RE:Octopus a real Doosey
HRBE
I have a real doosey.
I built an Octopus, along the lines of the one which was featured in the Dec
2007 issue of QST. This article was written by Geoff Haines N1GY.
The Octopus is a 4 band rotatable dipole, using either Hamsticks or Workman
mobile whips
I fitted the following Workman whips: 75m, 40m, 20m & 15m.
I can tune all but the 20m whips, there is naturally an interaction between
each of the whips, but 20m simply cannot be tuned. its resonance is around
13.6mhz, and no matter what I do with the whip length it cannot be bought
into resonance.
I stress that 75m, 40, and 15m are great.
I have tried the 20m whips on their own as a single antenna and they tune
well, but as soon as I offer the second whip to complete the dipole, the
resonance drops to 13.6mhz.
When I refit all the other elements, it makes not much difference to the 20m
whips as far as their resonance is concerned, it does shift a bit which is
understandable.
I have tried rearranging the whips out of sequence around the hub, but that
made no difference.
Am I missing something, or are the Workman whips poorly designed?
Is there a preferred way of arranging the whips so that I do minimise
interaction between the elements?
Why are the 20m whips the only ones giving me trouble?
I am using an MFJ269 Pro to tune each band.
Thanks,
John
VK2KCE
Cecil Moore
> Why are the 20m whips the only ones giving me trouble?
Probably what is happening is that you are getting a
self-resonance on 14.2 MHz from the 75m or 40m hamsticks.
That doesn't happen with 1/2WL fan dipoles but it certainly
can happen with hamsticks. For instance, if the self-resonant
feedpoint impedance of the 75m hamstick dipole is lower on
20m than the 20m hamstick dipole, you will not be able to
adjust the 20m hamstick dipole for a 1:1 SWR.
Using only the 75m hamstick dipole, find the 20m impedance.
Then do the same for only the the 40m dipole. You can
probably diagnose the problem but I don't know of a
solution.
--
73, Cecil http://www.w5dxp.com
Howard W3CQH
"HRBE" <hr...@bigpond.net.au> wrote in message
news:oI8Rj.5583$ko5...@news-server.bigpond.net.au...
up.
73's de Howard W3CQH/r
Dale Parfitt
"Cecil Moore" <nos...@w5dxp.com> wrote in message
news:G39Rj.23213$%41....@nlpi064.nbdc.sbc.com...
I agree with Cecil,
One of the lower freq whip pairs is resonating on 20M and masking the 20M
whips' resonance.
Try this: remove the 20M whips and look for resonance around 20M When you
locate the pair of whips that is tuning up, run your finger up and down one
of their inductance coils.
You will likely find a great upset in VSWR along the coil- if more than one
is found, select the one closest to the base point. Mark this point and then
use something like copper tape and a 12" wire to capacity couple to that
point. This will have close to zero effect on the band the whip is intended
for, but, because it is a voltage loop on 20M, will have a major effect. The
idea is to move this whip pair's resonanace well below 14MHz.
Dale W4OP
Cecil Moore
> I agree with Cecil,
Dale, I hope you know that assertion can get you
ploinked by the gurus. :-)
Dale Parfitt
Bring them on. When you're right, you're right.
Dale W4OP
HRBE
I re-read N1GY's original and he mentioned in his article about the actual
placing of the whips.
I followed his directions and placed the 75m and 40m whips at right angles
to each other and the ather two pairs of whips in a like manner.
This then made the whole job a little easier, but the 20m whips are not as
sharp in tuning as the other pairs.
I took Dales advice and actually slid a 1" piece of aluminium tubing over
the whip and slid them over the whip and found that the VSWR was reduced at
a point midway over the loading coil in the whip.
So I have got it working, thanks to you antenna specialists. I appreciate
the time taken to reply to my problem.
Regards
John
VK2KCE
"Howard W3CQH" <hsgo...@comcast.net> wrote in message
news:7NednSphZNq6YYjV...@comcast.com...
Fred McKenzie
"HRBE" <hr...@bigpond.net.au> wrote:
> I re-read N1GY's original and he mentioned in his article about the actual
> placing of the whips.
>
> I followed his directions and placed the 75m and 40m whips at right angles
> to each other and the ather two pairs of whips in a like manner.
>
> This then made the whole job a little easier, but the 20m whips are not as
> sharp in tuning as the other pairs.
John-
I built the Octopus, but decided to go one further. I added a Ten Meter
vertical whip on top, thinking the other elements would act as an
effective counterpoise. It didn't work! Apparently a Ten Meter
counterpoise element is needed.
Just using the basic 4-band design, it seemed to have quite good SWR
dips, and responded to tuning. It did not have your problem. It does
use whips like (or close to) the ones in the article, not true Hamsticks.
However, I kept trying to add Ten Meters. By changing one of the ground
elements to a hot element, and adding two 45 degree upward-tilted
grounded elements on a bracket, I was able to "share" one of the
dipoles. If one band's hot element is on the north end, then its
grounded element is the southern 45 degree element, and vice-versa for
the second band.
I tried various pairings of Ten, Fifteen and Twenty, but there was
always one band of the two that suffered with either a poor SWR dip
and/or a center frequency shifted downward. I ended up sharing Fifteen
and Ten Meters, with Ten Meters being the band that suffered. Even with
a poor dip, the SWR is below 2:1 over the entire Ten Meter band.
So far, Ten Meters is the only band I've gotten signal reports on, and
only for local contacts. In that situation, it seems to compare
favorably to a mobile installation with a single whip.
Fred
K4DII
John VK2KCE
Today I tried out the 40m band and got great reports from a mobile op who
was over 400miles away to the south of me and another op in his shack about
the same distance north, both said I put in a very good signal, and
strangely enough I was the same signal strength in comparison to another
operator who was operating from his shack 15 miles due west of me, to both
other other operators.
I intend to put a 2m/70cm antenna on top, but to do this I will have to do
some machining to the hub as it was machined out of a solid billet of
aluminium.
It has certainly exceeded my expectations.
73
John
VK2KCE
"Fred McKenzie" <fm...@aol.com> wrote in message
news:fmmck-2D248F....@nntp.aioe.org...
Fred McKenzie
"John VK2KCE" <hr...@bigpond.net.au> wrote:
> I intend to put a 2m/70cm antenna on top, but to do this I will have to do
> some machining to the hub as it was machined out of a solid billet of
> aluminium.
John-
Yours sounds like it is relatively weatherproof. Mine is definitely
not. It may be possible to use a round cap for a 4 or 5 inch PVC sewer
pipe to construct one that is. It might also be possible to mount all
10 elements of a 5-band antenna along the circumference.
A vertical 2m/70cm antenna would be worth a try. If it doesn't work
against the grounded HF elements, adding a few short resonant radials
should correct it. Worst case, a second feed line might be needed.
It occurred to me that my criteria for a "good dip" may be different
from yours. I consider it to be one that is distinct, at least 1.5:1
SWR but not necessarily down to 1:1. Using a rig that has an
auto-tuner, useable bandwidth should be at least as good as the spread
between 3:1 SWR points.
I made one additional modification to mine, swapping one insulated
element with one grounded element on the bracket. By pairing Ten and
Fifteen Meter elements as before, the previous problem seems to have
gone away.
(I'm having trouble posting, so this may appear twice!)
73,
Fred
K4DII