antenna length question
 

If a longer antenna is better then why do they sometimes have to be cut
shorter when tuning?

JArthur wrote:
If a longer antenna is better then why do they sometimes have to be cut
shorter when tuning?


I guess you're getting anal about a 1:1 SWR?

Most mobile antennas have an SWR of about 1.5:1 when tuned properly.
Tune for maximum field strength, not lowest SWR. The two don't
always happen at the same place.

My CB antenna runs about 1.7:1. I talk anywhere I want to. I don't
even know what my 2 meter beam is at. It works just fine.

Shorter makes an antenna capacitive.
Longer makes an antenna inductive.

This length is frequency dependent.

May I suggest a trip to your local library to pick up some books on
RF/antenna theory? Try ARRL handbooks and others somewhere around 630.

On 11 Jul 2005 19:28:01 -0700, "JArthur" wrote:

If a longer antenna is better then why do they sometimes have to be cut
shorter when tuning?


Because wavelength changes with frequency. The higher the frequency, the shorter
the antenna needed.

Vinnie S.

Vinnie S. wrote:
On 11 Jul 2005 19:28:01 -0700, "JArthur"
wrote:

If a longer antenna is better then why do they sometimes have to be cut
shorter when tuning?

Because wavelength changes with frequency. The higher the frequency, the
shorter the antenna needed.

Vinnie S.

The lower the freq, the bigger the tree! :)

no Scott I'm not anal about low SWR's. I am just not too knowlegeable
when it comes to antennas and was just curious is all. I like to learn
things

"JArthur" wrote in message
oups.com...
If a longer antenna is better then why do they sometimes have to be cut
shorter when tuning?


Actually, as long as you have an antenna tuner, the antenna need not be
resonant.

Years ago, the military used rhombic antennas for point to point HF long
haul communications. The monsters on Guam were 600 feet on a leg (they were
in a diamond shape) and 200 feet in the air. They were used on a large
range of frequencies (one fixed size antenna) and simply had the reactance
tuned out to present a good load for the transmitter. These things had
gains that would put most Yagis to shame.

That said, most of us use tuned antennas and then feed the things with 50
ohm coax and we don't need antenna tuners. 1.5 to 1 or below is excellent.
2 to 1 is about the highest you want to see without adjusting the antenna to
bring it closer to resonance (if, indeed, that is the problem - a vertical
over salt water will show around 36 ohms impedance when it is exactly
resonant, rather than the nominal 50 ohms). Surrounding metal, imperfect
grounds, and other problems can cause the antenna impedance to be
above/below 50 ohms and exhibit some SWR. As long as your 1.5 or below,
you're fine!


73 from Rochester, NY
Jim

"Jim Hampton" wrote:
"JArthur" wrote in message
oups.com...
If a longer antenna is better then why do they sometimes have to be cut
shorter when tuning?


Actually, as long as you have an antenna tuner, the antenna need not be
resonant.

Years ago, the military used rhombic antennas for point to point HF long
haul communications. The monsters on Guam were 600 feet on a leg (they
were in a diamond shape) and 200 feet in the air. They were used on a
large range of frequencies (one fixed size antenna) and simply had the
reactance tuned out to present a good load for the transmitter. These
things had gains that would put most Yagis to shame.

Longwires rock!

JArthur wrote:
no Scott I'm not anal about low SWR's. I am just not too knowlegeable
when it comes to antennas and was just curious is all. I like to learn
things


Antennas have so many facets, I could not even begin to dive in
and give you a simple start, that's why I suggested a trip to
the library, or a Google session. Start absorbing data and you'll
start to understand it in a while.

Scott in Baltimore wrote:
JArthur wrote:
no Scott I'm not anal about low SWR's. I am just not too knowlegeable
when it comes to antennas and was just curious is all. I like to learn
things


Antennas have so many facets, I could not even begin to dive in
and give you a simple start, that's why I suggested a trip to
the library, or a Google session. Start absorbing data and you'll
start to understand it in a while.

Or do like Jim said and try to keep it under 2 to 1 on 11 meters.

Try this site. A bit technical but absorb it. Be sure to look at the
Antenna Basics
page too. Tuning is a challenge that anyone with a bit of practice can
do.
http://www.signalengineering.com/ult...ax_basics.html