The purpose of this blog is to help new hams and promote amateur radio. I will do my best to keep the postings understandable for new ham with little or no technical background. Because of that there will be some things that might not be technically exact. However, I will endeavor to keep the information well within accepted practice and of practical use for entry level hams.
Tuesday, February 7, 2012
WEAK SIGNAL AND LOW SWR
There are a number of things that can cause this problem.Here is a list of common ones.
Placement of the antenna
Very long feed line High feed line loss Signal Ground Issue Local terrain
These apply differently at different frequencies.I’ll note this as I go.
Placement of the antenna - above 50 MHz
Higher is always better.Always place an antenna for VHF/UHF as high as your budget, safety, and local ordinances will allow.Keep it clear, as much as possible, of metal sheeting, metalized shingles, wires and cables, trees, even some types of glass in windows.Running a handy talky from inside a house, vehicle, or camp trailer is not going to work well at all. If you are lucky enough to be close to a repeater, you may get away with it.But simplex operation will be very marginal to non-existent.
Very long feed line - especially above 30 MHz
Feed line loss can seriously reduce the amount of power that arrives at the antenna.Use as low loss feed line as you can afford.Feed line loss is discussed in another post, “Feed Line - Introduction to coax”.
High feed line loss - gets worse as frequencies get higher
A feed line with high loss can mask a high SWR.Any signal on a high loss feed line loose power.It does not matter whether the signal going toward the antenna or being reflected back from it.(Remember SWR is a ratio of the RF leaving a source and that being reflected back toward the source.)When you measure your SWR at the transmitter end of the feed line, the reflected RF is lowered by the feed line loss too.So the SWR may be at an acceptable level but there still may be a mismatch at the antenna that is seriously impacting the signal strength.For this reason it is best to check SWR of a new antenna with a short piece of coax that is known to be good before you connect a long feed line or a feed line with considerable loss.
Local terrain - all frequencies but especially above 30 MHz
Generally radio signals are absorbed by soil.However, radio signals also reflect off soil and rock that is highly mineralized.So as a general guideline the best I can say is give it a try.You just never know.HF antennas placed in deep valleys or close to mountain ranges may suffer from poor performance.
Signal Ground Issue - especially below 30 MHz
I find an analogy first told to me by my Elmer (ham speak for mentor) helpful.“Radio signals want something to push against to get off the antenna.”A poor signal ground will reduce a signal’s strength just as effectively as high loss coax.Verticals at all frequencies are most susceptible to this problem.Long wire antennas also need good signal ground, especially random wire (length) antennas.Most VHF/UHF antennas have signal ground built in as radials that do not require tuning.(This doesn’t mean VHF/UHF antennas don’t need tuning.)Signal ground is the subject of another post.