Category: Transmission lines

Does VSWR damage HF ham transmitters

This Jan 2011 article has been copied from my VK1OD.net web site which is no longer online. It is for reference in further articles discussing the popular reflections explanations. The article may contain links to articles on that site and which are no longer available.

The statement is often made to the effect that:

VSWR will damage a HF ham transmitter, and the mechanism is that the ‘reflected power’ in a standing wave will be absorbed by the Power Amplifier (PA), increasing heat dissipation and damaging the PA.

There are two problems with this statement: Continue reading Does VSWR damage HF ham transmitters

Last update: 13th October, 2015, 10:05 AM

Transmission line loss under mismatch explanations – the missing TWLLC model

At Transmission line loss under mismatch explanations I wrote that there is a lot of woolly thinking amongst hams about transmission line loss under mismatch and worked a simple example that could be done ‘by hand’ to show that formulas that some authors have produced as implementations of their explanations don’t stack up.

I also gave a solution to the Zo*3 scenario using TWLLC, but not the Zo/3 scenario which a few eagle hounds have pounced on as evidence that the solution would not support the article.

Not at all, the Zo/3 TWLLC solution was not given so as to keep the article short and within the attention span of modern hams though it was eventually a quite long article, and for that reason I will address it separately, here. Continue reading Transmission line loss under mismatch explanations – the missing TWLLC model

Last update: 25th May, 2016, 8:31 AM

Transmission line loss under mismatch explanations

There is a lot of woolly thinking amongst hams about transmission line loss under mismatch, perhaps exemplified by Walt Maxwell (Maxwell 2001):

The power lost in a given line is least when the line is terminated in a resistance equal to its characteristic impedance, and as stated previously, that is called the matched-line loss. There is however an additional loss that increases with an increase in the SWR.

This article probes the folk lore with an example scenario designed to expose the failure of such thinking. Continue reading Transmission line loss under mismatch explanations

Last update: 10th October, 2015, 6:36 PM

Mini60 antenna analyser

There seems a never ending stream of low end antenna analysers appearing.

The Mini60 antenna analyser is one in that vein, and is sure to prove popular because of its low price. As is common, there does not appear to be an English language user manual and the specifications in eBay ads are not very reliable (eg weight: 200kg).

Clip 010

Above is a screenshot from an online demo of the Mini60 on a 7MHz antenna. Continue reading Mini60 antenna analyser

Last update: 11th October, 2015, 5:13 PM

An interesting case study – in service evaluation of coax loss

A poster sought advice of the forum experts about in service evaluation of the loss of some coax feed lines…

Has anyone tested old coax cable to see if the loss increased over time? I just tested two different coax cables at 146 Mhz with the use of a Bird Model 43 Wattmeter. Power measurements were taken at the input of each cable followed by the output. The load in both cases was a 146 Mhz Ground Plane.

The test results seem to show losses similar to new coax although Berk-Tek foam coax may have had a lower loss when new.

1. Berk-Tek 6211 RG-8X Ultra Flex Foam Coax – 68 feet

Measured 25 watts in and 11.7 watts out which represents a 3.3 db loss. …

Assuming that the stated measured power is in fact the indicated forward power on the Bird 43 directional wattmeter and given that the actual Zo of the line should be very close to the calibration impedance of the Bird (50+j0Ω), then the Matched Line Loss (MLL) is very close to 10*log(PfIn/PfOut)=10*log(25/11.7)=3.3db which is significantly above the expected 2.6dB for ‘ordinary’ RG-8/X and warrants re-measurement as it suggests that the cable might have degraded a little. In fact, the OP later reports 10.7W out for 25W in which is MLL of 3.7dB against spec of 2.6dB… a more convincing case for replacement! Continue reading An interesting case study – in service evaluation of coax loss

Last update: 13th July, 2015, 4:08 AM

Matching a quarter wave monopole with two variable caps

Two recent correspondents have discussed matching a quarter wave monopole with two variable caps.

Two capacitor shunt/series match

The matching scheme involves a shunt variable cap at the end of the coax feed line, and a series variable cap to the monopole base. The radials are of course connected to the feed line shield.

This type of matching scheme requires that the monopole feed point has sufficient +ve reactance, ie the monopole is longer than resonant. Lets assume the R component of feed point Z is 35Ω.

This scheme incorporates the simple shunt match, and the value of the shunt capacitor can be found knowing the R value to be matched to 50Ω.

Screenshot - 08_07_2015 , 06_43_07

Above is a Smith chart of a model of the match at 14MHz. The monopole has been lengthened to have 100Ω reactance along with 35Ω resistance. In this case a series cap of 148pF and shunt cap of 150pF are required. Continue reading Matching a quarter wave monopole with two variable caps

Last update: 8th July, 2015, 2:10 PM

Differential and common mode components of current in a two wire transmission line

A pair of conductors in proximity of some other conductors or conducting surface (such as the natural ground) can operate in two modes simultaneously, differential mode and common mode.

Differential mode is where energy is transferred due to fields between the two conductors forming the pair, and common mode is where energy is transferred due to fields between the two conductors forming the pair together and another conductor or conducting surface.

The currents flowing in the two conductors at any point can be decomposed into the differential and common mode currents.

Differential current Id is the component that is equal but opposite in direction, it is half the difference in the two complex line currents I1 and I2.

Common mode current Ic is the component of the line currents common to both conductors, it is half the sum of I1 and I2.

  • Id=(I1-I2)/2
  • Ic=(I1+I2)/2
  • I1=Ic+Id
  • I2=Ic-Id

So, for example, if I1=2A and I2=-1A, Id=(2–1)/2=1.5A, Ic=(2-1)/2=0.5A.

A line that is operating with perfect current balance has only differential current, ie common mode current is zero. It is unlikely that a feed line in a practical antenna system is perfectly balanced, but with due care, it can have very low common mode current, 20dB or more less than the differential component.

Last update: 1st July, 2015, 12:05 PM

Using the AIM to measure matched line loss

A correspondent wrote seeking explanation of difficulty he was having measuring line loss using the advice given in the AIM manual using a scan with either O/C or S/C termination:

Note the one-way cable loss is numerically equal to one-half of the return loss. The return loss is the loss that the signal experiences in two passes, down and back along the open cable.

Because my correspondent was using one of the versions of AIM that I know to be unreliable, I have repeated the measurements on a cable at hand using AIM_900B to demonstrate the situation.

The test cable I have used is 10m of RG58C/U which I expect should have matched line loss (MLL) of 0.26dB, but I expect this to be a little worse as it is a budget grade cable that I have measured worse in the past. Continue reading Using the AIM to measure matched line loss

Last update: 4th June, 2019, 10:53 AM

AIM 885A produces internally inconsistent results

 

AIMuhfFurther to AIM 885 produces internally inconsistent results…

A new release, AIM885A appeared recently.

In the common theme of one step forward, two steps backwards, this version produces error popups when started.

Screenshot - 15_04_2015 , 17_41_08

The above popup appears twice when starting AIM885A. Just another symptom to undermine confidence in the system. It doesn’t make sense to me, and the program appears to otherwise start and run. Continue reading AIM 885A produces internally inconsistent results

Last update: 17th May, 2015, 6:57 PM