RadioBanter - View Single Post

January 11th 05, 03:22 AM

On 10 Jan 2005 06:32:51 -0800, wrote:

Howard wrote:
On 8 Jan 2005 18:19:19 -0800, wrote:

How would you go about tuning a ground plane antenna? Is a ground
plane
a good antenna for 2 meters/440?

On another note, I have read about discone's being very broad
banded.
Are these the best antenna for scanner listening?

A 1/4 wave ground plane is tuned by varying the length of the
radiating element. Calculate the length, add a bit for good measure,
build it then tune it by pruning the length (in small increments).
The tuning can be done using an SWR analyzer or as most of us do,
with
a radio and an SWR meter.

Discone's do offer broadband coverage, though I don't have direct
experience to say they actually work well, great or poorly. I do
know
that for my scanner I use a 1/4 wave ground plane that was built for
220 MHz and it works fine - for a reference point though I am in an
urban area and most of what I listen to is within a 15 mile radius.
In order to decide what's best you should first establish your
purpose; are you in a fringe area, do you need directional coverage,
do you scan a wide range of frequencies or are your listening
interests all in the same "neck of the RF spectrum"?

What if you are trying to tune a ground plane for the aircraft band? I
mean, you cant transmit on this. I just want to listen.

Figure out the middle frequency of the range you are interested in,
cut your radiator & radials for that length, build it, install it,
listen with it. I wouldn't bother trying to tune it. Figuring that
traffic on VHF air is typically @ 120 - 136 MHz the end result is to
cut elements about 22 inches long. Being as you indicate you are also
interested in the 2 meter ham band you're probably about as well off
to just make it for that band and you can use it for both.

I really suggest you spend a few dollars on the ARRL Antenna book, you
will find all your answers and more in that text as well as gaining a
better overall understanding of antennas that will serve you well.