1/4 wave stub

[paul]

Hello everyone,

I am new to the aimUHF antenna analyzer and making antennas in general, so I have some basic questions.

I need to make a 1/4 wave and a 3/4 wave coax stub resonate on 432 MHz. I want to use these stubs as a balun as described by I0QM.

At first I see a 1/4 wave stub feature in the aimUHF, however when I activate it, it says the highest target frequency is 170 MHz.

My second option was to use the distance to fault feature. I used this procedure.

I created a coax cable with a connector on one end and measured the length at exactly 2 feet. This measurement includes the connector i.e. exactly end to end is 2 feet. I don't know if I should include the connector or not.

Now since I don't know the exact velocity factor, I did trial and error on the velocity factor until the distance to fault was extremely close to 2 feet. This gave me a velocity factor of 0.705.

I thought that I could now start triming, however I notice that I must have a cable length greater than 1 foot. Unfortunately for me, a 1/4 wave stub at 432 is less than one foot.

So the simple question is how can I use the tool to measure a 1/4 wave stub at 432 Mhz?

As a follow on, since the I0QM balun also uses a "T" connector, does the physical size of the "T" eventually affect the stubs lenghts so that it would need to be adjusted yet again? How would it be possible to determine the overall balance of the balun once it is built?

Thanks for any help in this project.

Paul, kg7hf.

[paul]

I think I just figured a way to do this, but would like to know if it is acceptable.

If I use the frequency of 144 MHz, that is exactly 1/3 of 432 MHz. Tuning a stub to 1/4 wavelength at 144 MHz should give me my 3/4 stub at 432 Mhz. To get my 1/4 wave, divide the 144 MHz 1/4 wave stub (aka the 3/4 wave 432 MHz stub) lenght by 3.

I don't see a problem with this procedure.

I would like to stil know about adding connectors and how that affects the lenght and if the aimUHF can give me useful information about that.
Thanks,

Paul, kg7hf

[Bob]

The 1/4 wave tuning function does have a limit of 170 MHz because it was written before the AIMuhf was available. This is a software problem which I have now updated. It has been extended to work into the uhf range with an AIMuhf model.

The Distance to Fault function has a comment about the upper limit being 1 foot. This is misleading because it does not take into account the AIMuhf. Functionally it is ok and it will work. I tested it on a cable 4 inches long. I have updated the comment so that will be correct in the next release.

The regular scanning procedure can be used to get the raw data to calculate the length of a stub. I've uploaded a picture that illustrates this.
www.w5big.com/Phase_Measurement.gif

Scaling the cable will work too. The technique of scaling also works when experimenting with antennas. A low freq antenna can be scaled up to a higher freq to reduce the amount of material it takes and to save space.

Connectors on the cable will have a significant effect. They add some length to the cable and they also add stray capacitance. These effects can be measured accurately with the AIM by making differential measurements (with and without the connectors).

Any change in the dimentions of the system will affect the phase shift. I tried an experiment with the 1 ft cable in the previously mentioned picture. Initially the resonant freq was 166.3 MHz. When I loosened the N connector about two turns, the resonant frequency went down to 165.6 MHz because the cable length increased slightly.

--73/Bob

[paul]

Hi Bob,
Thanks a bunch. I did find it a little odd that length to fault would work, but 1/4 stub woudln't, it seemed like a bug!

I also stumbled upon the same thing with the less than 1ft for length to fault when I just tried it with my 1/4 wave stub.

So now I have my I0QM (http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF) 1:1 balun made. I thought that I could simply take a 50 Ohm carbon composit resistor and put it on the end and perform a frequency sweep. My thougth was this would produce a low swr at the resonant frequency, somewhat like a band pass filter. This doesn't seem to be the case, but this also could be because of the resistor I have used doesn't behave so well at uhf.

I ended up putting a female "N" connector on the end, then using the provided 50 ohm calibration standard. I tried the Measure Zo feature and it indicates a 27 Ohm resistance. But I'm not sure what it should indicate.

[Bob]

Hi Paul,
Glad to hear you are getting good results.
For a termination, two 100 ohm carbon comps in parallel will be better than a 51 ohm by itself. I used to use the 1/4W carbon resistors a lot before I started using surface mount. For higher power, metal film resistors are recommended but 2x100 in parallel is nearly always better, even with little surface mount resistors. The 50 ohm cal resistor that comes with the AIMuhf is made with a pair of 100 ohm resistors.

--73/Bob