Category: Transmitters

Exploiting your antenna analyser #23

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.

Screenshot - 03_08_16 , 14_12_56

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

Last update: 3rd August, 2016, 2:46 PM

InsertionVSWR of Grebenkemper’s Tandem Match

The findings at InsertionVSWR of Revex W560 on HF and the suggestion that the low frequency problem is characteristic of poorly designed Sontheimer couplers (Sontheimer, C & Frederick 1966).

These couplers were popularised by (Grebenkemper 1987)  in his Tandem Match – An Accurate Directional Wattmeter and have appeared in ARRL handbooks over the decades, and may have inspired the many commercial implementations of the coupler.

Grebenkemper claims his meter is ‘good’ down to 1.8MHz, but does not clearly claim any particular InsertionVSWR. There is limited value in an instrument that can measure down to 1.05 when it causes significantly higher VSWR itself.

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Lets drill down on Gebenkember’s article, specifically the coupler design.
Continue reading InsertionVSWR of Grebenkemper’s Tandem Match

Last update: 25th July, 2016, 8:21 AM

KG-UV920P repair / modification for driver FET failure

The KG-UV920P is infamous for failure of the driver FET, they run excessively hot and clearly outside of safe operating limits.

I repaired one for a friend some years ago, and the dealings with Wouxun were enlightening. If I had little confidence in them before that experience, after it I would not give consideration to purchase of any Wouxun radio.

My repair / modification notes have been copied literally from the old VK1OD.net webside, and may contain stale links etc, but if it is of use to hams with a broken radio, see KG-UV920P – a repair / support story.

I have seen lots of articles on this problem over the years since, including ones that try to add a heatsink on top of the driver FET. The driver FET is meant to lose its heat through the bottom metal pad, and heatsinking the plastic encapsulation will not be very effective. Bottom line is to reduce the operating voltage on the driver (as per the factory advice), and keep the radio cool.

Don’t operate the radio sitting on the desk or the like, the bottom is the heatsink.

Wouxun are not alone in manufacturing radios that run red hot, see my notes on supplementary cooling for an IC-220H: https://owenduffy.net/blog/?s=IC2200H+cooling.

 

Last update: 22nd July, 2016, 7:56 PM

Can a hand held analyser be used to evaluate Insertion VSWR of a VSWR meter?

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.

  1. 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);
  2. 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).
  3. 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);
  4. 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);
  5. 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).
  6. 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?

Last update: 8th July, 2016, 9:15 AM

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

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

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

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

Last update: 6th June, 2016, 8:09 AM

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 VSWR50 is 3, the input impedance is 32.50-j22.86Ω and input VSWR50 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

Last update: 21st May, 2016, 3:45 PM