- FPS_Sage
- Oct 25, 2007
-
This was a triumph
-
Gun Saliva
|
|
#
¿
Feb 22, 2014 18:52
|
|
- Adbot
-
ADBOT LOVES YOU
|
|
|
#
¿
May 5, 2024 14:19
|
|
- FPS_Sage
- Oct 25, 2007
-
This was a triumph
-
Gun Saliva
|
PICS INC.
Let's design an antenna. Since people are getting started here, we have decided to build a simple 2-meter (144 MHz band) vertical antenna that would be suitable for putting up on a pole, in your attic, whatever.
We'll do a simple focus on trying to optimize the Standing Wave Ratio (SWR) which indicates how tuned the antenna is to the feedline and radio. 50 ohms is a perfect 1:1 match. We'll shoot for 1.5 or lower across the band.
DANGER WILL ROBINSON, HAM RADIO SOFTWARE ON THE HORIZON. PREPARING FOR BAD USER INTERFACE.......
Welcome to MMANA-GAL, written by an insane japanese ham. It's one of my tools for basic design and optimization.
Let's add a wire. We'll start with a 1/4 wavelength wire and feed it at the bottom. The "w1b" signifies the feedpoint at bottom left.
Aww, it's so cute.
Let's click that "Calculate" tab. Only pay attention to the top line for now, I was playing with other antennas - MMANA fills this newest-at-top. Looks like our antenna, when placed on the ground and fed against it, has about 1.4 SWR. Hey, not bad! We wouldn't hear much on a VHF antenna sitting on the ground, though.
After we click "Plots", MMANA will show us the projected radiation patterns. Pretty much by the book so far.
The SWR tab shows that we have decent, but rising SWR, and our antenna is possibly too long (if it's too long it'll be a better match at lower freqs, opposite for too short)
Let's get this thing off the ground! Add 5 meters of height and OH GOD WHAT THE gently caress JUST HAPPEN
We lifted it off the ground, and MMANA keeps track of all the electrical connections. By raising the antenna, we disconnected it from its ground plane. See how important the ground plane is?
Let's add a ground plane. Remember that a decent ground plane can be made by four 1/4 wave wires sticking out horizontally.
More interesting looking now.
You might have seen that "Optimize" button. This is where the magic happens. Here you can enter parameters to wiggle, criteria to wiggle them by, and metrics to judge the performance of the antenna, which you can individually weight.
What i'm doing here is wiggling the length of the vertical element (wire 1). Then, because the radials should match the element at this point, we 'bind' the radial lengths to the element length (the 'Associated' column). Now the program will wiggle all five elements' lengths in 5mm increments, measuring the SWR at each iteration. If it lengthens and the SWR goes up, it goes back and shortens.
Hey, better match now! You see the bottom line (right after we added the wires) and the top line (after we optimized). That SWR is still a little high, though. What else can we do to tune this?
Bend those radials! As your ground radials get bent down, your impedance rises - and that's just what we need here.
Let's go back and work that Optimize tab some. We'll set it up to wiggle the vertical element length, as before, but this time we're going to change the zenith angle (vertical tilt) of the radials, pivoting them around their start point (the feedpoint) between 90 degrees and 180 - straight out to straight down. START!
Holy crap, that worked perfectly! Look at the SWR below. Almost a perfect 1:1 match.
What's it look like now?
Looks like MMANA bent them down right around 35 degrees to get that match. Easy to reproduce on the real antenna.
And here's the SWR curve if we go back to Plots, a very well-tuned antenna that will give us less than 1.5:1 SWR (this is our general benchmark for usability) across the entirety of the 2 meter band.
best Lets Play i've seen in a while
|
|
#
¿
Feb 27, 2014 18:57
|
|
#
¿
Feb 22, 2014 18:52
