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
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 SOL 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 SOL calibration of the instrument itself, and some support SOL calibration using the client software (Antscope in this case). This article demonstrates use of Antscope with SOL 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
Measure MLL using the half ReturnLoss method – a spot test with a hand held analyser
At Exploiting your antenna analyser #14 I gave an explanation of the method of approximating MLL of a line section by taking the average half Return Loss with o/c and s/c terminations.
This article demonstrates the technique using the Rigexpert AA-600 analyser in hand held mode.
The task is to assess whether a section of RG58A/U coax has MLL at 3.5MHz similar to specification or not.
The specification loss of 10.13m of RG58A/U has MLL=0.29dB.
Above, the first test with an o/c termination. Return Loss is 0.4dB. Continue reading Exploiting your antenna analyser #15
Assessing the Q of a half wave dipole antenna system explained that Q can be a valuable indicator of antenna system health.
This article uses a recently published VSWR curve for a 15m half wave dipole antenna system as an example to demonstrate the technique.
The following graph is from a Sark100 style antenna analyser, and it is quite a poor start to diagnostics, but using it draws out what is desired for further analysis.
Above, the captured VSWR(50) sweep. Continue reading Assessing the Q of a half wave dipole antenna system – a real world example
Q can be a valuable indicator of antenna system health
The Q of an antenna can be a useful statistic in assessing whether it is operating as it should.
The Q of half wave dipole antenna system on HF depends to some extent on conductor size, its environment (height, type of ground, nearby structures and vegetation, and feed line / matching loss. Nevertheless, it should usually fall in the range of 10 to 13 for good wire dipoles, and if you measure a half wave dipole antenna system to have Q significantly outside that range, it is probably significantly less efficient than it should be. Continue reading Assessing the Q of a half wave dipole antenna system
This article documents a field strength survey of an M40-1 short helical vertical on 40m.
This test is more a feasibility study of the experimental method and apparatus than an absolute measure of the antenna.
The antenna under test is described at AUT – MobileOne M40-1 40m helical.
Field strength was measured using a small square untuned loop and VK3AQZ RF power meter (RFPM1), and data was captured using A prototype data logger for RFPM1.
VK3AQZ RF power meter (RFPM1) described my build and calibration of the RFPM1.
Above is the RFPM1, shown with two probes, but only one probe is required for this procedure, the other is disconnected. The RFPM1 directly reads input power in dBm.
The loop antenna used was described at (Duffy 2007). It is a small square loop (600mm sides) fed in one corner with a 1:1 voltage balun. Continue reading Field strength survey of an M40-1 short helical vertical on 40m