On Sat, 14 Feb 2004 11:08:39 -0000, "Richard"
wrote:

Hi. Is there a standard way to measure antenna bandwith? Is it within 3dB
down of maximum gain, or is it between SWR limits?

It could be either. Between 3dB points is the standard for
Electronics systems (filters, amplifiers, and such). Between 2:1 SWR
is standard for antennas, but could be pushed out to the 3dB margins.

I'm looking at an FM receiving yagi that is advertised as covering the
frequency range 87.5 - 108 Mhz. And I'm wondering what this means. Is the
received signal going to be 3dB down at the band edges or what?

If it is strictly stated as being used for receiving, the manufacturer
is simply offering a "vanilla" style design (i.e. cut to halfwave
length against the usual formula to a center frequency). It could
also be a specification for the more expensive Log Periodic which
offers a more honest wide band coverage with a consistent gain. Then
it could also be for a hybrid design that employs some elements of Log
Periodic elements. Other options include thick or fat elements that
naturally broadband the design (but are not particularly gain-ful).
An antenna with a folded element (useful for matching) provides some
of this "thickness."

I notice that there are commecial yagis (that are probably suitable for
transmission) that have 150-174 Mhz in their description. Yet they are
stated as having a bandwidth of say 1.3 Mhz. What's the deal here?

There, the bandwidth is for optimal results (like matching, gain, F/B
and so on). Outside of this bandwidth, the mismatch is tolerable
(professional systems employ system components that ignore mismatch
issues) and the customer accepts the degraded gain and other
characteristics that went south. For commercial applications, if the
circuit is effective, the antenna is simply a connector, even if a
sloppy one. Note, the same discussion of Log Periodicity or thickness
of elements applies here too.

73's
Richard Clark, KB7QHC