Why does SWR of a dipole decrease the closer it is to the ground?

Question

I experimented with a 20m dipole made of a 1:1 balun connected to 2x ~5-meter lengths of copper wire, running parallel to the ground (by "ground" I mean "pavement", not quite the electrical ground). I wanted to make sure that placing it higher is better, but it turned out that SWR readings (on a NanoVNA) were lower the closer the wires were to the ground! This surprised me, so I thought I'd ask why that is? Shouldn't lower SWR generally indicate a better antenna placement?

I tried placing it between 30cm and ~2.5m above ground.

Certainly the performance of this antenna was better when it was placed higher (in terms of received FT8 signals), but I hoped I could rely on SWR readings to find the best spot for it. It turns out I was (probably) wrong.

Answer 1

I presume you are running a 50 Ohm system (radio & trasmission line), and while the characteristic impedance, in free-space, of a dipole is ~73 Ohms, at various distances from the ground, the impedance can vary significantly;

So using a 1:1 BalUn, as you are, at a certain height you may be close to 50 Ohms, where at other heights you may be closer to the characteristic impedance of 73 Ohms or upwards of 100 Ohms, or low, around 37 Ohms; which means you could have anything between 1:1 - 2:1 VSWR on a 50 Ohm system. With your Nano VNA, you can check the antenna resonance, irrespective of VSWR, by looking at the R ± jX on the Smith Chart function, making sure your jX part is close to 0, or at least crosses 0 at a frequency you want as your "center" of resonance. If it stays positive through the entire frequency sweep of the band, your antenna is too long, if it stays negative through a frequency sweep, it's too short.

You can make a 1.5:1 BalUn so you can match 73 or 37 Ohms if you genuinly have 1.5:1 VSWR because of your antenna height, and not a resonance issue. I've used this guy's hifi1200 1.5:1 UnUn, and wired it to make it a BalUn. If you do not have a tuner, getting your entire antenna feed system to 50 Ohms really makes a difference in output power of radios with transistor finals. The other thing is that whether or not you have a tuner, if you have an antenna feed-point impedance mismatch, you will have the shield of your coax carrying current and radiating until or unless you put a choke on it; whether that's ferrites or coiling the coax.

Just keep in mind, that as long as a center-fed 1/2λ dipole, or 1/4λ antenna with a 1/4λ counterpoise (like a ground-plane antenna, or a 2 element so-called "dipole" that doesn't have a BalUn), is resonant, impedance can be adjusted with antenna height and the angle between the elements or impedance transformation with a BalUn or UnUn.

Answer 2

Shouldn't lower SWR generally indicate a better antenna placement?

No. Lower SWR generally indicates lower SWR. It doesn't tell you anything about an antenna's radiation pattern or efficiency.

Without seeing specific numbers it's hard to say what's going on, but an antenna's resonant frequency usually increases somewhat as you increase its height, so a good guess would be that your antenna is a little bit short, so it resonates above the frequency you're testing — and as you raise it higher, the resonant frequency moves even further away, so the SWR you measure goes up.

However, a couple graphs would be a million times more informative.

Answer 3

SWR tells you how well the radio is transferring energy into the antenna. While an antenna with a high SWR that transfers energy poorly might not work well, if what does get out goes where you want it to go, it's a better antenna. But remember that a dummy load has a perfect SWR and doesn't radiate at all, so SWR is not the only question.

Typically, a bigger concern is that if the SWR is high and you don't have a tuner, that means your radio is going to overheat because it has to eat some of the energy bounced back from the antenna.

Your goal is to make contacts, meaning the energy gets not only out of the antenna but to someone else who can hear it. There are at least three ways that energy goes wrong:

• High SWR means it doesn't leave the antenna, but bounces back to the radio (and bounces off the radio back to the antenna and back to the radio...and a little escapes at each end on each bounce).
• Low efficiency ironically can improve SWR, but it means that the energy is turned into heat instead of radiating out of the antenna. (Loss resistance vs. radiation resistance.) This is your dummy load.
• Poor radiation pattern -- either the antenna is isotropic, radiating equally poorly in all directions, or the direction it is radiating is different from where you want it to go. For example, up instead of horizontally, or east west when you want north south, etc.

For example, I have a disc cone antenna that has a 1.0 swr from 140mhz - 900+mhz. At 2m, it does pretty well, maybe with a gain of 3. But at 440mhz, the swr is still 1.0, but the gain is maybe 0.7 - 1, because the radiation pattern lifts up as the frequency goes up, sending more into the sky and less to stations on the ground.

If you have a horizontal antenna above the physical ground, if that ground is a poor conductor, then it is a resistor. As the antenna gets closer to the ground, more of the energy is absorbed by the ground and less is radiated, so the SWR goes down but so does the gain. Additionally, if the ground is reflective, then as the antenna gets closer to the ground, more energy goes up into the sky (NVIS?) and less towards the horizon.

Low SWR is good for the radio and good for your feed line (both of which absorb more at high swr), but good for the antenna is resonant -- which may or may not be where SWR is low (as explained in the other answer). And good for transmission depends on radiation pattern in addition to resonance.