Answer Page · Tuning · Feedline

Why Your SWR Changes When You Turn Up the Power

The antenna is a linear load

A Greyline radiator is aluminum tube, fiberglass insulators and stainless hardware. There are no traps, no loading coils and no ferrite in it, so there is nothing in it to heat, saturate or arc at amateur power levels. Its feedpoint impedance changes from band to band, which is how one feedpoint covers 160 through 6 meters, but on any one band it does not change with power. A component whose behavior shifts as power rises is called nonlinear. The antenna is not one. The usual suspects are below, most likely first.

What does change with power

1. RF on the outside of the coax. Common-mode current, the RF that rides back down the outside of the coax shield, is what the ferrite choke is there to stop. A dipole should end at its tips; without enough choking, the coax shield joins the antenna. At 5 watts there is too little of it to matter. At 50 to 100 watts it can reach the tuner's SWR sensor and its electronics, and the readings go haywire.

2. Stacked baluns and chokes. Some stations add high-impedance 1:1 baluns or chokes at both ends of the coax, on top of the ferrites. A ferrite core sitting in front of a high-reactance load can heat or saturate as current and flux rise. A core that is fine at low power changes its behavior at high power, the impedance the tuner sees shifts, and the match walks away.

3. Tuner headroom. A compact automatic tuner is built for its rated power into moderate mismatches. Into a high-reactance load at full power, its small relays and capacitors run out of room: they match at 5 watts and arc or drift at 100. A full-size wide-range tuner, in the shack or at the base of the antenna, has the headroom. Going up in power? Step up to a tuner rated for it. Shack tuner or remote tuner? →

4. A connection under stress. A loose or corroded PL-259, adapter or feedpoint terminal can arc or heat once real current flows.

The ten-minute test

Reading R and X: a worked example

Many tuners and analyzers show two numbers: resistance (R) and reactance (X). Here is a real reading from an owner's station on 15 meters: R = 41 ohms, X = +144 ohms (inductive). Put those through the reflection formula and the SWR is about 12 to 1, which is what his meter showed, so the meter was reading true.

That looks alarming, and it is an easy match. The resistance is already close to 50 ohms. All a tuner has to do is cancel the inductive reactance, which on 15 meters takes about 52 pF in series, and the SWR drops to roughly 1.2 to 1. That is why his tuner, in manual mode, matched it at 5 watts.

A large reactance is not a fault. It is how a multiband antenna with no traps works: the impedance changes from band to band, and the tuner cancels the reactance and transforms the resistance. As Walt Maxwell, W2DU, explains in Reflections, on a low-loss line with a tuner, SWR is a matching number, not an efficiency number.

How much choking is enough?

The five snap-on ferrites in every Greyline feedline kit give about 1,250 ohms of common-mode choking. That is the baseline, and it suits most 100-watt stations. Some installs want more, and adding ferrites, up to about 11, is a normal adjustment; stations running an amplifier want a dedicated choke rated for the power. The upgrade is easy and it is your choice. Test first, then add choking only if the test points there.

Go deeper

Jim Brown, K9YC, A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing, and his Choke Cookbook for transmitting chokes (free PDF): k9yc.com →

Roy Lewallen, W7EL, Baluns: What They Do and How They Do It (free PDF): eznec.com →

The ARRL Antenna Book chapters on transmission lines and matching, and Walt Maxwell's Reflections. The full shelf, and why each one matters: The Bookshelf →

The whole feed side, end to end: Your RF System, End To End →

Ham Radio is fun again! Pass it on… 73, Jon KL2A · Greyline Performance · Sun Valley, Idaho