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).

 

Clip 201

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

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.

Screenshot - 01_06_16 , 07_46_16

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

Exploiting your antenna analyser #20

Finding resistance and reactance with some low end analysers #2

Exploiting your antenna analyser #8 was about finding resistance and reactance with some low end analysers that don’t directly display those values of interest. The article showed how to calculate the values starting with |Z| from the analyser and included links to a calculator to perform the calcs.

This article describes an extension to that calculator Find |Z|,R,|X| from VSWR,|Z|,R,Ro to use R, VSWR, and Ro as the starting point. Note that the sign of X and the sign of the phase of Z cannot be determined from this starting point, there just isn’t enough information.

You will probably not find the equation for |X|(R,VSWR,Ro) in text books or handbooks, and the derivation is not shown here but if there is interest, I may publish a separate paper.

Lets say you knew VSWR=2, R=75Ω, Ro=50Ω, what is |X|?

Screenshot - 24_05_16 , 08_20_01

Above, entering the values in the calculator we find that |X|=35.4Ω. Continue reading Exploiting your antenna analyser #20

Analysis of a certain dipole animation – OCF implications

A correspondent having read Analysis of a certain dipole animation questioned the validity of the lossy transmission line model of the dipole, citing the case of an OCF half wave which has an approximately resistive feed point.

Since the OCF lacks the symmetry exploited in earlier study, we must consider each half of the OCF dipole and combine them. To assist, I have produced a similar plot of the transmission line but note the changed X axis.

The scenario is again a 2mm diameter copper wire, 3m above ground at 1MHz.

Zo can be approximated as 138*log(2h/r)=138*log(2×3/0.001)=521Ω.

Clip 199

Above is a plot of calculated V and I at displacements from the open end, and calculated phase of V/I. Continue reading Analysis of a certain dipole animation – OCF implications

Exploiting your antenna analyser #19

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

Analysis of a certain dipole animation

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.

Dipole_antenna_standing_waves_animation_461x217x150ms

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

Exploiting your antenna analyser #18

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.

Aa600BalunRg316

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

Exploiting your antenna analyser #17

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

Exploiting your antenna analyser #16

Measure inductor using OSL 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 OSL calibration of the instrument itself, and some support OSL calibration using the client software (Antscope in this case). This article demonstrates use of Antscope with OSL 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.

Aa600Fix02

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 BN-43-202 binocular balun core. Continue reading Exploiting your antenna analyser #16

Improved cooling for the ATR-30

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