On 16 May 2007 20:21:29 -0700, dykesc wrote:

On May 16, 12:46 am, Richard Clark wrote:


Look at your data comparison again and ask: Should it be 4:1 or 1:4?


Richard, I guess I'm not seeing something obvious. Its a 4:1 balun
transforming the high side impedance down by a factor of 4. The 4:1
impedance measurements (line on the high impedance side of the balun,
analyzer on the low impedance side) should be down by roughly a factor
of 4, right?
I can't see anything wrong with how I presented the data.

Hi Dykes,

Well then, your feed point Zs are inordinately low. (Get rid of the
idea of using the 4:1.)

The OCF feedpoint (at least by design) is near (but not on) the peak
of a current loop on 80, 40 and 20 meters (even harmonics). Don't
current peaks occur at points of low impedance along an antenna? My
EZNEC current traces confirm this. Shouldn't my even harmonics
impedance measurements all be low rather than high?

As OCF/OCD antennas have an infinite variety of feed points (where as
the balanced dipole has only one), there is nothing set in stone about
any of this until you define the degree of "off-center."

There are MANY OCDs that exhibit conventional Hi/Lo/Hi/Lo spectrums in
contrast to your Lo/Lo/Lo.... Certainly, they offer no advantage over
the conventional balanced dipole in this respect. Too often OCD
claims are made without corresponding supporting details.

By the way Richard the frequencies my impedance measurements were
taken at are not "resonant" frequencies. The resonant frequencies
(zero reactance) occur at 3.56 Mhz, 8.05 Mhz, and 15.7 Mhz.

You might note, then, that meaning of harmonics has been similarly
distorted in this thread.

A little
too low on 80M. Too high for 40M and 20M. The tuner gets me the match
in the 40 and 20 amateur bands.

I apologize for not stating that my sloping OCF is fed at 35% of its
total length from the high end. High end at a height of about 40 feet
and low end at a height of about 10 feet. Total antenna length is 136
feet.

Modeling against these criteria reveal nothing like your measurements
with:
(zero reactance) occur at 3.56 Mhz, 8.05 Mhz, and 15.7 Mhz.
(that is the short story)

However, modeling does suggest the antenna should perform for 80/40/20
in much the way your second set of data with the reactance driven out.
To say the least, you still have a hodge-podge of results probably
dominated by the contribution of unbalanced currents on the feed line
(a classic condition for this design that demands considerable
choking).

All that remains to confirm this last observation is to someway
determine the lobe patterns. If they exhibit weak nulls, then the
feed line is part of the radiator. Given the choke appeared to work
at the feedpoint (I assume this is what you did); then a second one is
warranted, and possibly a third. The usual placement advice is 1/4
wavelength from the feed point, but given this is a multi-band
antenna, that is out the window and you will have to experiment (I
would start with 1/4 of the shortest wavelength away).

However, if nulls are immaterial to the enjoyment of the antenna, then
theory has been satisfied and you can chalk that one up.

And to confirm my forecast of operation between 40M and 20M, it should
be exhibiting a significant SWR throughout.

73's
Richard Clark, KB7QHC