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

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

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


Because a longer antenna isn't necessarily better. When you hear that
a longer antenna is better it's usually a comparison between lengths
of loaded and unloaded antennas of the same -electrical- length. An
unloaded CB antenna is 9', it's the longest 1/4-wave you can get, and
it's also the most efficent. Shorter antennas (firestiks, wilsons,
etc) are still a 1/4-wave long -electrically- but are less efficient
because they are shorter -physically-.

You tune your antenna to get it into a zone where it's most efficient,
where it's as close to a 1/4-wave as possible. That 'zone' is actually
pretty wide, and a 2:1 or better SWR across the band is a good
indication that you are in that zone; i.e, your antenna is tuned. If
you want a more efficient antenna then get one with the same
-electrical- length and a greater -physical- length.





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ARRL books, etc are 621.384 in Dewey Decimal System.

On 12 Jul 2005 21:53:40 GMT, Steveo wrote:

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! :)


I was hoping I got that right. They cut my Firestik very long before I went to
the Imax.

Vinnie S.

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?
********

First off there is physical length and then there is electrtical
length. The two can be the same or they can be different.

Electrical length is the length that at some frequency the antenna is
self resonant. This is found for a half wave antenna by the following
formula

(299.8 meters / frequency in MHz.)/2 = resonant length of a half wave
antenna.

Other things can affect tuning of the antenna like nearby buildings,
how close the antenna is to the Earth, and many other variables. These
variables will alter the above formula from as little as 2% to as much
as 10%.

There are techniques that can actually shorten the physical length
while maintaining proper electrical length. One method is linear
loading of an antenna. Another is to introduce some reactance to alter
the physcal length while maintaining proper electrtical length. This
reactance is often in the form of a coil. This coil can be located
pretty much anywhere in the antenna. Coils are generally not
reccommended at the far ends of the antenna from the feed point.

Ideally one should have an antenna that has the physical and
electrical length the same. This is not always possible but the most
desirable. Reactances in the antenna alter radiation patterns off the
antenna and in most cases reducing overall efficiency of the antenna.

Antennas can be somewhat simple to very complex.

A good refereence book to pick up would be the ARRL Antenna Handbook.
There are many out there that range from introductory to very
technical.

james