Further to MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz this article presents some other models of expected performance of the MFJ-1786 loop.
One of, if not the most popular loop calculator cited by hams is that by AA5TB. It is especially praised by ham loop enthusiasts.
Above is a screenshot of AA5TB’s calculator with the real antenna dimensions and “Added Loss Resistance” to calibrate the model to the measured 8kHz half power bandwidth. It predicts an efficiency of 30.6%, 2.9 times that of the NEC model. Perhaps it is popular because it provides overly generous estimates, IMHO it lacks credibility for many reasons. Continue reading MFJ-1786 loop antenna – other models at 10.1MHz
My friend Carlos, VK1EA, made some measurements of an MFJ-1786 SUPER HI-Q 36″ (0.914m) DIA 10-30 MHz loop at 10.1MHz.
This article presents some modelling and analysis of the antenna principally to estimate its performance.
The loop was located at 2m above natural ground away from other conducting objects.
He tuned the loop for minimum VSWR at around 10.1MHz and took a sweep with a EU1KY antenna analyser looking through 0.5m of RG223 50Ω cable saving the results to a s1p file which was imported to Antscope.
Measurement of the real antenna
Here is the impedance plot (excuse the |Z| plot as it is Rigexpert’s concession to hams who do not understand impedance and I cannot disable it).
Above, the impedance plot. The cursor is at point of minimum VSWR, and the associated R and X values at the measurement point are not very useful. Continue reading MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz
Variants of loops have been designed and promoted as having certain advantages, and one of those is the so-called figure 8 loop.
This article describes an NEC-4.2 model at 14MHz of an antenna similar to a commercial example.
The graphic shows the geometry. In this case the source is at the bottom of the lower loop, and the blue square is the tuning capacitor. The loop conductor is 22mm copper tube, the loop diameters are 1m, and the capacitor connection is 100mm wide. Commonly these are fed by a low loss auxiliary loop at the bottom of the lower loop, but the direct feed is quite fine for modelling the loop performance. Continue reading NEC model of figure 8 transmitting loop
Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):
It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a non-resonant antenna to resonance while also providing an impedance match for the output of the transceiver.
Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.
This was recently cited in a discussion about techniques to measure high impedances with a VNA:
WHEN the L and C’s of the tuner are set to produce a high performance return loss as measured by the vna, then in essence, if the tuner were terminated (where the vna was positioned) with 50 ohms and we were to look into the TUNER where the antenna was connected, we would see the ANTENNA Z CONJUGATE. Wow, that’s a mouth full. The best was to see this is to do an example problem and a simulator like LT Spice is a nice tool to learn. Or there are other SMITH GRAPHIC programs that are quite helpful to assist in this process. Standby and I will see what I can assemble.
The example subsequently described set about demonstrating the effect. The example characterised a certain antenna as having an equivalent circuit of 500Ω resistance in series with 4.19µH of inductance and 120pF of capacitance (@ 7.1MHz, Z=500-j0.119, not quite resonant, but very close). A lossless L network (where do you get them?) was then found that gave a near perfect match to 50+j0Ω. The proposition is that if you now look into the L network from the load end, that you see the complex conjugate of the antenna, Z=500+j0.119.
I asked where do you get a lossless L network? Only in the imagination, they are not a thing of the real world. Continue reading The system wide conjugate match stuff crashes out again
This article documents the RF electronic part of a HF current probe project.
The AD8310 module was bought on eBay for about $10. Continue reading AD8310 module for HF current probe
At Series match for a half wave dipole I mentioned that RG6/U may have a copper clad steel (CCS) centre conductor, and may have significantly more loss at HF than expected based on datasheets and calculators.
Above is a comparison of matched line loss based on measurement of a length of RG6/U Quad Shield CCS cable and prediction from Simsmith of Belden 8215 (also CCS). The ripple is due to measurement system error. Continue reading RG6/U with CCS centre conductor at HF
An online poster was demonstrating the effect of varying line length on a half wave dipole on VSWR(50) and by mistake configured the line be of of Zo=75Ω.
He asked the question
In the general case, if you are trying to match 50 Ohms, would you be better off feeding a normal backyard dipole with 75 Ohm coax if you are willing to prune it to a specific length after the fact?
Continue reading Series match for a half wave dipole
The Rigexpert AA-600 has an inbuilt calibration which is convenient to use. It is capable of OSL calibration, but this article discusses only the inbuilt calibration.
The reference plane is the plane at which the instrument calibration is correct, at other locations there is a transmission line impedance transformation applied.
The pic above shows the reference plane, but where exactly is it and why do you want to know? Continue reading Rigexpert AA-600 reference plane
This article shows the use of SimSmith in design and analysis of the input circuit of an MGF1302 LNA.
The MGF1302 is a low noise GaAs FET designed for S band to X band amplifiers, and was very popular in ham equipment until the arrival of pHEMT devices.
An important characteristic of the MGF1302 is that matching the input circuit for maximum gain (maximum power transfer) does not achieve the best Noise Figure… and since low noise is the objective, then we must design for that.
The datasheet contains a set of Γopt for the source impedance seen by the device gate, and interpolating for 1296MHz Γopt=0.73∠-10.5°.
Lets convert Γopt to some other useful values.
The equivalent source Z, Y and rectangular form of Γopt= will be convenient during the circuit design phase. Continue reading SimSmith – looking both ways – an LNA design task
Jacobi’s Maximum Power Transfer Theorem
Jacobi’s law (also known as Jacobi’s Maximum Power Transfer Theorem) of nearly 200 years ago stated
Maximum power is transferred when the internal resistance of the source equals the resistance of the load.
Implied is that the internal resistance of the source is held constant, it does not work otherwise. The source must be one that can validly be represented by a Thevenin equivalent circuit. This was in the very early days of harnessing electric current, direct current initially.
Later adaptation dealt with alternating current and it became
Maximum power is transferred when the load impedance is equal to the complex conjugate of the internal impedance of the source.
Again a necessary condition is that the source must be one that can validly be represented by a Thevenin equivalent circuit. Continue reading Transmitter / antenna systems and the maximum power transfer theorem