Category: Antennas

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

Last update: 17th June, 2015, 11:55 AM

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

Last update: 16th June, 2015, 11:52 AM

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

Last update: 12th June, 2015, 9:20 AM

Enhancement of Calculate small transmitting loop gain from bandwidth measurement

At Efficiency and gain of Small Transmitting Loops (STL) I explained an approach to assessing the gain the efficiency of STL, and provided a link to a calculator to perform the calcs.

This expands on application of the concepts and introduces an enhanced calculator to perform the calculations.

Clip 222

Firstly, this technique applies to antennas where the VSWR characteristic is consistent with a feed point or virtual feed point where around the frequency of minimum VSWR, X varies with frequency much more than R. The simplified analysis assumes that R is constant, and change in X is the reason for the VSWR characteristic. See VSWR curve of a simple series resonant antenna for more information. Continue reading Enhancement of Calculate small transmitting loop gain from bandwidth measurement

Last update: 13th June, 2015, 8:17 PM

VK3IL’s 3m circumference LDF4-50B loop on 40m

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

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 150David offered measurement of VSWR around centre frequency for the loop matched on 40m. He has measured the VSWR=2.6 (the half power) bandwidth shown between markers 2 and 3 to be 22kHz. Continue reading VK3IL’s 3m circumference LDF4-50B loop on 40m

Last update: 12th June, 2015, 9:20 AM

VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

Peter, VK3YE, describes a small transmitting loop (STL) in his video at https://www.youtube.com/watch?v=Cv_RnLpZ9gw.

? 100 watt 7 MHz magnetic loop for units and apartments - YouTube - Mozilla Firefox firefox 04/08/2015 , 07:24:25

As far as I can glean from the video, it is made from a 3m length of copper tube 19mm diameter, and uses about 1.8m of RG213 to tune it, and appears to have its centre 0.7m above ‘ground’ .

Let us firstly look at a free space model of the antenna using Reg Edwards’ RJELoop1 tool.

Screenshot - 07_06_2015 , 09_53_12

This model has its limitations, but the calculated inductance is of interest. We can calculate the inductive reactance to be 118Ω. The capacitive stub of RG213 will need around 107Ω reactance, and solving for RG213, we find that 1.94m gives 0.19-j107Ω. The resistive component is important as it is ignored by the above model. The stub resistance is a loss resistance, and we need to recalculate the efficiency. Efficiency=Rrad/Rloss=0.005/(0.19+0.0351+0.005)=2.17% (-16.6dB). We can also calculate the Q as 107/(0.19+0.0351+0.005)=465 and half power bandwidth as 7100/497=15.3kHz. Continue reading VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

Last update: 4th August, 2015, 7:34 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

Feeding at a current maximum, and three other options

Feeding at a current maximum visited the common practice of designing to feed a multi band dipole with open wire feed at or very near to a current maximum.

I explained that feeding at the current maximum may provide sub-optimal performance on the popular T-match ATU as its losses tend to be worst with low R loads, aggravated by the use of 4:1 baluns for even lower R.

On the other hand, feeding at a voltage maximum might exceed the ATU’s voltage capacity, or perhaps be outside of the matching range of the ATU.

Well if neither of these is optimal in all cases, what about half way between. It has been done, the odd eighths wave feed line on an 80m half wave is another of the recipes you will hear.

Lets explore the options of a half wave dipole at 3.6MHz with four different feed line lengths (Wireman 551). Continue reading Feeding at a current maximum, and three other options

Last update: 9th March, 2015, 4:47 AM