A common question in online forums relates to inability to reconcile analyser measurements of an antenna system with the transmitter system antenna facing VSWR meter.
The cause is often that the antenna system was changed significantly to connect the analyser.
Seeing recent discussion by the online experts of how the measure the impedance of an antenna system looking into a so-called balanced feed line gives advice that is likely to cause reconciliation failure.
I will make the point firstly that the line is not intrinsically balanced, it is the way the it is used that may or may not achieve balance of some type. I will refer to that type of line as open wire line.
Let’s explore the subject using some NEC models.
I have constructed an NEC-4.2 model of an approximately half wave dipole at 7MHz, it is 20m above the ground, and fed slightly off centre with open wire line constructed using GW elements. At the bottom, I have connected a 2 segment wire between the feed line ends, and two sources in series. Continue reading Antennas – disturbing the thing being measured – open wire lines
The article MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz observed of the plot of loop impedance:
It looks quite different to the expected behavior of the underlying loop, but it does contain an arc albeit rotated and offset. In fact it can be transformed in two simple steps.
Continue reading MFJ-1786 loop antenna – a study of the matching scheme
Further to MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz, the question arises as to what commonly used tools readily permit the transformations and analysis.
Some relveant theory: for a load where R is approximately constant and X varies, the half power points occur where R=|X|, and following on from that s11=0.2±j0.4, s11=0.4472∠63.43°, |s11|=-6.99dB, ReturnLoss=6.99dB (yes, the +ve sign is correct), VSWR=2.618 etc.
Finding the points where ReturnLoss is approximately 6.99dB with the cursor on the above diagram is quite easy. Continue reading MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz – analysis tools
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