Different paramaters

[paul]

Hello all,

This is more of a general antenna question rather than specific to the aimUHF.

The question is, once you have performed an antenna analysis, what does the various information tell you, and which is "most" important? -- I've attached two plots of an antenna I have been working on. It is a 13 element 432 Mhz yagi I have built.

The SWR is reasonable, but are there any other pieces to the plot I should be looking at in order to make this antenna better? For example, what is the return loos paramater showing me? What does the theta value mean to me? If there is a place I can look up these terms and understand their meanings and importance, I would be happy to read about them. (this is an experimental antenna which may go into a 16 x 13 EME array).

The other plot I have attached is of the feedline from the aimUHF to the antenna. I felt it was important to at least know the characteristics of the transmission line to the antenna before making measurements.

Thanks,
Paul, kg7hf

[paul]

Interesting discovery. Testing on a different antenna, another homemade 14 ele 4.0 wavelength K1FO style, it seems the Rs and theta values both cross the zero mark at the lowest swr, and the return loss is also the largest near the zero cross of Rs and theta.

[Bob]

Hi Paul,
The SWR is probably the most interesting parameter for evaluating an antenna. A low value of SWR does not necessarily mean the antenna will work good. (A dummy load has a 1:1 swr). Also a high value of swr does not necessarily mean the antenna cannot be used. High swr many require a turner to match to the transmitter. If the transmission line has very low loss, then high swr may be acceptable.

The values of Zmag, theta, Rs and Xs can be used to design matching networks. A nice program for designing tuner circuits is available on the AIM utilities menu.

Xs and theta are zero at the same time because theta is the ArcTangent of Xs/Rs. Ideally at the input to the coax, the impedance would be Rs=50, Xs=0.

When adjusting a tuner, the AIM can be connected instead of the transmitter so you don't generate QRM and you don't stress the output of the transmitter with a bad load condition. The AIM Point Data Tune mode gives you a continuous display of SWR while making adjustments.

By calibrating at the far end of the transmission line, you can read the impedance right at the feedpoint of the antenna.

The feedline and the antenna combine to determine the swr. Even when these components are perfect, the swr can be greater than 1. For example, a lossless piece of 50 ohm coax connected to a perfect dipole of 75 ohms, will have an swr of 1.5 to 1.

--73/Bob

[paul]

Thanks Bob,
I actually found a nice tutorial on swr and return loss. As I was discovering, it seems they are mathematically related and are pretty much the same measurement.

I also ended up doing what you suggested in another thread and performed a custom calibration at the end of the coax cable which as you know eliminated the effects of the coax.

I'm just trying to get my feet under me on this. I know what you mean about having a good swr but not a good radiator. That was the crux of my question actually. Is there a set of paramaters which can help determine if you have a good radiator? The reason is that the two yagi's I am testing have very different driven element matching mechanisms and I'm curious if one scheme is better than the other and if the aimUHF can provide me any insight to that.

BTW, an unrelated question. Is it possible to buy another three 50 Ohm calibration plugs? If so, how much would that cost? I want to use them on a 4 port power divider.

Thanks,
Paul

[Bob]

Hi Paul,

The AIM is a very powerful tool for analyzing the details of the antenna driving point impedance because you can cal out the coax itself. Then focus on a matching network that will go at the antenna feedpoint. This will reduce the loss since the coax does have some loss at 432 MHz, it isn't negligible. By matching right at the antenna and operating the system with a low swr, you reduce the transmission line loss.

Since the 70 cm antennas are not too large, you can probably get a pretty good idea of the overall effectiveness by mounting it well away from other objects, like trees, other antennas, buildings, etc. Physically large antennas are tricky because they are likely to interact with other objects.The ground conductivity will be a factor and the height above ground is a consideration too.

The 50 ohm loads are not very hard to make. Get the desired connector and use two 100 ohm surface mount resistors in parallel. I use the 0805 size resistors but 0602 or 1206 can be used too. Two 100 ohm 1% resistors are much better than a single 49.9 ohm resistor. Typically the values are within 0.5%. You can also get 0.1% resistors.

Do not use Minicircuits 50 ohm terminations.
They are off about 2 or 3%, even at low frequencies.

--73/Bob