Stubs

Stub…  Your toe, maybe, out pounding the pavement?

Or maybe it has something to do with radio?

At these difficult times, finding something useful to do can be quite challenging. Not the least of reasons being the fact that you can’t get to a radio rally and get a few odds and ends for the project you had in mind.

The tuning stub.

Well here is an idea. The stub. An interesting device, incredibly simple to make, and has a lot of uses. Just exactly what IS a stub? A short length of transmission line, (open wire, flexible twin, co-ax, etc. anything you could use to connect a radio to an antenna). So…  Do you have a few feet of co-ax lying around doing nothing?

I’ll be discussing here the ¼ wave stub. This is going to  be an ELECTRICAL quarter wave length of  co-ax. The electrical length of a transmission line is different from its physical length, but fortunately, this is easy to determine. All you need to do is find out the PHYSICAL length, then multiply that by the velocity factor. The result will be the ELECTRICAL length.

As an example, at 145Mhz, a PHYSICAL ¼ wavelength is 517mm. ( 300/145 x 0.25)

The cable we wish to use has a velocity factor of 0.66.(Sometimes shown as “66”).

The ELECTRICAL length becomes 341.3mm. (517 x 0.66).

So when using THAT cable, a ¼ wavelength of it for 2M is 341.3mm long. (Good luck on getting it that accurate, Hi!).

The following table lists common cables and their characteristics:

https://www.febo.com/reference/cable_data.html

A stub, when cut to a suitable length, can make a very good filter. It is the way the stub is used that allows this. Simply put, a stub is connected in PARALLEL with an existing feeder. That is to say, the inner of the stub is connected to the inner of the feedline, and the outer, to the outer…. In other words, you “teed” it. It will look like this:

 T

 Imagine a piece of cable with a plug either end that you use as a patch lead. That is the “top” part of the “T”. From the middle, coming down, is the stub. It is as simple as that. Now the elegant way to do this is with a “T” connector in the middle, and plugs. But you have not been to a rally….  If you are very careful, you could just cut the patch lead (and it does not need to be in the middle), join all three centre conductors together, insulate, then do your best to rejoin the outers of all three over the top of the centre join.

OK, so far, so good. Now this monstrosity has been constructed, what can be done with it? Well that rather depends on what you would LIKE to do with it. (Stick it up the back end of a covid and run a kW maybe?  J )

If you SHORT CIRCUIT the bottom of the “vertical” part of the “T”, and that stub is ¼ ELECTRICAL wave length at 2m, your patch lead is now only going to pass 2M signals. As it happens, stubs work at three times, five times, seven times the frequency etc. So it would also work on 70cms.

So what happens if you leave the end of the stub OPEN circuited? Well, that patch lead will pass signals from one end to another, but NOT at 2M, nor 70cm etc as above.

Stub uses.

The uses? Many. But here are a couple. Your 2M signal flattens your DAB, or FM tuner. Place an open circuit stub in the patch lead for the tuner antenna. This will go a long way to preventing your 2M signal getting to the tuner in the first place.

A station just starting out with a limited budget may purchase a Baofeng. (For a variety of reasons, one of the best radios I ever had, (I’ve got four!). But a decent receiver is not within its grasp). If you put an external antenna on it, and you live on top of a hill, you will very soon find out that it has some “deficiencies”. The front end of that radio is wide open to pretty much all the VHF and UHF bands, and it has a low dynamic range.

There is not a lot you can do about the dynamic range, (and this will show up when other Amateur stations on different frequencies cause issues). But there is a LOT you can do regards “out of band” signals. Stop them getting in the radio in the first place, by including a stub in the antenna cable to the radio.

Multiple stubs.

You can use more than one stub, and this will “sharpen” the response. Imagine you have a patch lead, with three stubs on it. Assume you used the same cable (0.66 velocity factor) to make the patch lead and the stubs. After calculating the lengths required, (I’ll use the example lengths above), you would have three stubs, each 341mm long, and they would be spaced 341mm apart on the main patch lead.

Looking like        TTT    with the plug either end, left and right.

Tuning.

This is incredibly simple, if you have a radio, a VSWR meter, and a dummy load. Make your stub slightly too long. Connect the radio to a VSWR meter, and the meter to the “modified patch lead”. Connect the far end to the dummy load. SET RADIO TO LOW POWER!

Transmit, checking the VSWR. Now start sicking a pin through the end of the co-ax, (across it, not down the end), and check VSWR again. (Your pin is providing a movable short circuit). Keep doing this, bringing the pin further away from the end each time. If you made your stub only slightly too long, you should not have to come back far from the end.

Now you have tuned it, if you want to pass signals at that frequency, cut at the pin location and short inner and outer together. If you want to prevent signals passing, don’t short the end out.

Once you have done this, you will have the exact length of the stub, and for that frequency, and that co-ax, you will know how long to make further stubs. (In the case of wanting three stubs, make one, tune as above, and then cut the other two exactly the same. And space them exactly the same.

How do stubs work?

 Now…  How does this work? Think first of an antenna. The end of an antenna is just that. Where the wire/metal/whatever finishes. As a result, no more current can flow, there is nothing to conduct it. So this becomes a “high voltage” point. ¼ wave back from the end of the antenna, the waveform carried by that antenna is such that there is very little voltage at that point. And if there is low voltage, yet there is power, there is a very high current, and very low resistance at that point.

 Now enter the stub. ¼ ELECTRICAL wavelength back from the end of the stub (if open circuited), there will be a short circuit. And if that end of the stub is connected in parallel with the “patch lead”, then AT THAT FREQUENCY there is a short circuit across the patch lead, which will cause a very high VSWR, causing that frequency to return from whence it came. Like back up the antenna feeder, and NOT into your DAB radio…

Now consider the short circuit stub. At its far end, it can have no voltage as it is shorted out. An ELECTRICAL ¼ wavelength back, this is translated to a very high impedance, but only at that frequency. As a very high impedance is connected across the main patch lead, it has no effect on the signals passing along that patch lead.

As a result of doing this, you have a band pass filter in your antenna lead. (Or for the DAB radio, a “band stop” filter).

You can, of course, make filters from coils and capacitors. The stub filter is better, because made from co-ax, it uses a lot more copper, and has lower losses, and sharper tuning.

The larger the co-ax the better. I have some 1” co-ax that I am going to play with, but I will need to dispense with the plugs in the centre of the “T”, as the plugs are £50 each! (And I have never seen a suitable “T” adapter).

Let me know how you get on, I’ll be interested in the results you get on a “poor” radio. Maybe we should have a “Baofengs on the air” night!

Apart from stubs and feeder cables, co-ax has other uses. Whilst beyond the scope of this article, how can you connect two, 50R antennas together? Two short lengths of 75R cable are used, which makes each antenna look like 100R, and two of those in parallel gives 50R… And a short length of co-ax can be used as a capacitor. Thick co-ax has a high voltage rating, so you get a high voltage, low loss capacitor…  Dumb old bit of co-ax?  Don’t think so, Hi!

Lastly…  What do you mean, you don’t have any spare 50R cable? Well for the UPRIGHT part of the “T”, any impedance will do. Including 75R. Go nick the patch lead from the TV, the XYL won’t mind….

73 de Stan.

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