Bandwith of antennas
 

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

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?

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?

no, there is no 'standard'. the bandwidth depends on what is important to
you. you could be interested in forward gain, f/b ratio, swr, or maybe
something else. manufacturers pick what they think their best qualities are
for advertising of course. then of course there is the gain rating
controversy, is it dbi or dbd or db-something-else, and it it measured
properly or just calculated... again, there is no standard and lots of
methods exist.

on the 150-174mhz antennas. in that band you typically buy an antenna for
your operating frequency as that area of the band is channelized and
licensed here in the states. a typical license may include a couple
channels, but they are usually close together so a single antenna will cover
them. in this are the local police and fire are all on channels between
153.75 and 155.3 so if they buy an antenna made for 154.5 or so it should be
ok over all their channels. these are not normally things stocked in retail
stores, you order them for the frequencies you want to cover and they are
tuned at the factory or distributer for you.

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

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?

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?

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

Use your common sense!
You are answering your own questions as you post.

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

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?

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?

Ian wrote:
Use your common sense!
You are answering your own questions as you post.


The question was is there a standard measure for bandwidth.

I gave a couple of possible measures.

This is not answering my own question.

So, the FM antenna that is advertized as covering 87.5-108 mhz, what
"Richard" wrote in message
...
Hi. Is there a standard way to measure antenna bandwith? Is it within
3dB down of maximum gain, or is it between SWR limits?

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?

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?

3dB is about 1/2 of an S-unit, not a big deal on receiving FM.


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

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?

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?

"W4JLE" w4jle(remove to wrote in message
...
3dB is about 1/2 of an S-unit, not a big deal on receiving FM.


Yes, but maybe the antenna is not being measured at 3 dB points.

I ought to ask the manufacturer. Maybe they won't say. :c)


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

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?

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?

Couldn't comment on the ccommercial antennas, but most ham radio antennas
are measured between 2:1 VSWR points. This, of course says nothing about
their potential performance as receiving antennas...

Mike KI6PR
El Rancho R.F., CA

"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?

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?

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?

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?

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?

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?


They are usually showing you that these antennas are available
anywhere in that frequency range but you have to specify what 1.3 mhz
part you actually want. The antenna will be cut to operate only in
that 1.3 mhz range. It is usually swr band width that it is specked
at. Gain band width may be different.

73
Gary K4FMX

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

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?

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?


What it means is that they have an antenna that will provide all the
signals
on a 1.3 Mhz frequency spread at the same time. It is incumbent on the
user to provide equipment that discriminates against unwanted signals
that the antenna supplies thru out the 1.3 Mhz frequency span even tho
it is resonant on only one frequency.
This makes it a very low 'Q' antenna where a very high 'Q' is desired
for a particular mode of communication is used such as audio.
Idealy one wants an antenna that is resonant on any frequency of
choice and where the band pass is tailored around audio carrier if
audio is the mode of communication. The latter is the antenna I use.



Cheers
Art