Matching the 2m Colinear for FM

Hi all,

I have described my efforts in building a colinear for 2m over several posts. The base unmatched colinear was sort-of ok at 146.5, but not very good at 144.1, soI set about performing a match for 144.1 using data obtained using BlueVNA and the mrs MiniVNAPRO vector network analyser. This has resulted in a fairly good match, with an SWR around 1.7 and the FT-817 showing only one SWR bar. I can live with that.

Considering a match for FM

Back at 146.5, the FT-817 shows 3 bars. This is not so good – its not a disaster match, but it is not too special either. My earlier analysis suggested that I could put a stub out a few cm further from the feedpoint, but this did not work. I think the reason was that the data I was getting from the MiniVNAPRO was not that good. Recently, I sought to rectify that, as discussed in this post, which the discussion relates not only to the extender but the base MiniVNAPRO as well.

With that in mind, I set up the 2m colinear and did some tests. Here is a screenshot of Zplots showing the data I obtained:

Zplots showing resistance and reactance of the 2m colinear at the BNC connector

Impedance at the BNC connector

Just for piece of mind, I ran the analysis with the extender, and this is what it shows:

Impedance at BNC connector obtained using extender, which shows very similar results

Impedance at BNC connector obtained using extender

The results are quite close, and any analysis done with one data set will pretty much be matched from the other.

The T piece for the 144.1 match

As well as the BNC connector for the radio (or a cable going there), I also have another convenient point to consider matching – the same place I have the stub for 144.1 I have put a BNC t piece here to connect the stub (and disconnect it when I don’t want it). I know I can’t use another stub for 146.5 right at this point on the transmission line, but I could use some surface mount components for a shunt, or add an extra piece of coax and put a shunt off that. We’ll first look at the data from this point:

Impedance at the T piece

Impedance at the T piece

The interesting thing about this is that this is an effective transmission line transformation. If I use zplots to consider the effect of 36cm of RG58 on the impedance, I get the very similar results, and there is about 34cm of RG58, plus the BNC connectors in the middle. The results are not exactly the same – the empirical results are a little flatter because of loss in the cable.

The impedance looks quite different, but if you consider the plot on the smith chart over on the right, it is simply rotated nearly halfway around the centre of the chart. This is the effect of a 50 ohm coax transmission line.

Now I can’t use a stub right here, but lets see what linsmith suggests I can do:

Match design in linsmitch showing the untransformed impedance then two steps of transformation

Match design in linsmitch

First up is to use 15cm of RG58 to get to a point where a series stub could match. The untransformed impedance is the pink dots – this is taken from the Zplots data. If you look carefully at the smith chart from “Impedance at the T piece”, you’ll notice a subset of that curve is shown here. The 15cm of coax transforms this to the green dots. Putting the series stub with a short 18cm away then transforms it to the red dots.

Series stubs are more difficult to create than parallel stubs. The inner conductor of the coax must be broken, but the outer conductor allowed to continue. The stub inner conductor must be connected (eg via solder) to one side of the line inner conductor and the shield to the other. This is logistically quite difficult.

I also provided about 5cm of extra coax with another BNC connector on the other end. This will allow me to easily put this segment into the colinear when I want to match to 146.5, or to take it out. When I am operating on 144.1, I do not want this segment with this stub.

Here is the data with the match in place:

2m Colinear matched in the FM band segment

2m Colinear matched in the FM band segment

I didn’t quite get the match quite right, the best match is around 147.3, however I can live with this. A FT-817 shows no SWR from 146.2 all through to 148.

Now why did it not quite match? I think it’s because I might have stubbed about 10mm too short. Such is life. Here’s a linsmith look at this assumption:

linsmith analysis of the match result

linsmith analysis of the match result

Note: I’ve added an extra bit of transmission line to account for the measuring point of the VNA – it’s not right at the stub point. This means the red dots are replaced with a second lot of green dots, with the new red dots after the extra coax transformation – note that these calculated red dots fairly match the empirical data shown in the zplots smitch chart in “2m Colinear matched in the FM band segment” above.

This analysis shows that I might actually be able to take off about 5mm from the stub and I’ll further improve the match. I’ll not do that straight way – I’ve got a big trip to the Australian Capital Territory coming up and the match is already fairly good – good enough that further improvement might have only marginal effect.

Anyway it’s pleasing to have a good match now at 146.5, and I’ll be using this a lot in the next week.

73, best regards, Wayne VK3WAM


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