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
Optimising a G5RV with hybrid feed
(Varney 1958) described his G5RV antenna in two forms, one with tuned feeders, and the more popular form with hybrid feed consisting of a so-called matching section of open wire line and then an arbitrary length of lower Zo coax or twin to the transmitter.
(Duffy 2005) showed that the hybrid feed configuration is susceptible to high losses in the low Zo line as it is often longish, is relatively high loss line and operates with standing waves. Varney did offer two options for the low Zo line:
any length 72Ω twin or coax. Continue reading Exploiting your antenna analyser #17
Measure inductor using OSL calibration
At Measuring balun common mode impedance I showed a method of backing out the effects of a text fixture using the “subtract cable” facility of Antscope software with the Rigexpert AA-600.
Some analysers (including the AA-600) support OSL calibration of the instrument itself, and some support OSL calibration using the client software (Antscope in this case). This article demonstrates use of Antscope with OSL calibration to measure a small RF inductor which has similar characteristic to good Guanella 1:1 HF baluns.
The text fixture used for this demonstration is constructed on a SMA(F) PCB connector using some machined pin connector strip, and SMA(M)-SMA(M) and N(M)-SMA(F) adapters to connect to the AA-600.
Above is a pic of the test inductor in the test fixture with adapters. The test inductor 6 turns of 0.5mm PVC insulated wire wound on a BN43-202 binocular balun core. Continue reading Exploiting your antenna analyser #16
In Improved cooling for the MFJ-949E I described a solution to a problem of demonstrated overheating of the ATU at rated power, indeed at a lot less than rated power.
Though I have never measured the ATR-30 temperature rise, and am probably unlikely to stress the 3kW rated ATU with a 100W transmitter, I have performed a similar cooling modification to the ATR-30.
Continue reading Improved cooling for the ATR-30