wrote:
Is this OCF dipole a single band antenna or a multiband antenna?
- 'Doc

It is alleged to be a multi-band antenna. Assuming that
the standing-wave current is 90 degrees out of phase
with the standing wave voltage, one can actually calculate
an approximation for the feedpoint impedance by dividing
the standing-wave voltage by the standing-wave current at
the feedpoint. With an OCF, the feedpoint impedance will
always be lower than Vmax/Imin and always higher than
Vmin/Imax and thus provide a reasonable SWR for 450 ohm
ladder-line.

The free demo version of EZNEC available from
http://www.eznec.com
will provide a reasonable guess at the feedpoint impedance.

Here are what EZNEC 4.0 says about the
feedpoint impedance of the following:

------------83'----------FP-----47'--------

3.8 MHz, 85.5+j98.5 ohms, SWR(300)= 3.9, SWR(450)= 5.5
7.2 MHz, 177-j181 ohms, SWR(300)= 2.5, SWR(450)= 3
10.125 MHz, 3569-j102 ohms, SWR(300)= 12, SWR(450)= 7.9
14.2 MHz, 96-j357 ohms, SWR(300)= 7.7, SWR(450)= 7.7
18.14 MHz, 797-j911 ohms, SWR(300)= 6.3, SWR(450)= 4.4
21.3 MHz, 933-j1239 ohms, SWR(300)= 8.8, SWR(450)= 6
24.95 MHz, 218-j652 ohms, SWR(300)= 8.5, SWR(450)= 6.7
28.4 MHz, 757+j1019 ohms, SWR(300)= 7.4, SWR(450)= 5.1

As you can see, the myth that this antenna has a 300
ohm feedpoint impedance is just an old wives' tale but
it does have reasonable SWRs when fed with 450 ohm
ladder-line.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Cecil,
I sort of thought that this OCF antenna was typically used for a
multiband antenna, that's typically the 'idea' behind using them.
Keeping that 'thought' in mind, how can you 'optimize' this multiband
antenna for one particular band without 'de-optimizing' it for
others? That seems sort of going at it from the wrong direction, why
not use an antenna that works well for the particular band of
interest. Oh, I know that assumes that you can have more than one
antenna, which isn't always the case. So having an 'un-optimized'
antenna is what you'll end up with in most (if not all) bands except
for one. Which is the 'fault' for almost all multiband antennas.
Isn't it?
- 'Do

wrote:
I sort of thought that this OCF antenna was typically used for a
multiband antenna, that's typically the 'idea' behind using them.
Keeping that 'thought' in mind, how can you 'optimize' this multiband
antenna for one particular band without 'de-optimizing' it for
others?

Given the SWRs that EZNEC predicts, feeding the OCF
at a current maximum point causes the XMTR (or tuner)
to see the following resistances looking into the
ladder-line.

3.8 MHz, 129 ohms; 7.2 MHz, 120 ohms; 10.125 MHz, 37.5 ohms;
14.2 MHz, 58 ohms; 18.14 MHz, 71 ohms; 21.3 MHz, 51 ohms;
24.95 MHz, 53 ohms; 28.4 MHz, 61 ohms

Let's say you wanted to optimize for 40m. You would
choose a 450 ohm ladder-line length of 67.5 feet and
expect 120 ohms looking into the ladder-line. Use a
tuner or transformer for a perfect match.

Oh, I know that assumes that you can have more than one
antenna, which isn't always the case. So having an 'un-optimized'
antenna is what you'll end up with in most (if not all) bands except
for one. Which is the 'fault' for almost all multiband antennas.
Isn't it?

Not for this one: http://www.w5dxp.com/notuner.htm
That antenna system is efficient on all HF bands.
It is fed with 1/2WL of ladder-line on 80m and 3/4WL
of ladder-line on 40m. Also appropriate lengths of
ladder-line on all other HF bands. The ladder-line
length averages around 100 feet.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

wrote in message
...
Cecil,
I sort of thought that this OCF antenna was typically used for a
multiband antenna, that's typically the 'idea' behind using them.
Keeping that 'thought' in mind, how can you 'optimize' this multiband
antenna for one particular band without 'de-optimizing' it for
others? That seems sort of going at it from the wrong direction, why
not use an antenna that works well for the particular band of
interest. Oh, I know that assumes that you can have more than one
antenna, which isn't always the case. So having an 'un-optimized'
antenna is what you'll end up with in most (if not all) bands except
for one. Which is the 'fault' for almost all multiband antennas.
Isn't it?
- 'Do

Seems to me it's not "optimized" for any band, but is just a fudged
radiator fed by ladder line, matched with a tuner or maybe a pi-net without
too much line loss. You can fiddle with it to your hearts content, but in
the end will it load up where you want it to and will you get out? It would
be nice to have an antenna analyzer to have some indication of YOUR
installation as you fiddle. Obviously you will have trouble using any of
the modern internal ATUs.

Years ago, I threw one up but fed it with coax. It didn't work so I rebuilt
it as parallel fed Dipoles for 80 and 40m.

Thanks for the NEC output, Cecil.

JB wrote:
Thanks for the NEC output, Cecil.

I showed how to optimize an OCF for a single band, e.g.
40m. If the 50/120 ohm transformer is link coupled, it will
cause a common-mode current node at the transformer on 40m.
Any antenna system can be optimized if one knows what they
are doing.

For instance, a shunt 1000 pf cap at the twinlead to coax
junction will optimize a G5RV for 75m.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

"Cecil Moore" wrote in message
news
JB wrote:
Thanks for the NEC output, Cecil.

I showed how to optimize an OCF for a single band, e.g.
40m. If the 50/120 ohm transformer is link coupled, it will
cause a common-mode current node at the transformer on 40m.
Any antenna system can be optimized if one knows what they
are doing.

For instance, a shunt 1000 pf cap at the twinlead to coax
junction will optimize a G5RV for 75m.


Of course but I'm too lazy to run the numbers if it isn't my project. My
favorite single band wire antenna is the Dipole. How is OCF an improvement?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference, considering
coax losses.

JB wrote:

...

Of course but I'm too lazy to run the numbers if it isn't my project. My
favorite single band wire antenna is the Dipole. How is OCF an improvement?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference, considering
coax losses.


A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my
humble opinion (or, IMHO ...)

Regards,
JS

"John Smith" wrote in message
...
JB wrote:

...

Of course but I'm too lazy to run the numbers if it isn't my project. My
favorite single band wire antenna is the Dipole. How is OCF an
improvement?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on
shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference,
considering
coax losses.


A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my
humble opinion (or, IMHO ...)

Regards,
JS

At least the "one size" will be very large. I once saw a truly monumental
commercial or military log periodic.

Tam/WB2TT

I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on
shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference,
considering
coax losses.


A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my
humble opinion (or, IMHO ...)

Regards,
JS

Then there are the discones and LPDAs where size matters.

The bug catcher worked well and still does despite many repairs, 15-40 m by
itself. 40 and 80 with the extension. It's a modified hamstick with 4"
dia. by 8" hand wound #14 coil in the center with scrap plastic forms.
There must be an unwritten law that says only the ugliest projects will
work. It was a pipe dream of flea clips, but the goal was to do all
matching on the antenna.

I used it mobile with a 365 pf broadcast variable on the mount and a remote
operator into the cab. Add a little series XsubL with a flea clip, matched
out with the cap to raise the impedance at the mount. The coil would self
resonate so 12 and 17 wouldn't work (perhaps a longer stepped pitch later).
Shorting around the coil got me 10m back. I would tune for min SWR
(actually ALC) and the difference between not heard with an OK SWR and heard
well after a fine tuning was repeatable over and over. Even with as little
as 15' of RG8x. Worked a lot of DX on the commute back and forth to
school. Had no time to do it any where else.