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.

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.

Screenshot - 01_02_16 , 11_40_57

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

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

A low cost home made USB CI-V interface with open collector and solid Windows drivers

This article describes an inexpensive USB adapter for Icom’s CI-V interface.

There are four common options for USB-serial adapters:

  • Prolific;
  • FTDI;
  • WCH; and
  • Silabs.

This article describes an adapter based on an inexpensive FTDI adapter (~$5 on eBay).

opc478ftdi01

You will need the module, a Schottky signal diode (eg 1N5711), wire and a 3.5mm TRS plug or two. I have connected two plugs, one wired for TS (CT-17) and one for RS (OPC-478x). Continue reading A low cost home made USB CI-V interface with open collector and solid Windows drivers

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.

Screenshot - 29_01_16 , 19_45_17

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

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).

UhfTL

Above, the test jig.

Screenshot - 29_01_16 , 14_15_27

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

APRS duplicate removal

I have written about duplicate position records assigned different timestamps in the APRS – APRS-IS – archiver network, and the corruption of mapping as a result of false timestamps which causes zig zags in the mapping.

Most recently, I demonstrated that at APRS – APRS-IS latency.

I have been challenged that this just does not happen.

As a by product of developing code to circumvent the defects in the design and implementation, I have been running a rover stationary on the driveway during the day to supply test data to the filtering code, and of course these exceptions have occurred.

Here is an extract of the raw packets log from aprs.fi for just one case.

2016-01-26 16:20:25 AEDT: VK2OMD-8>APT314,WIDE1-1,WIDE2-2,qAR,VK2OMD-3:/052024h3429.91S/15027.12E>110/000/A=002263
2016-01-26 16:23:17 AEDT: VK2OMD-8>APT314,VK2RHR-1,WIDE1,VK1RGI-1,VK2RWG-1,WIDE2*,qAS,VK2KAW:/052024h3429.91S/15027.12E>110/000/A=002263

Inspection will reveal that both records have the GPS derived timestamp 052024h, and the message contents are identical, they are both a result of a single posit report at 052024UTC, the relay via VK2KAW has been assigned a timestamp some 173s (almost 3m) later and were the rover moving, this corrupted report would cause a track back and the next valid report would leap forward.

In six hours of operation, there were 36 posits transmitted, and 11 corrupt posits via VK2KAW logged by APRS-IS, that is 31% of valid posits have a false and corrupted posit generated in the wider network.

APRS fails by design, the core infrastructure does not detect these anomalies and discard them. Note that this scenario is ONLY provable because the tracker sends the HMS format timestamp… which almost no-one does.

This is only one source of corruption of posits, vulnerability to this one is designed into APRS, but it is detectable and fixable in core infrastructure.

 

APRS – APRS-IS latency

A fundamental problem with APRS-IS is that archives assign a timestamp to a position report (posit) when the posit is received from the APRS-IS network. They do this even when the posit contains a timestamp which is almost always derived from the GPS and is quite accurate.

The archives such as findu.com and aprs.fi are not formally part of APRS-IS, but they are the visible interface to APRS-IS for many and are treated as part of the wider APRS-IS network in this article.

The APRS-IS network is designed to permit, encourage even, multiple submissions of the same originating posit via multiple iGates over multiple digipeater radio links.

You might expect that the maximum latency of two or three digipeater links might be a couple of seconds but not only is it ordinarily longer than that due to an overcommitted network, but defective infrastructure using old and misconfigured software can introduce much larger delays.

The effect of delayed duplicates is that they create false back and forth zig zags in a track, and make the mapping hopelessly inaccurate.

For this article, I will use data gained from a recent 100km rural trip using VHF APRS. For all of the trip, the rover was in range of a prominent digipeater which is monitored by many iGates, and for about half of the trip, the rover was in working distance of one of the iGates. About 200 posits were captured by APRS-IS, and they have a GPS derived HHMMSS timestamp.

Lets look at the latency.

Clip 114

Above is a chart of the difference between APRS archive timstamp and the GPS timestamp. A very large number of posits takes more than a couple of seconds to reach the APRS-IS archives. Continue reading APRS – APRS-IS latency

Graphic demonstration of loss under standing waves #3

Having recently published Graphic demonstration of loss under standing waves, I have received a stream of emails asking whether some online calculator or another is subject to the errors discussed in the article.

Clip 080If we look at the graph from the article showing the example of a load of 500+j0Ω on 5m of RG58A/U, test the left hand data point at 1MHz and see what you get. If the loss is greater than the matched line loss (around 0.08dB), the calculator probably uses one of the flawed formulas discussed in the article… in any event it is wrong.

Here is an example from KV5R.com.

Screenshot - 22_01_16 , 10_11_37

The calculated loss is around 4 times what it should be. Continue reading Graphic demonstration of loss under standing waves #3

VSWR expressed as 1:n

At Expression of VSWR as a simple decimal real number I put the case for expressing VSWR simply as a real (ie a decimal) number rather than in the ratio form.

Lets remind ourselves of the meaning of VSWR (SWR).

(Terman 1955) gives a meaning for the term SWR (or VSWR).

The character of the voltage (or current) distribution on a transmission line can be conveniently described in terms of the ratio of the maximum amplitude to minimum amplitude possessed by the distribution. This quantity is termed the standing wave ratio (often abbreviated SWR)…

Standing-wave ratio=S=Emax/Emin

Terman has not dealt with the complication of short lines and lossy lines.

Note that the use of capital E implies the magnitude of voltage, so Emax/Emin must always be a positive number greater than or equal to 1.0 under that definition. Under that definition (and it has shortcomings), VSWR expressed as a ratio of m:n (and n is usually 1), m MUST be equal to or greater than n.

Screenshot - 20_01_16 , 11_35_41

Above is an extract from the brochure for Icom’s newest offering, the IC-7300. Continue reading VSWR expressed as 1:n

Exploiting your antenna analyser #11

Backing out transmission line

Often we make measurements through a section of transmission line, and the measurements are wrt the reference plane, which for many analysers is the connector on the instrument.

Some analysers, or their associated software allow the effects of the transmission line to be backed out.

Screenshot - 17_01_16 , 09_16_18

Above is a Smith chart view of measurement of a test antenna through some length of RG58. The antenna will have R<50Ω at minimum VSWR, so the angle of the complex reflection coefficient Γ will be close to 180° at the feed point. Antscope uses a different notation, but shows here the angle at the point of measurement to be -15.1°, so we need to increase it by 180–15.1=195.1°, which will take about half that electrical length of line, 97.6°. From TLLC, I calculate the length involved is 7.6m of RG58, which is an estimate that gives a starting point for backing out the cable. Continue reading Exploiting your antenna analyser #11