I have a question that's probably easy for anyone with practical experience
building UHF antennas. I need to make a shortened dipole (from tubing) at
450 MHz without sacrificing too much gain compared to full-size.

I have a design based on EZNEC. Total shortened "half-wave" dipole length
is 0.32 lambda. Half of that of course is in each of the two elements.
There is a loading coil in the middle of each element. EZNEC assumes zero
length, non-radiating coils. In reality, the coil length is significant
compared to the element length.

So, how do I build this thing considering the real coil length? Can the
coil be inserted in the middle of each element without reducing the element
length by the length of the coil? In other words, would final element
length be 0.32 lambda plus coil length? Or, should the elements be
shortened to maintain overall 0.32 lambda end-to-end length?

Thanks.

George K6GW

George. You can describe your loading coils to Nec by taking the
physical length of the coil, say, 1", and creating a number of wires
that divide into 1", say 5.
THen put a load on all 5 wires, the total load amount would be the sum
of the 5 loads.

This technique is used in one of my sample antenna files,
'mobile.ant' at:
www.nec2go.com

Regards Pat W0OPW

I have a design based on EZNEC. Total shortened "half-wave" dipole length
is 0.32 lambda. Half of that of course is in each of the two elements.
There is a loading coil in the middle of each element. EZNEC assumes zero
length, non-radiating coils. In reality, the coil length is significant
compared to the element length.



Welcome to real world. If you want to read more about the loading coil
controversy, go to my web page www.K3BU.us and snoop around ARTICLES, you will
get the picture. Eznec doesn't understand real coils, you can try to substitute
the coil with hairpin stub of the same inductance for modeling.

With 450 MHz dipole you will "research" this subject faster by taking piece of
wire or tubing, make about 3 turns in the middle, put the dipole up, measure
the resonant frequency and then snip the ends untill you get it where you want
it. Fine tuning can be done by squeezing or expanding the coil turns.

To make it more efficient, you can just bend the ends 90 deg. in L or T shape,
instead of using coils.

Yuri, K3BU.us

The only critical length of a dipole is the end-to-end length. The length
of the loading coil when inserted in the antenna should not affect this.

Locate the centre of the loading coil at the point where it is supposed to
be along the antenna and reduce the lengths of the antenna elements on
either side of it by half of the length of the coil.

In other words, the total reduction in length of the antenna elements is
equal to the coil length, leaving the original dipole end-to-end length
unchanged.
----
Reg, G4FGQ

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

"George" wrote in message
link.net...
I have a question that's probably easy for anyone with practical
experience
building UHF antennas. I need to make a shortened dipole (from tubing) at
450 MHz without sacrificing too much gain compared to full-size.

I have a design based on EZNEC. Total shortened "half-wave" dipole length
is 0.32 lambda. Half of that of course is in each of the two elements.
There is a loading coil in the middle of each element. EZNEC assumes zero
length, non-radiating coils. In reality, the coil length is significant
compared to the element length.

So, how do I build this thing considering the real coil length? Can the
coil be inserted in the middle of each element without reducing the
element
length by the length of the coil? In other words, would final element
length be 0.32 lambda plus coil length? Or, should the elements be
shortened to maintain overall 0.32 lambda end-to-end length?

Thanks.

George K6GW

Though I haven't actually done this, the way I'd approach it is to
determine the desired resonant frequency of that element, standing
alone. It's not so much the length of the element as the element's
reactance at the operating frequency that's important, and if you make
an element the same diameter and length as in the simulation, and trim
the coil so that your physical element resonates at the same frequency
as the one in the model, you should be OK. That's my reasoned
approach, but for sure listen to those with experience before taking
my advice on it. You should be able to determine the resonant
frequency by modelling just that element separately, with a source
somewhere near the center.

When you posted earlier about your shrunken design, I had the mental
image of a loading coil in each side of the element, and the other
poster's comment about using a shorted stub shorter than 1/4 wave to
get the same inductive reactance gave me the idea for what should be a
pretty rugged, and easy to build, way of doing it...the center of the
element would be tubing, and the ends would be rod. The rod would go
into the tubing, forming the coax stub on each side. They could screw
in, so you could store the antenna in a very small space. I don't
recall ever seeing such a design, but it should work, and avoids the
trouble of coming up with a good mechanical arrangement for the
loading coils. Same thing could be done for shortened-element HF
beams, too.

Cheers,
Tom

"George" wrote in message hlink.net...
I have a question that's probably easy for anyone with practical experience
building UHF antennas. I need to make a shortened dipole (from tubing) at
450 MHz without sacrificing too much gain compared to full-size.

I have a design based on EZNEC. Total shortened "half-wave" dipole length
is 0.32 lambda. Half of that of course is in each of the two elements.
There is a loading coil in the middle of each element. EZNEC assumes zero
length, non-radiating coils. In reality, the coil length is significant
compared to the element length.

So, how do I build this thing considering the real coil length? Can the
coil be inserted in the middle of each element without reducing the element
length by the length of the coil? In other words, would final element
length be 0.32 lambda plus coil length? Or, should the elements be
shortened to maintain overall 0.32 lambda end-to-end length?

Thanks.

George K6GW

Hi Yuri

What's the best way to measure the resonant frequency of the 450 MHz
dipole?

I've got a simple UHF return loss bridge and a counter.

What else would I need?

Thanks

MikeN ZL1BNB

On 15 Apr 2004 14:04:09 GMT, oUsama (Yuri Blanarovich)
wrote:


I have a design based on EZNEC. Total shortened "half-wave" dipole length
is 0.32 lambda. Half of that of course is in each of the two elements.
There is a loading coil in the middle of each element. EZNEC assumes zero
length, non-radiating coils. In reality, the coil length is significant
compared to the element length.



Welcome to real world. If you want to read more about the loading coil
controversy, go to my web page www.K3BU.us and snoop around ARTICLES, you will
get the picture. Eznec doesn't understand real coils, you can try to substitute
the coil with hairpin stub of the same inductance for modeling.

With 450 MHz dipole you will "research" this subject faster by taking piece of
wire or tubing, make about 3 turns in the middle, put the dipole up, measure
the resonant frequency and then snip the ends untill you get it where you want
it. Fine tuning can be done by squeezing or expanding the coil turns.

To make it more efficient, you can just bend the ends 90 deg. in L or T shape,
instead of using coils.

Yuri, K3BU.us

Mike, ZL1BNB wrote:
"I`ve got a simple UHF return loss bridge and a counter.

What else do I need?"

To measure the resonant frequency of the 450 MHz dipole, you need an
appropriate variable frequency signal source.

Return loss is the difference between incident and reflected powers at
the antenna. At resonance, antenna reactances cancel removing a large
impediment to power acceptance by your antenna.

Energy absorbed by the antenna is not reflected back toward the signal
source so you have the greatest difference between incident and
reflected powers near resonance (greatest return loss).

Best regards, Richard Harrison, KB5WZI

Resonance will be the frequency at which the feedpoint reactance is
zero. You may have difficulty with your simple equipment determining
that point, but it will be vanishingly close to the frequency at which
the SWR is minimum -- or the point of maximum return loss -- on a 50
ohm line feeding a dipole. Be sure the line is properly decoupled
from the antenna: use a balun. Generally you won't need a counter,
because the resonance will be quite broad and there's no practical
need to get the frequency closer than a percent or even a few percent.

Cheers,
Tom

MikeN wrote in message . ..
Hi Yuri

What's the best way to measure the resonant frequency of the 450 MHz
dipole?

I've got a simple UHF return loss bridge and a counter.

What else would I need?

Thanks

MikeN ZL1BNB

On 15 Apr 2004 14:04:09 GMT, oUsama (Yuri Blanarovich)
wrote:


I have a design based on EZNEC. Total shortened "half-wave" dipole length
is 0.32 lambda. Half of that of course is in each of the two elements.
There is a loading coil in the middle of each element. EZNEC assumes zero
length, non-radiating coils. In reality, the coil length is significant
compared to the element length.



Welcome to real world. If you want to read more about the loading coil
controversy, go to my web page www.K3BU.us and snoop around ARTICLES, you will
get the picture. Eznec doesn't understand real coils, you can try to substitute
the coil with hairpin stub of the same inductance for modeling.

With 450 MHz dipole you will "research" this subject faster by taking piece of
wire or tubing, make about 3 turns in the middle, put the dipole up, measure
the resonant frequency and then snip the ends untill you get it where you want
it. Fine tuning can be done by squeezing or expanding the coil turns.

To make it more efficient, you can just bend the ends 90 deg. in L or T shape,
instead of using coils.

Yuri, K3BU.us