that the 4160 analyzer was most accurate even at reasonable reactive loads in these tests.
73, Phil
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w4dnr
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Post by w4dnr on Aug 21, 2014 20:43:45 GMT -5
Phil,
I also have a OIB-3 and a "poor-man's" Potomac ( FT-757GT ) and I appreciate your reminder to null the reactance if it is high . I use the AIM4170 to record the finished ATU settings and at low reactances that I am used to, its darn close. I'll be playing with a 75 meter skirt fed tower soon, and your information will be very helpful.
Don W4DNR
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Post by smokchsr on Aug 24, 2014 4:37:24 GMT -5
As a regular user of the power Aim at AM frequencies, I think I'll have to play with what you're seeing and see what happens. My gut reaction is that you may want to verify or better "refresh" your calibration files. For any critical measurements I'll always recalibrate just before making them.
What I use for an AM test cable is about 6' of RG-400 that splits into 2 10" insulated flat braid leads with 30 amp clips on the end. This cable seem to measure pretty close up to 12 MHz. After that it's just useless data.
Typically when I get into high Z situations the AIM gives me much better numbers than the OIB. Of course the main drawback is that the AIM at a tower base can't excite a DA to make to make proper operating measurements. What I have done in those situations is to get an accurate (at least I think it is) measurement, is to set everything up so that I have a 50j0 on the input of the ATU. Once that's done I terminate the ATU input with a 50 ohm resistor and measure the conjugate.
As for the poor mans generator, I have a FT-747. I don't know about the 757, but with the 747 I found I need to run it through a T network to clean up the output. If you look at the output of the 747 with a scope you'll see it has a stair stepped sine wave that puts some error into bridge readings. Once through the T network it's nice & clean, and reads well. It's a bit more trouble to take into the field, but the 40 or so watts of test signal makes it real easy to get a good reading. With an SD31 with an OIB you pretty much have to use an external detector which throws in another error.
Alan KD4QOF
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Burt
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Post by Burt on Sept 9, 2014 11:56:20 GMT -5
Alan,
I also have trouble measuring Hi Z towers with the SD-31/RX-31 combination. Moving cables can drastically change the readings due to changes in distributed capacitance and just crosstalk in cables. I use the best cables I can to try and minimize the crosstalk aspect. What I have found is that using a Stoddart NM-25T as the detector, I get more consistent and "good" readings than with the RX-31. Moving cables around does not affect the readings nearly as much as when using the RX-31. I suspect the NM-25T is a much better receiver with respect to shielding. When I need to use an external detector with my OIB-3, It's generally the NM-25T that I use.
The AIM-120 has given me some very good measurements in High Z situations, and they're generally repeatable. As a normal practice, I, like you, always calibrate before making important or critical measurements.
It was you that convinced me of how well the AIM-120 would work in the presence of other strong signals.
By the way, the "Phil" that I now see on this forum, is that "our" Phil?
Thanks,
Burt, K6OQK
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Post by smokchsr on Sept 13, 2014 4:57:15 GMT -5
Ever since I realized how big the error can be using an external detector with an OIB, I just don't trust doing it. I usually use the FIM-41 as the external detector, and saw big differences between the null points of the internal detector and the FIM. So for the most part I just gave up on using an external detector. Actually I think I've only used the OIB 2 or 3 times since I bought the Power AIM.
And no, I don't think that's Phil from Indy.
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phil
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Post by phil on Jan 26, 2015 11:39:02 GMT -5
No, this is Phil from NJ, WB2IQG, I work as an engineer for a large and well known DC based "AM Detuning" firm for the wireless and tower industries. External null detection with any standard impedance bridge has always been tricky business. The suggestion to use the NM 25T is a good one and it seems to have the best shielding from external sources, even better than the FIM 41. The quality of BNC cables has a lot do with it also. However as someone pointed out there is always the "uncertainty" of the measurements using an external detector with any bridge. In the good old days in the 60's and 70's when there were a lot of AM consulting engineers around we were lugging communications receivers as null detectors and a huge 30 pound generator (GR 1330A). I still have my GR 1330A generator/1606A bridge from back then. We struggled back then with this problem and seemed to come up with a better answer with better one point grounding (read strap/heavy braid) of all the test equipment. Our company is now using the new Potomac FIM 4100 field strength meters. I would like to experiment with one of those as a bridge null detector and see how the shielding is on it.
The Power AIM 120 is deadly accurate with no interference, however with the flea power it uses it is more than difficult to get really good sweeps of the BC band (or portions) on any AM tower in most places even in the daytime. Ron Rackley, Tom Jones and others are now using high power amplifiers and a directional coupler (Tunwall) with standard (expensive) HP or equivalent VNA's. They are getting very good results which is now required for the Method of Moments directional antenna proofs.
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phil
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Post by phil on Feb 8, 2015 17:49:22 GMT -5
Alan has suggested in a separate correspondence to operate the AIM 120 with averaging turned on at 256 and using very small frequency steps, as low as 10 Hz. I am anxious to try that and see what we arrive at using this method. I pulled out of mothballs and did try my NM 25T and the difference for null detection is quite significant from the FIM-41 due to better shielding using the OIB-3. The only trouble is my NM 25T Ni-cads are shot. It only works on AC. Also, the NM 25T can be hard to find if you don't have one already. Keep your eye out on the flea markets. When they come up on e-bay they are expensive. These were made by other companies than Stoddart. They are all the same high quality by Singer, and others. Made to military spec. Sorry for dragging this off topic.
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Burt
Junior Member
Posts: 51
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Post by Burt on Feb 13, 2015 12:57:24 GMT -5
Phil and Alan,
Quite often I set up my AIM to take small steps. Yes, it takes longer to do a scan, but there are times when I want to go back to a particular file (scan) and take a closer look at something. For example, the electrical length of a transmission line or the +/-45 degrees from 0+j0. It's handy to have that resolution.
I've had my NM-25T and NM-12T battery packs rebuilt at a local Battery house called BatteryPlus. I usually hand it to them and tell them to, "make it better", and a couple of hours later my pack has been rebuilt. My two units have been running fine for over 5 years since last re-built by them. On a 12 hour charge they'll run about 40 hours continuous.
Burt, K6OQK
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Post by smokchsr on Mar 24, 2015 0:42:30 GMT -5
BTW Burt, Hope I'll see you in April, Phil you too if you are headed to the show.
Alan KD4QOF
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