Thursday, May 3, 2012
SWR Changes When I Increase My Transmit Power
Have you ever tuned up your rig running low power only to have it change when you increased the power? (It is recommended that you reduce you rigs power to its lowest setting when you tune up your antenna. This is so you will not cause accidental interference to ongoing QSOs or overheat your transmitter.) This becomes more of a problem as you begin to work the HF bands, but it may also happen at VHF/UHF particularly if you run a hundred watts or more.
What is the cause of this problem?
The cause is usually the RF from your antenna is coupling into some surrounding conductor that is not part of the antenna. This can be almost any piece of metal from a nearby wire to a metal building. It may not be noticeable on all bands but the cause is still the same. You may think the wire clothes line below your antenna is not a problem. But if it is within 15% of some part of the wave length you’re running, it may become resonant and start to radiate thus effecting you antenna. Remember this is how a yagi works. Wire or long metal objects such are metal rain gutter running in parallel to the antenna are most often the cause for most hams.
Should you be concerned?
Probably yes. 1) It will impact the way your signal leaves you antenna thus resulting in your antenna “not working as advertised”. 2) It can cause all the problems associated with high SWR. 3) Depending on what it has coupled to, there is a potential for someone getting RF burns, could cause RF on the feed line, and/or RF interference with other electronic equipment.
What is one to do?
The easiest solution is move the antenna. Try to determine what the antenna is coupling to. Metal objects are almost always the offender. Hidden wiring in building walls/ceilings or metal siding/roof are some of the more common problems. However, sometimes trees can cause this especially if they are beside the antenna. Trees at the ends of dipoles aren’t usually a problem nor are metal masts/towers. It is always good practice to use several feet of rope at the end of an antenna to isolate it from whatever you use to support it. (Don’t forget to use an insulator. Wet rope will conduct RF.)
However, things are not as bad as they might seem though. Generally moving one end of the antenna a few feet away from the offending conductor is sufficient to resolve the problem.