Key Points
- An SWR meter at the radio shows how well the radio is matched to what is plugged into it, not whether the antenna is working.
- Coax loss makes the SWR at the radio look better than it is at the antenna: a 3:1 antenna through coax that loses 3 dB reads about 1.7:1.
- A suspiciously flat 1:1 across a whole band is a clue that something lossy is in the system.
- An SWR that changes when you touch or move the coax usually means current on the outside of the shield; a choke at the feedpoint usually fixes it.
- Measure at the antenna end with an analyzer when you can. Most modern radios are happy below about 2:1.
Almost every new ham learns to watch the SWR meter. Get the needle near 1:1 and all is well, or so the thinking goes. The meter isn’t exactly wrong, but it only answers one narrow question: how well is the radio matched to whatever is plugged into it? That isn’t the same as asking whether your antenna is working.
The Meter Is at the Wrong End
Your meter sits at the radio. The antenna is at the far end of the coax, and coax loses some of the signal on the way. It loses the reflected power too, and the reflected power has to make the trip twice: out to the antenna and back. So the meter at the radio always sees less reflected power than there really is at the antenna, and the SWR looks better than it is.
An example: an antenna with a 3:1 SWR at its feedpoint, fed through coax that loses 3 dB at that frequency (a long run of thin coax on the higher bands can do it), will read about 1.7:1 at the radio. Most people would call that fine. In fact half your power is being lost as heat in the coax before it reaches an antenna that isn’t matched well anyway.
A Perfect SWR Can Mean a Perfect Heater
A dummy load shows a beautiful 1:1, and it radiates almost nothing. Anything lossy in the system pushes your SWR reading toward 1:1: very long or very thin coax, water in the coax, a corroded connector, a lossy balun. If your SWR is suspiciously flat across a whole band, that’s a clue, not a victory. A real resonant antenna shows a dip near its best frequency and rises on either side.
Cheap Meters Have Limits
Many inexpensive meters are only accurate over part of their range. At low power there may not be enough signal for the meter to read reliably. A meter designed for HF can read badly on 2 meters or 70 centimeters. And the meter has to match your coax: a 50-ohm meter on 50-ohm line.
If the Reading Changes When You Touch the Coax
If the SWR changes when you move the coax, touch it, or add a few feet of line, something is wrong. In a well-behaved system, adding coax only adds a little loss. A reading that jumps around usually means current is flowing on the outside of the coax shield, so the coax itself has become part of the antenna. A common-mode choke at the feedpoint (a few turns of coax through ferrite, or a proper choke balun) usually settles it down.
A Tuner Fixes the Radio’s View, Not the Antenna
An antenna tuner at the radio makes the radio happy. It doesn’t change what’s happening on the coax between the tuner and the antenna. If the mismatch up there is large and the coax is long, the loss is still there. The tuner simply hides it from the meter.
How to Find Out the Truth
Measure at the antenna end when you can. An antenna analyzer, or an inexpensive NanoVNA, connected right at the feedpoint tells you what the antenna itself is doing. If you can’t get up there, measure through a short, known piece of good coax and keep the losses in mind. Check your meter against a dummy load now and then. And look at the SWR across the whole band, not just at one frequency.
You don’t need 1:1. Most modern radios are happy below about 2:1, and many start reducing power as SWR climbs above that. What matters is that the power leaves the antenna instead of warming the coax.