Much is written about ATU efficiency, about the need for them or not, and often in subjective terms like “lossy ATU”, and most of it lacking quantitative detail.
The little quantitative detail is almost entirely for purely resistive loads… as if that is typical of real life conditions.
The most common configuration used today is the ‘high pass T match’, but a range of other configurations are seen as being superior… though usually without quantitative evidence.
More Hams use MFJ-949s than any other antenna tuner in the world! Why? Because the worlds leading antenna tuner has earned a worldwide reputation for being able to match just about anything.
… so let’s make some measurements with a reactive load on a MFJ-949E. Capacitive loads tend to be very common for antenna systems at lower HF, so let’s choose a load of 50Ω with a 100pF silver mica cap in series at 3.6MHz. The reactance of the cap is -442Ω, so the load is 50-j442Ω, and the 50Ω part is a RF power meter (RFPM1).
The test setup then is:
a standard signal generator (SSG) on 3.6MHz with 20dB precision attenuator so that we are confident that Zs=50Ω (important to the adjustment of the ATU for maximum power as indication of 50Ω match);
The SSG was adjusted for -10dBm out directly into the RFPM1, then the ATU+cap inserted and ATU adjusted for maximum power indication. Power indicated was 1.4dB lower, so InsertionLoss and TransmissionLoss are both 1.4dB.
Above is a simulation of the T network in RFSim99, component values are adjusted for a match and inductor Q is calibrated to the measured loss of 1.4dB. Continue reading ATU efficiency
RG-6 has become a popular 75Ω transmission line for ham stations, and I have used it to good effect in many applications.
(Duffy 2007) extolled the virtues and gave implementation information, but cautioned:
Some types of RG−6/U use a CCS centre conductor and will have higher loss at low frequencies that shown in Fig 1, depending on the thickness of the copper cladding which may vary from cable to cable.
I have used RG-6/U with solid copper centre conductor widely on HF, and measured performance has always been consistent with expectation.
However, RG-6/U with solid copper centre conductor has become very hard to obtain, and products that remain available such as Belden 1694A are quite expensive.
This article documents measurements at low HF on a 100m roll of Quad shield RG-6/U purchased for UHF TV cabling.
The method used was to measure input impedance of the open circuit terminated 100m line section at a range of resonant and antiresonant frequencies, and from those to calculate Matched Line Loss (MLL) in dB/m.
Above is an example measurement around 3.74MHz. Zin is 213.4Ω at 3.74MHz. In this case I have used an AIMuhf one port analyser, but any instrument that can measure impedance in the range 10-500Ω would suit this particular scenario. Measurement of short low loss cables will yield more extreme impedances and may not be within range of some instruments. Continue reading RG-6/U for lower HF
The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a weep hole would be advisable in a permanent outdoor installation).
Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated.
The interior shows the layout. The wires use XLPE high temperature, high voltage withstand, moderate RF loss insulation. Two short pieces of 25mm electrical conduit serve to position the balun core against the opposite side of the box, and a piece of resilent packing between lid and core holds the assembly in place.
Differently to the example shown in the earlier articles, this prototype uses twisted PTFE insulated wires which have voltage breakdown higher than the XLPE shown earlier.
The self resonant frequency of the built balun was measured as 7.4MHz and the predictive model above calibrated. The balun has high choking impedance on the lower half of HF.
This Jan 2012 article has been copied from my VK1OD.net web site which is no longer online. It is for reference from other related articles. The article may contain links to articles on that site and which are no longer available.
One often wants to identify the type of material used in a ferrite core for use at radio frequencies. This article captures advice that the author has offered in online fora stretching back more than a decade, yet it seems uncommon knowledge.
The most common method is to make some measurements to determine the initial permeability µi, usually at audio frequencies, and to compare that to a table of µi for common core materials. This method might well indicate several mixes that have similar µi, but each may be quite different at higher frequencies.
The suitability for use at RF usually depends much more on complex permeability at radio frequencies than it does on µi at say 10kHz.
A correspondent asks about the effect of RCA connectors at HF on his proposed noise bridge. The question is very similar to that considered at Exploiting your antenna analyser #13 for UHF series connectors.
I have made a simple measurement of a BNC 50Ω termination (to check calibration) then inserted a BNC-RCA and RCA-BNC adapter.
Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.
To jump to the chase, it also has a Smith chart plot up to 200MHz that suggests it might be well modelled by a TL segment of 30-35Ω.