"Robert11" wrote in message . ..
Hello,

Trying to learn a bit more about antennas. New at this.
A very confusing subject.

Here's a very basic question.

For the 2m band, and the VHF bands in general, it is possible to find
vertical whips that are omni
and offer, apparently, 3 db gain (or more). Do receiving only.

It is also possible to find other omni antenna types over this range, and
wider, that offer the same Gain numbers, or lower.

Also, there are Yagis (directional) for these ranges, again with the same
types of gain numbers.

So, seeing the convenience of a vertical whip of a few feet or so, what
advantage would most any other configuration or type that also offers, e.g.,
3 db of gain, offer ? Why not use a vertical whip for most (all) VHF
receiving ?


First of all we need to define some terms here. 1. Some antenna
manufactures will list their antenna using the term DBi,, the i stands for
isotropic. The isotropic antenna is a theoretical antenna that doesnt really
exist. It is an antenna that supposedly radiates in all directions equally,
much as a free standing light bulb radiates light in a spherical pattern.
No antenna like that really exists. Manufacturers like to use that term
because they can use a higher number for gain than the old standard
which is DBd, which stands for dipole. A dipole is a real antenna, but it
does have a radiation pattern kind of doughnut shaped. ( A vertical dipole)
Now we get into gain numbers. Real vertical type antennas are either
a 1/4 wave which realistically has a gain of 1. Sometimes they call this
unity gain. A 1/2 wave vertical will have a DBd gain of 2.7, most everybody
rounds this up to 3. A 5/8 wave vertical will have a DBd gain of somewhere
between 3.5-4. The gain comes from flattening of the radiation pattern from
a round doughnut to a more flat or a squashed doughnut shape. A Yagi
is a beam antenna that focuses the radiated field moreso than an omni
directional. The no. of elements of the Yagi determines the gain, i.e.
the more directional. I had a 2 meter 12 element beam that had a gain
of somewhere around 9-10. For every 3 db you are doubling the
signal. And yes antennas tend to work on receive as they do on transmit.
BTW that 12 element Yagi I had was a BIG antenna, most people dont
want a monster like that on their house, unless you really have a need
for it. Unless you live in a remote area you dont need a beam.
A vertical will work fine in most cases. A discone antenna is a very
good compromise for scanner use because it is so widebanded,
and you can use the discone to transmit on 2 meters and 440 if
you want, I use to do it all the time.