shortened dipole loaded
 

October's QST has an article "Designing a Shortened Antenna" pp 28-32. It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole
length of the half-sized dipole [p30 "a second example"] is 10.61 meters.
This is 20 feet off ground using #12 wire. The formulas give a solution of
XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each
leg.


I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of
the dipole, rather than +j1776?

--
Jerry

Jerry wrote:
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole
length of the half-sized dipole [p30 "a second example"] is 10.61 meters.
This is 20 feet off ground using #12 wire. The formulas give a solution of
XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each
leg.


I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of
the dipole, rather than +j1776?

Get a copy of VE3ERP's Hamcalc,it includes a program that lets you set
the total length and "slide" the loading coil anywhere inside that length..
Hamcalc is available on the CQ magazine website.
73 de VE3JUA....

Jerry,
I tried an EZNEC simulation also and did not have the same results. I got
74 ohms at 1912 ohms inductive reactive. I resonated the simulated antenna
with 28 uH, getting an SWR of 2:1
I will send my simulation to you. Please send me yours as we are not getting
the same thing from the QST numbers.
Bob,

On Sat, 27 Sep 2003 10:05:06 -0500, "Jerry" wrote:


I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of
the dipole, rather than +j1776?

--
Jerry

Hi Jerry,

I can confirm your first pass analysis is close enough (probably
differs only by wire gauge from mine).

I would suggest you simply add the loads you speculate and find out
for yourself, which is, after all, the point of modeling.

73's
Richard Clark, KB7QHC

On Sat, 27 Sep 2003 19:18:26 GMT, "Bob Walker" wrote:

Jerry,
I tried an EZNEC simulation also and did not have the same results. I got
74 ohms at 1912 ohms inductive reactive. I resonated the simulated antenna
with 28 uH, getting an SWR of 2:1
I will send my simulation to you. Please send me yours as we are not getting
the same thing from the QST numbers.
Bob,


Hi Bob,

For a 40M dipole (short or long) that is only 20' off the ground?

73's
Richard Clark, KB7QHC

Jerry wrote:
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole
length of the half-sized dipole [p30 "a second example"] is 10.61 meters.
This is 20 feet off ground using #12 wire. The formulas give a solution of
XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each
leg.

I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of
the dipole, rather than +j1776?

Yes, if you were going to put the loading coils at the center of the antenna.
That is akin to a base-loaded mobile antenna. But you are going to put the
coils in the center of each leg. That is akin to a center-loaded mobile
antenna and that requires about double the inductive reactance that a base-
loading coil requires.
--
73, Cecil http://www.qsl.net/w5dxp



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For design of short coil-loaded centre-fed dipoles begin with a short loaded
1/4-wave vertical and, in-effect, connect a pair of them back to back.

Download programs VERTLOAD or LOADCOIL or ADDALOAD. For even shorter loaded
dipoles download MIDLOAD. All these programs will tell you how may turns are
needed on a former of given length and diameter at whatever position along
the wire you chose to locate it.

In most cases the length of the coil is taken into account in the overall
length.

When using the design of short-verticals to design short-dipoles the ground
loss estimates can be assumed zero.

Some of these programs include assistance with bottom-end tuner design
which, of course, is of no use when a pair is connected back to back. It
should be remembered the feedpoint resistance of a short resonant dipole
will be considerably less than the desirable 50 ohms and a poor match to a
coax feedline will exist. But the mismatch may not be excessive.

The main disadvantage is single-band working whereas an unloaded short
dipole can usually be effective on several bands by using a 450-ohm or
600-ohm feedline and a good tuner. But you will need a tuner with whatever
length of dipole, loaded or unloaded, you end up with.

As for myself, when using short antennas, I prefer not to use heavy-weight
loading coils in antenna wires but concentrate on keeping home-brew tuner
losses to a minimum.

Programs can be downloaded in a few seconds and run immediately.

As were Bolton & Watt's condensing steam engines - Made in Birmingham.
----
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================

Jerry,

I ran EZNEC on this, using the QST dimensions, and got what seem to be
reasonable results.

Resonant frequency = ~7.11 MHz
Impedance = 21.15 + j1.17

For those who obviously did not read the QST article, the 25 uH loading
coils are 1.7 meters from the ends. Total length is 10.64 meters, and height
is 6 meters. Over real ground, maximum gain is about 6dbi, straight up, and
about 3 dbi at 30 degrees elevation.

Tam/WB2TT
"Jerry" wrote in message ...
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole
length of the half-sized dipole [p30 "a second example"] is 10.61 meters.
This is 20 feet off ground using #12 wire. The formulas give a solution
of
XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from
each
leg.


I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007
khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg
of
the dipole, rather than +j1776?

--
Jerry

Whoops, I see I did "Building the Antenna", not "Second Example". There is
something wrong with "Second Example", because the dimensions are about the
same as "Building", but the coil is almost twice as big.

Tam/WB2TT
"Tarmo Tammaru" wrote in message
...
Jerry,

I ran EZNEC on this, using the QST dimensions, and got what seem to be
reasonable results.

Resonant frequency = ~7.11 MHz
Impedance = 21.15 + j1.17

For those who obviously did not read the QST article, the 25 uH loading
coils are 1.7 meters from the ends. Total length is 10.64 meters, and
height
is 6 meters. Over real ground, maximum gain is about 6dbi, straight up,
and
about 3 dbi at 30 degrees elevation.

Tam/WB2TT
"Jerry" wrote in message ...
October's QST has an article "Designing a Shortened Antenna" pp 28-32.
It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole
length of the half-sized dipole [p30 "a second example"] is 10.61
meters.
This is 20 feet off ground using #12 wire. The formulas give a solution
of
XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from
each
leg.


I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007
khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each
leg
of
the dipole, rather than +j1776?

--
Jerry

It's disappointing to see that people have gotten so many different
results modeling such a simple antenna with EZNEC.

Jerry's results are correct. You'll get slight variations with differing
segmentation and ground conductivity, by they won't be large differences.

I haven't followed through the article's math, but there's either a
computational error or something fundamentally wrong with the equations
to produce a value of j1776 for the antenna's reactance. The fact that
the author's reactance is just about twice what it should be points to a
likely error in computation.

Jerry, what the EZNEC result means is that you'd need a *total* of j881
ohms at the feedpoint to resonate the antenna. You could do this by
adding half the amount to each leg, or the total to one leg. Or, you
could move the coils out toward the end, but you'd then need more
inductance. Of course, you've still got a feedpoint resistance of about
11 ohms, plus coil resistance, to transform into something close enough
to 50 ohms to make your rig happy.

Roy Lewallen, W7EL

Jerry wrote:
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole
length of the half-sized dipole [p30 "a second example"] is 10.61 meters.
This is 20 feet off ground using #12 wire. The formulas give a solution of
XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each
leg.


I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz
gives impedance =11.33 - j 881.2 ohms.

If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of
the dipole, rather than +j1776?