Amateur Radio – Page 65

Exploiting your antenna analyser #22

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.

The problem

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

Last update: 30th June, 2016, 9:40 AM

End Fed Half Wave / Inverted L / Half Wave Dipole

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 (σ=0.005, εr=13): 10m;
lossless balun / matching device.

Key differences:

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

Last update: 24th June, 2016, 10:40 AM

The magic of End Fed Half Waves / EFHW

I have noted recently the increasing popularity of the so-called End Fed Half Wave antenna, though the term often includes harmonic operation of the antenna.

It seems that at the heart of common ham understanding of this antenna system is that some kind of two terminal feed device creates a scenario with current on the nominal radiator, and zero common mode current on the feed line. If that feed device is small, its contents bears little influence on the current distribution on the feed line and radiator (the device behaviour approaches that of a simple circuit node).

Above is the kind of current distribution envisaged by many. The equivalent source is shown at the end fed feed point The red curve is the magnitude of current, the horizontal line represents the nominal radiator, and the vertical line represents the common mode conductor formed by the feed line. The feed line is often of arbitrary length, arbitrary route, and it may connect to real ground via an arbitrary impedance. Pretty much everything about this antenna system is random save the length of the nominal radiator. Continue reading The magic of End Fed Half Waves / EFHW

Last update: 22nd May, 2019, 7:09 AM

Exploiting your antenna analyser #21

A correspondent wrote about the apparent conflict between Exploiting your antenna analyser #11 and Alan, K0BG’s discussion of The SWR vs. Resonance Myth. Essentially the correspondent was concerned that Alan’s VSWR curve was difficult to understand.

K0BG’s pitch

For convenience, here is the relevant explanation.

By definition, an antenna’s resonant point will be when the reactive component (j) is equal to zero (X=Ø, or +jØ). At that point in our example shown at left, the R value reads 23 ohms, and the SWR readout will be 2.1:1 (actually 2.17:1). If we raise the analyzer’s frequency slightly, the reactive component will increase (inductively) along with an increase in the resistive component, hence the VSWR will decrease, perhaps to 1.4:1. In this case, the MFJ-259B is connected to an unmatched, screwdriver antenna mounted on the left quarter panel, and measured through a 12 inch long piece of coax. This fact is shown graphically in the image at right (below).

Note that the graph is unscaled, and that frustrates interpretation. The text is also not very clear, a further frustration. It is easy to draw a graph… but is the graph inspired by a proposition or is it supporting evidence. Continue reading Exploiting your antenna analyser #21

Last update: 19th June, 2016, 7:12 PM

Chinese AD8307 power measurement module #4

Desk study of opportunity to improve linearity.

At Chinese AD8307 power measurement module #2 I showed measurement of the linearity of an AD8307 based RF power meter.

The specification linearity is +/-1dB, which is poorer than one might like in a power measuring instrument.

The diagram above from the AD8307 datasheet shows the internal architecture, including 9 stages of cascaded log detector cells that attempt to give a log response over around 100dB range. The issue is that in the transition region between detector cells, error is worse than well inside an individual detector cell’s range.

Above is a sweep from -65 to -6dBm at 10MHz 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 (well within spec at this frequency, temperature etc).
Continue reading Chinese AD8307 power measurement module #4

Last update: 19th June, 2016, 10:27 AM

W2AU 1:1 voltage balun

A very long time ago, I purchased a W2AU 1:1 balun on the maker’s claims that it was good from 1.8 to 30MHz.

These were very popular at the time, but as voltage baluns they achieve good current balance ONLY on very symmetric loads and so are not well suited to most wire antennas.

Above is W2AU’s illustration of the internals.

Mine barely saw service before it became obvious that it had an intermittent connection to the inner pin of the coax connector. That turned out to be a poor soldered joint, a problem that is apparently quite common and perhaps the result of not properly removing the wire enamel before soldering.

Having cut the enclosure to get at the innards and fix it (they were not intended to be repaired), I rebuilt it in a similar enclosure made from plumbing PVC pipe and caps, and took the opportunity to fit some different output terminals and an N type coax connector.

W2auBalun01 Above is the rebuilt balun which since that day has been reserved for test kit for evaluating the performance of a voltage balun in some scenario or another. Continue reading W2AU 1:1 voltage balun

Last update: 26th June, 2016, 11:49 AM

Expected ambient noise – in practice

This posts shows a measurement of ambient noise and comparison with the data given at Expected ambient noise and its more detailed references.

The test scenario is my 40m station, a G5RV inverted V dipole with tuned feeders, a balun and ATR-30 ATU. Antenna system losses are less than 1dB.

The chart above gives a range for expected ambient noise at 40m.

Above is a screen shot from a spectrum analyser measuring power in 1kHz bandwidth from 7.0 to 7.1MHz. The band is mostly unoccupied, and the mean noise power is about -99dBm, it would be 3dB higher in 2KHz bandwidth (ie -96dBm). Continue reading Expected ambient noise – in practice

Last update: 8th June, 2016, 8:39 PM

Expected ambient noise

One of the casualties of the cessation of VK1OD.net was an article on expected ambient noise.

The original work was based on ITU-R P.372-8 which has been updated to -10, -12 and now -13, but the updates do not alter the basis for the original article.

Since the work was a reference cited on my FSM pages, it has been updated and copied to Expected ambient noise level. The graphics and tables in the article and the PDF file all refer to ITU-R P.372-8 but remain correct wrt ITU-R P.372-13 (2016).

Last update: 14th January, 2018, 4:18 PM

Review of Dunlavy’s STL patent gain claims

(Dunlavy 1967) sets out his description of a wide range tunable transmitting loop antenna and makes a broad efficiency claim of better than 30% (-5.3dB) for his system.

Minimum efficiencies of 30 percent are attainable with practical designs having a diameter of only 5 feet for 3-15 Megahertz coverage.

In a context where extravagant claims are often made for such antennas, his claims warrant review.

Dunlavey gives an example embodiment in approximate terms.

Practical loop designs for use in the range of 2-30 megahertz will utilize copper or aluminum tubular conductors having a diameter of 3 inches to 5 inches. A typical design for 3 to 15 Megahertz operation would be constructed as shown in FIG. 2 with a primary loop 4 having a diameter of about 5 feet and tuned by a high voltage vacuum capacitor 5 having a capacitance range of approximately 25 to l,000 picofarads. The tuned primary loop should be made of aluminum or
copper tubing having a diameter of approximately 4 inches-5 inches. The diameter of the feed loop, which is designated by the reference number 6, for 50 ohms impedance should be approximately l0 inches.

Lets take a perimeter of 4.8m (dia=5′) and copper conductor diameter of 100mm (4″) as the dimensions for further exploration.

Above, Dunlavy’s Figure 5 gives gain relative to a monopole above perfectly conducting ground. Continue reading Review of Dunlavy’s STL patent gain claims

Last update: 1st June, 2016, 11:39 AM

On copper loss in transmitting baluns

Designs appearing in the ham literature and online articles tend to espouse relatively large diameter conductors, conductors that can be challenging to wind onto the toroidal cores often used.

This article analyses the copper losses in a practical Guanella 1:1 balun where a fabricated twisted pair line is used.

Total losses comprise core losses and transmission line losses. Continue reading On copper loss in transmitting baluns

Last update: 26th May, 2016, 9:54 PM