RESONANT ANTENNAS
 

Howdy,

What's all this stuff about resonant antennas? Some great antenna designs do
not use a resonant length.

73 de Jack, K9CUN

What's all this stuff about resonant antennas? Some great antenna designs
do
not use a resonant length.


All antenna 'systems' are resonant because they all present a resistive load
to the transmitter. QED.

Very broadband antenna systems, such as log-periodics, are still resonant.
They are a collection of different-frequency resonant elements.

Another way of looking at it, a collection is broadband because as a whole
it has a very low resonant Q.

Resonant circuits have an effective Q or a collection of Q values even when
the impedance-frequency response is flat-topped. (As inside a double-tuned
455 KHz IF transformer can.)

At sufficiently high and sufficiently low frequencies the reactive component
of the input impedance always predominates.

The definition of resonance as being adjusted to present a resistive load to
the transmitter should not infringed.

Of course, it is quite possible to operate a transmitter with a non-resonant
load, ie., the load impedance having a reactive component in addition to the
essential resistive load. But if only for economic reasons this condition is
nearly always avoided.
----
Reg.

=======================================

"Dave Shrader" There's a world of difference between a resonant antenna
and a resonant
antenna system!!

BTW, is a Log Periodic Antenna, example Tennadyne T8, resonant across
the frequency range of 13.5 MHz to 30 MHz?? No tuning required on any
frequency between 13.5 and 30 MHz, and VSWR 1.7:1 across the range!

Deacon Dave, W1MCE
+ + +
Reg Edwards wrote:

What's all this stuff about resonant antennas? Some great antenna
designs

do

not use a resonant length.



All antenna 'systems' are resonant because they all present a resistive
load
to the transmitter. QED.

Reg Edwards wrote:
All antenna 'systems' are resonant because they all present a resistive load
to the transmitter. QED.

Actually, when the transmitter circuitry folds back, it means that the
antenna system is not resonant. When I change bands with my screwdriver
and start tuning, my antenna system is certainly not resonant.
--
73, Cecil http://www.qsl.net/w5dxp
"One thing I have learned in a long life: that all our science, measured against
reality, is primitive and childlike ..." Albert Einstein



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Richard Harrison wrote:
. . .
If the antenna is operated off-resonance, it still works but with less
vigor due to diminished current opposed by inherent reactance.
. . .

This is true only if no effort has been made to match the antenna to the
transmitter. If it's matched, it will have the same current and "vigor"
as a resonant antenna. Assuming negligible loss, if 100 watts is applied
to resonant and non-resonant antennas by any means, 100 watts will be
radiated from each.

Roy Lewallen, W7EL

What's all this stuff about resonant antennas? Some great antenna designs do
not use a resonant length.

73 de Jack, K9CUN

Hi Jack,
When you posed this same query back in June of 1999, (myth od the resonant
antenna) you got 193 responses. You trying to beat your own record?
That was about the time I happened upon this Newsgroup, I really enjoyed that
particular thread. I have gone back and re-read it a couple of times.
73 Gary N4AST

Roy, W7EL wrote:
"This is true only if no effort has been made to match the antenna to
the transmitter."

It is true with every transmitter which occupies more than zero
bandwidth. Reactance is zero at one point in the frequency spectrum.
Off-resonance, an antenna system accepts less current than it does
exactly on resonance. But, the difference is usually less than 1 db.

I wrote that if the antenna is operated off-resonance (excited by a
frequency other than its resonant frequency) it works but with less
vigor etc.

If the antenna`s power factor has been externally corrected for some
frequency other than its natural resonant frequency, then it is
resonanat at a new frequency. Its vigor will not be subdued by inherent
reactance at the new resonant frequency.

When I said an antenna operated off-resonance works with less vigor, I
tried for a statement true with a solid rod without connections and in
free-space, a receiving antenna, and a transmitting antenna, all
operated at a frequency other than their resonant frequencies. All are
transmitting antennas because they all radiate when excited, no matter
how the excitation is delivered.

I think I succeeded in saying it correctly but failed in saying it well
if it was misunderstood.

Best regards, Richard Harrison, KB5WZI

Actually, when the transmitter circuitry folds back, it means that the
antenna system is not resonant.
==================================

Wrong ! The antenna 'System' IS resonant, by definition, if it has a purely
resistive input impedance. If that interfering nuisance of your fold-back
circuit springs into action then it means the pure input resistance is
something other than 50 ohms. But it is still resonant.

Actually, in YOUR case, the antenna is NEVER resonant. You make sure the
antenna is NOT resonant by making the whole system resonant by varying the
length of your transmission line.
---
Reg,

Reg Edwards wrote:
Actually, when the transmitter circuitry folds back, it means that the
antenna system is not resonant.

==================================

Wrong ! The antenna 'System' IS resonant, by definition, if it has a purely
resistive input impedance. If that interfering nuisance of your fold-back
circuit springs into action then it means the pure input resistance is
something other than 50 ohms. But it is still resonant.

Reg, the chances of a foldback being caused by a resistive antenna is
about 1 in 360. Actually less than that because the transmitter will
not fold back between 25 ohms and 100 ohms.
--
73, Cecil http://www.qsl.net/w5dxp
"One thing I have learned in a long life: that all our science, measured against
reality, is primitive and childlike ..." Albert Einstein



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If that interfering nuisance of your fold-back
circuit springs into action then it means the pure input resistance is
something other than 50 ohms.

Hi Reg,
That foldback circuit is an interfering nuisance agreed, but it comes in
handy at times. My homebrew mobile antenna is difficult to keep tuned, much
less in one piece doing 100 km/hr down the US Interstate system. My HF
transceiver folds back if things are not right with the antenna system.
The fold back circuits tell me I need to do something with the mobile
antenna, before I smoke the final semiconductors in my rig. It doesn't tell me
what I need to do, I use other stuff for that. I used a couple of your
programs in the design of this beast, Thanks!
73 Gary N4AST