(Dunlavy 1967) sets out his description of a wide range tunable transmitting loop antenna and makes a broad efficiency claim of better than 30% (-5.3dB) for his system.
Minimum efficiencies of 30 percent are attainable with practical designs having a diameter of only 5 feet for 3-15 Megahertz coverage.
In a context where extravagant claims are often made for such antennas, his claims warrant review.
Dunlavey gives an example embodiment in approximate terms.
Practical loop designs for use in the range of 2-30 megahertz will utilize copper or aluminum tubular conductors having a diameter of 3 inches to 5 inches. A typical design for 3 to 15 Megahertz operation would be constructed as shown in FIG. 2 with a primary loop 4 having a diameter of about 5 feet and tuned by a high voltage vacuum capacitor 5 having a capacitance range of approximately 25 to l,000 picofarads. The tuned primary loop should be made of aluminum or
copper tubing having a diameter of approximately 4 inches-5 inches. The diameter of the feed loop, which is designated by the reference number 6, for 50 ohms impedance should be approximately l0 inches.
Lets take a perimeter of 4.8m (dia=5′) and copper conductor diameter of 100mm (4″) as the dimensions for further exploration.
Above, Dunlavy’s Figure 5 gives gain relative to a monopole above perfectly conducting ground. Continue reading Review of Dunlavy’s STL patent gain claims
Designs appearing in the ham literature and online articles tend to espouse relatively large diameter conductors, conductors that can be challenging to wind onto the toroidal cores often used.
This article analyses the copper losses in a practical Guanella 1:1 balun where a fabricated twisted pair line is used.
Total losses comprise core losses and transmission line losses. Continue reading On copper loss in transmitting baluns
Finding resistance and reactance with some low end analysers #2
Exploiting your antenna analyser #8 was about finding resistance and reactance with some low end analysers that don’t directly display those values of interest. The article showed how to calculate the values starting with |Z| from the analyser and included links to a calculator to perform the calcs.
This article describes an extension to that calculator Find |Z|,R,|X| from VSWR,|Z|,R,Ro to use R, VSWR, and Ro as the starting point. Note that the sign of X and the sign of the phase of Z cannot be determined from this starting point, there just isn’t enough information.
You will probably not find the equation for |X|(R,VSWR,Ro) in text books or handbooks, and the derivation is not shown here but if there is interest, I may publish a separate paper.
Lets say you knew VSWR=2, R=75Ω, Ro=50Ω, what is |X|?
Above, entering the values in the calculator we find that |X|=35.4Ω. Continue reading Exploiting your antenna analyser #20
A correspondent having read Analysis of a certain dipole animation questioned the validity of the lossy transmission line model of the dipole, citing the case of an OCF half wave which has an approximately resistive feed point.
Since the OCF lacks the symmetry exploited in earlier study, we must consider each half of the OCF dipole and combine them. To assist, I have produced a similar plot of the transmission line but note the changed X axis.
The scenario is again a 2mm diameter copper wire, 3m above ground at 1MHz.
Zo can be approximated as 138*log(2h/r)=138*log(2×3/0.001)=521Ω.
Above is a plot of calculated V and I at displacements from the open end, and calculated phase of V/I. Continue reading Analysis of a certain dipole animation – OCF implications
Critically review your measurements
A recent post on an online forum provides a relevant example to discussion of this subject.
I have personally seen ratios similar to 3:1 or higher at the feed point become 1:1 at the rig over 100 or so feet of coax cable.
First point is that in good transmission line, it takes an infinite length to deliver the observations made above. Less might deliver almost VSWR=1 at the input end of the line.
Let us consider a practical scenario, 100′ of RG58A/U with a load of 150+j0Ω at 14MHz, the load end VSWR(50) is 3, the input impedance is 32.50-j22.86Ω and input VSWR(50) is 2.01. In this scenario, the line loss is 2.5dB which might be unacceptable for some applications. Continue reading Exploiting your antenna analyser #19
Modern people look for videos and animations for their learning, and these are often not from reputable sources and raise more questions than they answer.
An example is an animation of a half wave dipole on the Internet, and being discussed on QRZ.
Above, the animated graphic.
Without trying to understand the problem, lets just extract two cases for further discussion, an analysis in the limits if you like. Continue reading Analysis of a certain dipole animation
At Chinese AD8307 power measurement module #2 I concluded that the modified AD8307 was useful on HF, and through to 54MHz depending on accuracy requirements.
This article looks at combining the AD8307 module with a display option based on an Arduino Nano.
Above is a demonstration of the display prototype. The module in the foreground is an Arduino Nano (~$6), and behind it a 16×2 LCD with I2C module (combined, ~$4). The Android tablet is connected to the Nano using a OTG cable (~$1) and is logging the measurements (optional) and powering the equipment. The red and black clips are connected to a power supply to simulate the voltage from the AD8307. The same configuration should work with any Arduino phone or tablet if it supports OTG. Continue reading Chinese AD8307 power measurement module #3
At Chinese AD8307 power measurement module #1 I documented the first phase of checkout of a low cost AD8307 module.
There are two requirements for accurate power measurement:
- input impedance of the power meter must be very close to 50+j0Ω (say input VSWR<1.2); and then
- gain from the SMA terminals to the AD8307 input terminals must be independent of frequency.
Though the module was clearly junk in terms of criteria 1 as supplied, it was possible to modify it to present a low VSWR 50Ω input impedance, and that was documented in the last chapter.
This article carries on with criteria 2 above, the amplitude response.
The application requires an adjustment of the AD8307 calibration to 20mV/dB with -90dBm intercept, meaning it will produce 1800mV at 0dBm input and have a slope of 20mV/dB.
Though the original circuit shows the necessary components R3 & R4, and R5 & R6, they are not fitted and must be fitted to the board. I have used 50kΩ 20t trimpots for R3 and R6, and 33kΩ for R5 and 47kΩ for R4.
The technique used to calibrate slope and offset is that described at (Duffy 2014).
Slope and offset calibration, and log conformance / scale linearity at 10MHz
Above is a sweep from -65 to -6dBm after calibration of slope and offset. The linear fit to the blue curve shows slope is 20mV/dB and intercept 1.8015 for 0dBm means the offset is -1.8015/0.02=-90.08dBm. Log conformance is 0.2dB.
Above is the fuller plot from -65 to 15dBm, and it can be seen the linearity degrades above -5dBm, but the error is small for this class of measurement chip.
The module was swept from 1 to 500MHz and response at 0dBm captured.
Above is the response from 1 to 50MHz. Response is down by 1.5dB at 55MHz, but within 0.5dB to 30MHz so quite suited to the intended application, a HF common mode current meter.
- The module as supplied was cheap Chinese junk, it had 35dB slope from 10MHz to 1MHz, input VSWR from 1.6 to extreme over the range 1-500MHz.
- Reworking the input circuit delivered very good input VSWR to 240MHz.
- Amplitude response with the reworked input circuit is within 0.5dB from well below 1MHz to 30MHz.
- Flat response at VHF – UHF would require an equalised input circuit and appropriate PCB layout.
I purchased a ‘ready to use’ AD8307 RF power measurement module on eBay for a project to develop a HF common mode current meter sensor for use with the RFPM1 (Duffy 2014). Price was A$22 incl post.
Above, the AD8307 module on a small PCB with shield enclosure. Note the prominent labeling DC-500MHz, but the abundant Chinese language must sound a warning. Continue reading Chinese AD8307 power measurement module #1
Measure velocity factor of open wire line
One of the measurement tasks that one often encounters is to measure the velocity factor of a transmission line.
Often this is an indirect task of tuning a tuned line section, my method is to often measure some line off the role, find the velocity factor (vf), and use that to cut line for the tuned section making appropriate allowance for connectors etc.
Measuring vf for an open wire line includes all that is done for measuring vf of coax, but requires measures to ensure that common mode current does not affect measurement significantly.
To minimise common mode current effects, I will use two measures:
- a high common mode impedance Guanella balun; and
- form the line section being measured into a loose helix supported on some fishing line to spoil any common mode resonance.
Above is the balun used, it is described at Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves. Continue reading Exploiting your antenna analyser #18