Transmission lines – Page 24

Thinking about SOTA, EFHW and EMR safety

There seems to have been a revival in use of the so-called End Fed Half Wave antenna.

The prospect that a small radio such as the FT817, a magic match box and 10m of wire makes a good 20m field station appeals to many a SOTA enthusiast.

Let us model a scenario with a FT817 powered by internal battery and sitting on an insulating platform (eg a pack) 0.3m above natural ground, a 10m wire strung up into a tree at a 45° angle, and a 1m long mic cord stretched up at 45° in the other direction. The is the popular so-called ‘no counterpoise’ configuration.

A simplified model of just the current paths without regard to the bulk of the radio, or the effect of the helix of the mic cord illustrates an approximate current distribution. The model uses 1W RF input to the antenna over ‘average ground’ (σ=0.005, εr=13).

Above is a plot of the current distribution. Current is a minimum at the open ends, a boundary condition for the problem, and maximum in the middle of the half wave. We expect H field to be greatest near the current maximum, and E field to be greatest near the current minima. Continue reading Thinking about SOTA, EFHW and EMR safety

Last update: 27th April, 2016, 6:00 PM

RG-6/U for lower HF

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

Last update: 18th September, 2016, 5:32 PM

KL7AJ quick quiz 21/02/2016

Eric posed a quick quiz for the masses to test their knowledge under his heading “Do you really understand impedance matching?”

For your convenience, I will quote his challenge here.

All connections are made with low-loss coaxial cable. The antenna tuner is high quality with negligible losses.
According to standard conversion charts, we find that 4:1 SWR will give us 36% reflected power. Keep that number handy.
Now, we set up the experiment. First, set the slugs on BOTH wattmeters to read REFLECTED power.
Turn on the transmitter, and adjust the antenna tuner for zero reflected power on Bird #1. Switch to forward power, and set transmitter output to exactly 100 watts. Readjust antenna tuner if necessary to achieve zero reflected power, while maintaining 100 watts forward.
Go to Bird #2 and confirm that reflected power is 36 watts.
Question: What is the FORWARD power on Bird Wattmeter #2? How you answer this question determines if you understand the conjugate match theorem or not.

Let us assume that the transmission lines are 50Ω, and that the Bird wattmeters are calibrated for 50Ω.

So, to extract the key information, we have a lossless system (KL7AJ is a lossless kind of guy) and the load is stated to be VSWR=4. Continue reading KL7AJ quick quiz 21/02/2016

Last update: 22nd February, 2016, 10:04 AM

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #4

Fourth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Packaging

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.

Next installment: Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5.

Last update: 13th February, 2017, 7:47 AM

Reconciling my #52 choke design tool with G3TXQ’s measurements

A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.

The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale). Continue reading Reconciling my #52 choke design tool with G3TXQ’s measurements

Last update: 10th February, 2016, 7:12 PM

Exploiting your antenna analyser #14

Insertion Loss, Mismatch Loss, Transmission Loss

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.

Above is a plot of VSWR when Zref is adjusted for the flattest response from DC, and it can be seen that with Zref=33, response is quite flat to 200MHz. Continue reading Exploiting your antenna analyser #14

Last update: 9th June, 2016, 9:33 AM

Exploiting your antenna analyser #13

Insertion Loss, Mismatch Loss, Transmission Loss

A correspondent having read Exploiting your antenna analyser #12 asks whether the measurement provides evidence of loss of the connectors, and referred me to (Arther nd) where he reports some measurements of UHF series adapters and conclusions.

Duffy

Let’s deal with interpretations of my own measurements first.

Above is a plot of ρ and ReturnLoss for the DUT. ReturnLoss curiously is plotted ‘upside down’ as ReturnLoss increases downwards… a quirk of AIM software, but remember that ReturnLoss in dB is +ve.
Continue reading Exploiting your antenna analyser #13

Last update: 7th March, 2017, 7:32 AM

Low noise Yagis and 50MHz noise

After reading my post Some thoughts on noise on 6m (50MHz)…, a correspondent asked about the content in the context of comments by G0KSC.

Under the altruistic heading “Low Noise Yagis Explained”, G0KSC takes a competitor to task over the accuracy of statements regarding the importance or not of G/T at 50MHz.

G0KSC says:

MISREPRESENTATION OF PERFORMANCE MEASUREMENTS SUCH AS G/T:

Have you ever heard the saying ‘A little knowledge is a dangerous thing’?…

What is G/T? Continue reading Low noise Yagis and 50MHz noise

Last update: 1st February, 2016, 7:50 AM

Voltage and current on a transmission line with standing waves

Folk often ask how to calculate the maximum voltage on an antenna feed line with standing waves, often to get a feel for the necessary voltage withstand of baluns, feed line, switches and relays, and ATUs.

Feeding at a current maximum outlines the method described in detail at (Duffy 2011), but the approach is more complex than a lot of hams want.

A simpler method is to treat the transmission line as lossless, and to simply find the worst case voltage and current that can occur… and design for that, or perhaps do the more detailed analysis depending on the outcome.

A new calculator, Calculate Vmax, Vmin, Imax, Imin for lossless line from Zload (or Yload) and Zo, does just that.

Above is the built-in example of a G5RV with tuned feeder on 80m with feed point impedance derived from a modelling package. The voltage and currents calculated are those for a long lossless feed line.
Continue reading Voltage and current on a transmission line with standing waves

Last update: 30th January, 2016, 10:15 AM

Exploiting your antenna analyser #12

Is there a place for UHF series connectors in critical measurement at UHF?

Seeing some recent discussion by a chap who was trying to construct a low power 50Ω termination on a UHF series plug, it bought to mind the futility of using some kinds of connector for critical measurement above perhaps 100MHz.

There is a lot of conjecture about the nature of UHF series connectors, whether they act line a simple transmission line section with fairly uniform Zo, whether they are really just a lumped shunt capacitance, whether it is even important at UHF etc.

To illustrate the issue, I have assembled a simple test jig comprising an N(M)-UHF(F) adapter, UHF(M)-N(F) adapter and a 50Ω N termination (which was also used to calibrate the analyser. This set was assembled and plugged onto a calibrated AIMuhf analyser and swept from 1-500MHz… just into the UHF range (which is 300-3000MHz).

Above, the test jig.

Above is an expanded scale centre of the Smith chart of the sweep. Continue reading Exploiting your antenna analyser #12

Last update: 9th June, 2016, 9:33 AM