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.

Screenshot - 24_07_16 , 15_31_41

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

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: http://owenduffy.net/blog/?s=IC2200H+cooling.

 

Fan controlled by humidity sensor

I have a problem with machine tools getting condensation on them when conditions in the shed read dew point.

A possible solution being explored is to circulate air with a fan, possible inducting outside air, when humidity approaches condensing conditions.

A quick search reveals the HDS10 resistive humidity sensor for a dollar or so on eBay.

Above is the HDS10 humidity sensor.

Most low cost humidity sensors use a humidity dependent capacitive element, the HDS10 is different in being a humidity dependent capacitive element and is therefore simpler to use with microcontrollers with ADC input.

HDS10-02

The above graph is from the datasheet. It is intended primarily for sensing high humidity (dew point, condensing conditions) which suits this application.
Continue reading Fan controlled by humidity sensor

Optimal Zo for TLT sections challenge – a solution

Optimal Zo for Guanella balun sections left the reader with a though exercise, a transmission line transformer used by PA0V in a 144MHz power amplifier output network.

Clip 206

The pair of tabs to the left are driven by FET drains, the upper pink centre conductor is grounded, the lower end connecting to C1 is the output to a nominal 50R load. The network shown near OUT is for fine load adjustment. There are two coax sections making this TLT, shields bonded all the way around and the centre conductors connected as shown. What is the optimal value of Zo for each the coax sections?

PAdetail

Above is a pic of the PA, and we are looking at the network to the right of the dual FET. Continue reading Optimal Zo for TLT sections challenge – a solution

Optimal Zo for Guanella balun sections

A Guanella balun may have several sections, and they may be connected in parallel on one side and series on the other side so as to achieve nominal impedance transformation ratios other than 1.

The question is often asked, what is the optimal Zo for these line sections?

Several answers exist in ham lore, but the answer is relatively simple and revealed by the most basic understanding of transmission lines.

If you do not want standing waves on a line section and its associated impedance transformation, then make sure that Zo=V/I… easy as that.

(Guanella 1944) explains it with examples:

Screenshot - 09_07_16 , 15_55_57

Note above that he refers to coil systems. He did not describe for instance (b) on a single core, a shared magnetic circuit which would be a single core system, but he states clearly two coil systems. (Sevick 2001) and lots of other hams say otherwise, but they are wrong. Continue reading Optimal Zo for Guanella balun sections

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?

Coax switches – is shorting unused port necessary for isolation?

At Ratings of coax antenna switches I showed characteristics of a home made switch which has very low InsertionVSWR, but poor isolation.

A couple of correspondents have offered an explanation that the unused port must be shorted to get good isolation.

If that was the case, then we would expect all coax switches that leave the unused port open to have poor isolation.

Let us look at a very good coaxial relay

 

Above is a Dowkey 402 series relay which has good performance to GHz. It does not short the unused port.
Continue reading Coax switches – is shorting unused port necessary for isolation?

Ratings of coax antenna switches

In a recent article I discussed how InsertionLoss implies InsertionVSWR in lossless devices.

This article looks at measurements of a few antenna switches at hand.

Daiwa CS-201G II

It is difficult to find comprehensive data on the very popular Daiwa CS-201 series switches.

CS201spec

Above is the data from the packet of one of these switches, a CS-201G II. The specifications are pretty loose, and one must depend on one’s own measurements.

CS201N01

Above, the CS-201G II, a basic CS-201 series switch with N connectors, advertised as useful to 2000MHz where InsertionLoss is given as 1.2dB (or better?). If there were no TransmissionLoss in the switch, that would imply InsertionVSWR=3.6, but there is probably some significant TransmissionLoss and InsertionVSWR would be somewhat less. Nevertheless, IMHO InsertionLoss=1.2dB indicates it as unsuitable such frequencies. Continue reading Ratings of coax antenna switches