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

Transmit performance of 2m hand held transceivers

This article documents measurements of transmit performance of three hand held 2m radio with several antennas.

Measurements of field strength were done with Lou Destefano’s (VK3AQZ) VK3AQZ RF power meter (RFPM1) and a small loop antenna.

RFPM1-VhfLoop

Above, the field strength meter, a RFPM1 with small loop antenna oriented for max gain in the direction of the DUT.  The instrument reads -73.5dBm with no signal, -69.5dBm with the strongest transmitter with the loop removed, and around -30dBm for the various transmitters with the loop in place… so the meter reading is predominantly due to the loop mode pickup.

All three transmitters have different power. The table below reports power into a 50Ω load and does not take account of mismatch with the various antennas.

Screenshot - 06_07_2015 , 12_53_56

Above a comparison of the configurations on a field strength test at 1λ. The relative column factors the different transmitter power and FS to obtain  a comparative figure independent of power. Mismatch is almost certainly a significant part of the explanation of different performance, but it is quite difficult to measure in this sort of application without disrupting the DUT.

BoafengINF641Antenna

It is interesting that there is little difference observed with the Baofeng on two different antennas, when the Boafeng antenna is clearly inefficient, see the thermograph above.

 

Balanced ATUs and common mode current

This Feb 2012 article has been copied by request from my VK1OD.net web site which is no longer online. The article may contain links to articles on that site and which are no longer available.

Many designs have a ‘balanced output’ or an option of a ‘balanced output’, but what does that mean, and are they effective in minimising common mode current in an antenna feed line?

ATUs achieve ‘balanced output’ in one of several ways, the common ones are:

  • a grounded impedance transformation network followed by an internal voltage balun;
  • a grounded impedance transformation network followed by an internal current balun;
  • a current balun followed by a symmetric impedance transformation network that may or may not be directly grounded at its centre;
  • a link coupled ATU where the output circuit is symmetric and may or may not be directly grounded at its centre.

Much has been written about the merits of one approach or another, mostly qualitative and often subjective, but there is little in the way of quantitative analysis of the impedance that the ATU offers to common mode current. Continue reading Balanced ATUs and common mode current

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.

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

Antenna span tensioner using a counterweighted halyard

Correspondents have asked about application of the technique used in Antenna span spring tensioner using Antenna wire catenary calculator to a span tensioned with a counterweighted halyard.

The scenarios bear a lot of similarity, the main difference being that the tension from the counterweight is constant up to the point that the counterweight travel reaches its limit.

The tension applied is the weight force of the counterweight with a little increase to allow for friction in the sheave block.

So, lets say the scenario is a 42m wire plus halyard that adds 1m to the span under no wind conditions and can pay out a further 1m at which it reaches its limit. Lets say the counterweight is 5kg weight so 49.0N tension. Continue reading Antenna span tensioner using a counterweighted halyard

Antenna span spring tensioner

A correspondent asked about application of the Antenna wire catenary calculator to a scenario with a spring tensioner in a simple span.

His proposed tensioner has a maximum length of 2.6m at a tension of 445N, and the intention is to tension the span with no wind loading to 178N at which the tensioner is 1.92m long. Minimum GBS of the tensioner is 1560N, WLL=456N (ie the tensioner is specified that with safety factor 3.5, it reaches its working load limit at full extension).

The following is a simple analysis that assumes the fixed supports are equal height, the tensioner has the same m/l as the wire, and 2mm (#12) 30% Copperweld is used for the wire which is 42m long, so the distance between supports is taken to be 45m. Continue reading Antenna span spring tensioner

Capacitive loading device for fine tuning wire dipoles

There are applications where you might want to make the tuning of a wire dipole adjustable.

Adjusting the length is often not convenient, especially in-service tuning which might be triggered by changing vegetation, ground moisture etc.

This article shows some simple means of attachment of a small capacitive load to deploy near the high charge points, and to adjust their effect by moving them to or fro on the main dipole wire. Continue reading Capacitive loading device for fine tuning wire dipoles

VK3IL’s 3m circumference LDF4-50B loop on 20m

David, VK3IL, describes a small transmitting loop (STL) at Portable magnetic loop antenna.

At VK3IL’s 3m circumference LDF4-50B loop on 40m. I reviewed his loop behaviour on 40m, and its efficiency was quite low… though typical of a loop of that size at that frequency.

Radiation resistance of a STL is proportional to the fourth power of frequency, and since it is often dwarfed by loss resistance, we should expect that doubling frequency will dramatically improve performance.

As far as I can glean from the article, it is made from a 3m length of LDF4-50B Heliax, and uses a Patterson match to tune it.

Clip 153David offered measurement of VSWR around centre frequency for the loop approximately matched (VSWR=1.24) on 20m. He has measured the VSWR=2.86 bandwidth shown between markers 2 and 3 to be 45kHz. Continue reading VK3IL’s 3m circumference LDF4-50B loop on 20m