Find coax cable velocity factor using an antenna analyser with OSL calibration
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that supports OSL calibration, an AIMuhf in this example.
Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.
There are a thousand recipes, I am going to demonstrate just one that suits the instrument and application.
We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).
The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.
Above is a pic of the test fixture with adapters (in this case on a AA-600). Continue reading Exploiting your antenna analyser #24
NEC requires the user to define a model structure as a set of geometry elements. It includes two powerful cards that make definition of the structure simpler and more reliable, they are the GM card for coordinate transformation and GX card for reflecting a structure in coordinate planes.
This tutorial demonstrates the use of these cards to define what might appear to be a fairly complex hypothetical NVIS antenna scenario quite simply, and more importantly, reliably. I say reliably because the logical definition of the model based on similar elements already defined, the more confident the developer can be that they are indeed similarly defined, the differences are explicit, and that they are properly connected.
Above is a model to explore coupling from a tx antenna to a nearby rx antenna, The scenario contains 52 wire elements which one could naively define using 52 GW cards.
Instead, we will define it with far fewer GW cards and use model symmetry, rotation and translation to define the model. Continue reading NEC GM, GX tutorial
This article is a post mortem review of a 144MHz splitter combiner that was made using RG6 coax. It is post mortem (ie post death) because the combiner was stored outdoors without checking that the connectors were protected from weather.
The combiner was used successfully for over 10 years on a 144MHz four over four antenna system (above) without any maintenance problems.
Above is a close up of the Tee point of the network. The coax cables are protected by HDPE sleeving to reduce the chance of damage at the hands of Sulphur Crested Cockatoos, in the event there was no damage.
Continue reading Post mortem review of a 144MHz combiner / splitter
A correspondent questioned the writings of an online expert who opined whilst discussing loaded monopole antennas:
… there is a formula circulating the Internet which states that antenna Q is equal to 360 times the frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz. One has to assume they mean antenna system Q, but that’s not a given. While this formula might give you a comparison between antenna A and antenna B (all else being equal), the actual Q of the antenna (system or otherwise) requires a textbook-full of formulas, and a lot more information than just the 2:1 bandwidth! Fact is, this formula is no more specific than the number of DX contacts a specific antenna garnered.
The formula given is:
Q=360*fc/B(VSWR=2) where fc is the centre frequency.
Continue reading A certain formula for antenna system Q
A common method of combining two 50Ω antennas to a single 50Ω feed is using a quarter wave transformer using 75Ω line from the common feed point to each antenna.
A recent posting to one of the ham fora raises the posters problems with making this really simple feed system work.
Above is his measured input characteristic with good 50Ω loads on each leg. Reading a hundred posts, it seems that he attributes this to legs of 0.167m length of RG11. The problem is that RG11 as most of us know it has a solid PE dielectric giving it a vf=0.66 and that 0.167m is 63° at 207MHz… so why the response above. Continue reading Tuning combiner lines
Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser.
Above is a sweep of an A/B changeover relay intended for HF application at up to 100W and lowish VSWR. The sweep is actually from 1 to 61MHz to be confident that there is not poor behaviour just outside of the HF range that might present on another implementation of the same design. Continue reading Exploiting your antenna analyser #23
End Fed Half Wave antennas are again very fashionable with hams, accompanied by extraordinary claims and somewhat sparse understanding (the way of modern ham radio).
To add some light I have created a set of NEC 4.2 models of a half wave antenna on 20m to give some insight into the behaviour of a bottom fed vertical half wave over real ground.
This analysis does not consider harmonic operation, antennas are a half wave at 14.2MHz.
Four models are used:
- 20mHW-VEP – bottom fed vertical above perfect ground;
- 20mHW-VEA – bottom fed vertical above real ground;
- 20mHW-VCA – centre fed vertical above real ground (ie ground independent feed);
- 20mHW-HCA – centre fed horizontal at 5m height above ground;
NEC 4.2 model description:
- no conductor loss;
- real ground assumed to have conductivity=0.005S, εr=13, of course results are dependent on these values;
- conductors are ~10m long, 20mm diameter;
- bottom fed vertical half wave uses a 10m x 20mm vertical driven ground electrode;
- centre fed vertical is raised 200mm above ground;
- feed line and feed line common mode current are excluded;
- the centre of all antennas is ~5m above ground (real or perfect).
Above are the patterns from the models for discussion. Continue reading End fed half wave – NEC models for 20m
Some recent articles here used a two port analyser to evaluate Insertion VSWR of some coax switches, and it raises the question about application of a hand held analyser and Insertion VSWR of a VSWR meter.
(Duffy 2007) listed tests for evaluation of a VSWR meter:
Testing a VSWR meter
The tests here need to be interpreted in the context of whether the device under test (DUT) has only calibrated power scales, or a VSWR Set/Reflected mode of measurement, and whether directional coupler scales are identical for both directions.
- Connect a calibrated dummy load of the nominal impedance on the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be 1. (Checks nominal calibration impedance);
- Repeat Test 1 at a selection of test frequencies and for each test, without changing transmitter power, reverse the DUT and verify that repeat the forward/set and reflected readings swap, but are of the same amplitude (checks the symmetry / balance of the detectors under matched line conditions).
- Connect a s/c to the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be infinite. (Discloses averaging due to excessive sampler length);
- Connect an o/c to the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be infinite. (Discloses averaging due to excessive sampler length);
- Connect a calibrated wattmeter / dummy load of the nominal impedance on the instrument output and measure calibration accuracy of power / ρ / VSWR scales at a range of power levels in both forward and reflected directions (Checks scale shape and absolute power calibration accuracy).
- Repeating Test 1 additionally with a calibrated VSWR meter connected to the input to the DUT, and measure the VSWR caused by the DUT at a range of test frequencies (Checks Insertion VSWR).
It is not unusual for low grade instruments to pass Test 1, but to fail Test 6 (and some others, especially Test 3 and Test 4) towards the higher end of their specified frequency range.
Item 6 in the list was to evaluate the Insertion VSWR. Continue reading Can a hand held analyser be used to evaluate Insertion VSWR of a VSWR meter?
On a transmission line with standing waves, the voltage varies cyclically along the line, and is dependent also on power.
This article explains a method to use an analyser to predict the peak voltage level at a point for a given frequency and power based on measurement or estimation of complex Z or Y at that point using a suitable antenna analyser.
Lets say you have some critical voltage breakdown limit and want to use your analyser to find any non-compliance at the proposed power level.
Let us assume that the not-to-exceed voltage at that point is 1000Vpk. Let’s allow a little margin for variation due to factors not fixed, let’s actually use 800Vpk as the limit. We will use the maximum permitted power in Australia, 400W.
Continue reading Exploiting your antenna analyser #22
The popular End Fed Half Wave is all things to all men, but this article compares an End Fed Half Wave, Inverted L, and Half Wave Dipole with some common parameters:
- frequency: 7.1MHz;
- flat top length: 20m;
- Height above ‘average’ ground (0.005,13): 10m;
- lossless balun / matching device.
- ground connection: Inverted L = 2Ω, End Fed Half Wave = 100Ω; and
- effective common mode choke used on the dipole.
Above is the modelled gain for all three. Continue reading End Fed Half Wave / Inverted L / Half Wave Dipole