"JN" wrote in message
...
I thought Richard's statement that modeling would tell you about how the
design might perform, was good advice. If you use EZNEC, I'd be willing
to help you if you dont already know how to use it to model your antenna.

Jerry KD6JDJ


Jerry, thanks for the offert.
I dont know how good these modelling programs are in this situation.
But if it is easy, so could somebody do the following:

Take 20m of wire, that is half of a dipole.
Put a 1/4 electric wavelength shorted stub for 10,1MHz so that the open
end is 7,25m from center and the shorted end pointing to the tip of
antenna. One side of stub is the 20m wire itself, like in the upper
picture. Ignore all velocity factors to make it sinple.
Now tell me what are the resonance frequencies of that kind of element and
if possible feed point impedande (80m and 30m)

I know I could do it myself but as you know learning to use a new program
reliably takes lot of time.

73 Jouko OH5RM

Hi Jouko

I am not sure I am reading your text properly. But, I did model a 40
meter wire with a trap on each side. You are welcome to E-mail me if you
want to see what EZNEC says about *my* model. Maybe you can help me model
your concept.
I have a friend in vasa Finland who can get you any information about
contacting me. Google Patrik Tast.

Jerry KD6JDJ

Jouko, OH5RM wrote:

Take 20m of wire, that is half of a dipole.
Put a 1/4 electric wavelength shorted stub for 10,1MHz so that the open end
is 7,25m from center and the shorted end pointing to the tip of antenna. One
side of stub is the 20m wire itself, like in the upper picture.

I did just that with EZNEC, but with a free-space dipole. Without the
stubs, the antenna resonated at 3.6 MHz. With the stubs, that dropped
to 2.75 MHz and additional low-impedance points were noted at 6.5 and
11 MHz. The patterns at 2.75 and 6.5 had the desired dipole shape,
but the 11 MHz pattern had multiple lobes.

Jim Bromley, K7JEB
Glendale, AZ, USA

Repeated message seeing that you both missed it:

73's
Richard Clark, KB7QHC

Hi Richard,

No everything red carefully.
I fully understand what you are suggesting.
Cutting the element and insertin stub at right angles
Sure works OK but that is no answer to my original question, HI

73 Jouko OH5RM

wrote in message
...
HI
Theory says that whatever condition exists at far end of 1/4 wave
stub will
appear at fed end. ie. if far end is left open a high impedance will
appear at fed end, at resonant freq. So it would appear that the
shorted end should be nearest
to feed point of antenna. The centre of dipole or bottom of a
vertical.
Hope that makes sense!................................Rod EI3CZ

You have it just backwards. A 1/4 wave stub is an impedance converter. i.e.
a short at one end appears as an open at the other end.
Dale W4OP

On Wed, 16 Apr 2008 09:36:27 -0500, Cecil Moore
wrote:
Is 24 dBi omnidirectional gain from a vertical
antenna enough to "get hyperbolic about
trivialities"?

Hysterical AND hyperbolic apparently.

On Wed, 16 Apr 2008 19:50:18 +0300, "JN" wrote:

Repeated message seeing that you both missed it:

73's
Richard Clark, KB7QHC

Hi Richard,

No everything red carefully.
I fully understand what you are suggesting.
Cutting the element and insertin stub at right angles
Sure works OK but that is no answer to my original question, HI

Your original question:
My question:
Is there any difference if the shorted end is pointing to the center or to
the tip of dipole.
My specific response:
Keep the line conductors at least 3, preferably more, wire diameters
from the radiator or themselves.

Certainly more was said, but you have shied from terse responses in
the past.

You have a tuned line, that is illustrated several times by you. You
have a radiator, that is illustrated several times by you. You have
asked about the line in proximity to the radiator several times, that
is illustrated several times by you (you really need to use fixed font
to do this, it is the protocol of allowing everyone to see what you
mean). I have responded about the proximity of the line to the
radiator several times with a very simple rule.

The line going radially away from the radiator (at right angle)
absolutely satisfies that rule. Keeping the line away from the
radiator (at a 0 degree angle, either up or down towards a tip) can
also satisfy that rule - but only if you observe the rule. If you do
not observe the rule you will have problems. You may have problems
even if you do - but your question lacks your requirements.

So, ostensibly, yes there is a difference - this is just the nature of
life. The real question is would it matter to you? Some folks worry
over half a dB, others can live with -10dB. This has all been
discussed to some length by others, in related topics about hairpin
matches. A general reading of the postings helps put specific
problems into context and relationship. This means browse the topics
that are active - not just your own.

Now, do you have an original question that has gone without an answer?

73's
Richard Clark, KB7QHC

Here is what I mean:

http://farm3.static.flickr.com/2208/...7fd3ba40_o.jpg

Measures taken from hat, but if one could say the resonance points and
impedances I would be grateful
Originally I asked the difference if you turn the stub 180 degrees.

73 Jouko OH5RM

Jim, K7JEB wrote:
Jouko, OH5RM wrote:

Take 20m of wire, that is half of a dipole.
Put a 1/4 electric wavelength shorted stub for 10,1MHz so that the open end
is 7,25m from center and the shorted end pointing to the tip of antenna. One
side of stub is the 20m wire itself, like in the upper picture.

I did just that with EZNEC, but with a free-space dipole. Without the
stubs, the antenna resonated at 3.6 MHz. With the stubs, that dropped
to 2.75 MHz and additional low-impedance points were noted at 6.5 and
11 MHz. The patterns at 2.75 and 6.5 had the desired dipole shape,
but the 11 MHz pattern had multiple lobes.

Great minds think alike. :-) I was doing the same thing
at different frequencies. I noticed that a 1/2WL dipole
with 1/4WL stubs on each end tends to resonate on frequencies
in a ratio of about 2.57:1. 18.14/2.57=7.06 so such an
antenna should be resonant on both 17m and 40m. EZNEC
agrees. Here are the approximate dimensions for the dualband
40m/17m antenna. Cutting and trying will be necessary for
fine tuning to resonance. Note that the antenna is about 13
feet shorter than a 1/2WL dipole on 40m. It has a dipole
pattern on both 40m and 17m and a 50 ohm SWR less than 2:1
on both bands. It appears to be linear-loaded on 40m and
stub-matched on 17m.

26.8' 26.8'
+-------------------------FP-------------------------+
| |
+------------ ------------+
13.3' 13.3'

Since 10.125/2.57=3.9 MHz, this antenna should be scalable
to become a dualbander on 75m and 30m.
--
73, Cecil http://www.w5dxp.com

On Wed, 16 Apr 2008 21:53:39 +0300, "JN" wrote:

Here is what I mean:

http://farm3.static.flickr.com/2208/...7fd3ba40_o.jpg

Measures taken from hat, but if one could say the resonance points and
impedances I would be grateful
Originally I asked the difference if you turn the stub 180 degrees.

Hi Jouko,

If we are looking at four wire segments to the left of the center of
the dipole, then this is a lattin design element. All my comments
about that stand.

As for speculating where it resonates, there is not enough
dimensioning to be exact (while there is little enough to make a poor
guess).

I would only ask, do you want it to act as a trap such that the
radiating portion of the antenna is confined to the right (as
illustrated in your link) of this construction (like a conventional
trap for a dual band operation)? Or do you want the element to act as
linear loading to make the entire structure resonate at a shorter
physical length?

If you turn the element, yes, it will make a difference. There is
still a long road to travel to make it "work" and it probably won't
look like your link's illustration and it won't resonate where you
expect it to.

You still have to observe the separation of the two lines by at least
3 times the largest diameter wire/rod.

73's
Richard Clark, KB7QHC

"JN" wrote in message
...
I thought Richard's statement that modeling would tell you about how the
design might perform, was good advice. If you use EZNEC, I'd be willing
to help you if you dont already know how to use it to model your antenna.

Jerry KD6JDJ


Jerry, thanks for the offert.
I dont know how good these modelling programs are in this situation.
But if it is easy, so could somebody do the following:

Take 20m of wire, that is half of a dipole.
Put a 1/4 electric wavelength shorted stub for 10,1MHz so that the open
end is 7,25m from center and the shorted end pointing to the tip of
antenna. One side of stub is the 20m wire itself, like in the upper
picture. Ignore all velocity factors to make it sinple.
Now tell me what are the resonance frequencies of that kind of element and
if possible feed point impedande (80m and 30m)

I know I could do it myself but as you know learning to use a new program
reliably takes lot of time.

73 Jouko OH5RM

Hi Jouko

I suspect that you either understimate your ability to learn or you arent
aware of how EZNEC works. I submit to you that a modeling program is the
best method of establishing the antenna design you are describing. You can
probably learn everything you need to know about your antenna within a few
hours of thinking on your own. You dont need to attend a class room course
on use of the EZNEC program.
I considered computer modeling to be beyond my ability till Richard Clark
encouraged me to 'just try it'. I tried it, and I consider the program to
be the best tool a HAM could have for developing antennas. I am sure you
will figure out how to model your "trap" antenna and learn how well it will
work for you within a very short time. You even have access to some EZNEC
experts from this rraa group. Try it, you'll like it.

Jerry KD6JDJ