Iain Kelly wrote:
Sounds like a scaled up version of an antennas I built from a design
called 'The Mighty Wide 6M Dipole'... The design came from the internet
I think, I seem to recall G0IER published it on his website. I have also
seen the scaling information for other bands...

Thanks, but it led to some questions.

Here is the web page I found for it: http://www.qsl.net/g3pto/6m_dipole.html

From my calculations, assuming his center frequency was 51 mHz, he used
a size factor of 142.494 for the total length of the antenna. This seems
odd to me, as it would translate to 467.5 feet which is number I've
never seen before. Either something is amiss in my calculations, or he
used a different center frequency or just an unusual number.

The shorted length makes perfect sense, the ratio of total to shorted is
..8425 which is a reasonable velocity factor for 300 ohm twinlead.

The length of the stub makes no sense to me at all, but I'm sure he had
his reasons.

Using the proportions he used, I wrote a perl program to calculate the
size for any frequency.

For six meters, it matches his dimensions with a center frequency of 50.1 mHz:

for frequency 50.100 mHz.
total length 9 feet 3 inches or 2.844 meters.
shorted length length 8 feet 10 inches or 2.396 meters.
difference length length 0 feet 8 inches or 0.224 meters.
stub length length 0 feet 11 inches or 0.298 meters.

For 20 meters (14.175 mHz center frequency) I got:
for frequency 14.175 mHz.
total length 32 feet 11 inches or 10.052 meters.
shorted length length 32 feet 9 inches or 8.469 meters.
difference length length 0 feet 7 inches or 0.792 meters.
stub length length 0 feet 5 inches or 1.052 meters.


Anyone have any ideas? Will this work? How do I calculate the stub
length?

The ARRL Handbook antenna (not to be confused with the ARRL Antenna
Handbook, which he used) was slightly different, although this would work too
if my calculations are correct.

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 IL Fax: 972-2-648-1443 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

For 20 meters (14.175 mHz center frequency) I got:
for frequency 14.175 mHz.
total length 32 feet 11 inches or 10.052 meters.
shorted length length 32 feet 9 inches or 8.469 meters.
difference length length 0 feet 7 inches or 0.792 meters.
stub length length 0 feet 5 inches or 1.052 meters.


Anyone have any ideas? Will this work? How do I calculate the stub
length?

Think you have some errant decimal points in the last three English/metric
conversions
W4OP

Dale Parfitt wrote:
Think you have some errant decimal points in the last three English/metric
conversions

Thanks for noticing that.

I had the feet part of them being zero, when they should have been
greater. It was stupid testing, it worked right at 51 mHz, where they
were zero. I didn't look at the 14.1 mHz numbers closely enough.

Here is the correct numbers:

for frequency 14.100 mHz.
total length 33 feet 1 inches or 10.106 meters.
shorted length length 32 feet 11 inches or 8.514 meters.
difference length length 2 feet 7 inches or 0.796 meters.
stub length length 3 feet 5 inches or 1.057 meters.

Just another proof that programmers should NOT do quality assurance on their
own work.

73,

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 IL Fax: 972-2-648-1443 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

Geoffrey S. Mendelson wrote:
From my calculations, assuming his center frequency was 51 mHz, he used
a size factor of 142.494 for the total length of the antenna. This seems
odd to me, as it would translate to 467.5 feet which is number I've
never seen before.

468/f is the length of a dipole adjusted for end effects.

Anyone have any ideas? Will this work? How do I calculate the stub
length?

Why not use a Xc = -j150 cap? I get 0.176 wavelength for a
capacitive stub which may be inconvenient at HF.
--
73, Cecil http://www.qsl.net/w5dxp

Geoffrey S. Mendelson wrote:
. . .
The shorted length makes perfect sense, the ratio of total to shorted is
.8425 which is a reasonable velocity factor for 300 ohm twinlead.
. . .

Actually, shorting the antenna at an intermediate point between the
center and ends rather than just at the ends doesn't make much sense.

The feedpoint impedance of a folded dipole consists of four times (or
other ratio if the conductors are different in size or there are more
than two folded conductors) the impedance of a standard dipole, in
parallel with two series connected shorted stubs. The dipole consists of
the two conductors in parallel. This behaves as a single fat wire which
has the effective velocity factor of ordinary insulated wire, around
0.97 or 0.98 that of bare wire. That's why a folded dipole is about the
same overall length as a standard dipole. The stubs have the physical
length of half the dipole, or a bit shy of a quarter free space
wavelength. Unlike the dipole part, they operate as transmission lines,
so their velocity factor is around 0.8 -- a value which varies somewhat
with cable construction. Folded dipoles are sometimes shorted about 0.8
of the way from the center to the ends in an apparent attempt to make
the stubs an electrical quarter wavelength, resulting in their impedance
being very high as seen at the feedpoint.

But if the intermediate short circuit isn't done, the effect of the
somewhat longer stubs is only to add a bit of capacitive reactance
across the feedpoint. This lowers the antenna resonant frequency roughly
50 kHz at 14 MHz, and has very little effect on the feedpoint
resistance. Antenna resonance can be restored by simply shortening the
antenna a little. So why bother with the intermediate short?

Roy Lewallen, W7EL

The 1986 handbook has the details for this type of antenna on page 33-14,
under the title of "Simple Antennas for HF Portable Operation." For 7.15
MHz, the value of the capacitor is 152 pF, and the length of the matching
stub is 6'11-1/2". An open-end stub, made from twin lead, of length 20'-1/2"
can be substituted for the capacitor.

"Roy Lewallen" wrote in message
...
Geoffrey S. Mendelson wrote:
. . .
The shorted length makes perfect sense, the ratio of total to shorted is
.8425 which is a reasonable velocity factor for 300 ohm twinlead.
. . .

Actually, shorting the antenna at an intermediate point between the center
and ends rather than just at the ends doesn't make much sense.

The feedpoint impedance of a folded dipole consists of four times (or
other ratio if the conductors are different in size or there are more than
two folded conductors) the impedance of a standard dipole, in parallel
with two series connected shorted stubs. The dipole consists of the two
conductors in parallel. This behaves as a single fat wire which has the
effective velocity factor of ordinary insulated wire, around 0.97 or 0.98
that of bare wire. That's why a folded dipole is about the same overall
length as a standard dipole. The stubs have the physical length of half
the dipole, or a bit shy of a quarter free space wavelength. Unlike the
dipole part, they operate as transmission lines, so their velocity factor
is around 0.8 -- a value which varies somewhat with cable construction.
Folded dipoles are sometimes shorted about 0.8 of the way from the center
to the ends in an apparent attempt to make the stubs an electrical quarter
wavelength, resulting in their impedance being very high as seen at the
feedpoint.

But if the intermediate short circuit isn't done, the effect of the
somewhat longer stubs is only to add a bit of capacitive reactance across
the feedpoint. This lowers the antenna resonant frequency roughly 50 kHz
at 14 MHz, and has very little effect on the feedpoint resistance. Antenna
resonance can be restored by simply shortening the antenna a little. So
why bother with the intermediate short?

Roy Lewallen, W7EL

John, N9JG wrote:
The 1986 handbook has the details for this type of antenna on page 33-14,
under the title of "Simple Antennas for HF Portable Operation." For 7.15
MHz, the value of the capacitor is 152 pF, and the length of the matching
stub is 6'11-1/2". An open-end stub, made from twin lead, of length 20'-1/2"
can be substituted for the capacitor.

It might be interesting to some to explain how/why this works.
On a feedline with reflections, there are purely resistive
current maximum points existing every half-wavelength up and
down the feedline. Since the resistance at the current maximum
point is often in the ballpark of 50 ohms, this is often
a logical point at which to connect the transmitter/tuner. That's
why I feed my 130 foot dipole at a current maximum point on each
HF band.

However, for a folded dipole with a 300 ohm resonant impedance
fed with 300 ohm feedline, there are no reflections and therefore
no current maximum points because there are no standing waves.
So the trick is to cause a 50 ohm current maximum point to occur
by causing reflections and standing waves on a short piece of
series matching section. A parallel capacitor can often accomplish
this function. Obviously, a capacitive stub can do the same thing.

Note: the following calculations were done with a paper Smith Chart
and therefore suffer from some inaccuracies.

A parallel capacitive reactance of -j150 ohms will shift the 300+j0
ohm impedance to 60-j120 ohms. Some may want to refresh their
memories on the parallel/series and series/parallel impedance
equations.

60-j120 ohms will cause an SWR of ~6:1 from the point where the
capacitor is installed on a flat 300 ohm feedline to the necessary
1:1 choke/balun. If we divide the 300 ohm Z0 by the SWR, we will
obtain the resistance at the 6:1 SWR current maximum point. That
value is 300/6 = 50 ohms and it exists ~0.062 wavelengths
farther along than the 60-j120 point where the cap is located.

The cap needs to cause an SWR of 9:1 if flat 450 ohm line is used
and 12:1 if flat 600 ohm line in used. Needless to say, the Z0 of
the line determines the value of capacitive reactance that is
necessary to accomplish that function.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 15 Feb 2006 22:44:00 GMT, Cecil Moore wrote:


It might be interesting to some to explain how/why this works.
....

Some thoughts on why it might not work to plan, though it should be
close to plan and capable of fine adjustment if needed.

Worth noting that the characteristic impedance of some lines is quite
different to their market labelling, especially 300 ohm and 450 ohm
lines.

In this part of the world, nominal 300 ohm TV ribbon is more like 360.
Wes measured a range of nominal 450 ohm lines to be from 360 to 400
ohms.

Although the calculated answer might be a shunt capacitor of 147pF, in
practice a close value would be used and both the dipole length and
feedline length could be juggled for better match if needed.
Alternatively, a shunt stub could be used and tuned along with the
series line length.

The feed point impedance of the dipole will depend on its environment
to some extent, and may be reactive if formulas are used to cut it to
length.

Of course, it is a one-band matching arrangment.

Owen
--