I guess I would do better if I just put together a regular linear loaded 80 meter dipole as an inverted V to solve small backyard issues and possibility better SWR curve. The feed point, the 1 to 1 balun is about 40ft high in a tree.
I have read about the morgain antenna.
I was thinking of using a 3 conductor wire on my next antenna build, I was under the impression that the 3 wire conductor could be shorter because the 3 wires are shorted together to make the over all length, longer. So is this why the COBRA uses so much over all wire because it uses a 3 conductor wire and 3 turn linear loading?
Here is a drawing of a
MOR-Gain antenna (if I can get the images to load):
Here is a
Cobra dipole:
Basically, the original MOR-Gain had additional dipoles in
parallel for the higher bands, and used direct coax feed.
The Cobra version took the same approach, since it gives a
resonance on two bands in half the normal length on the
lowest band, and fed it with ladder line for multi-band use.
All the people I know who used the Cobra Sr. on 160m
(which, granted, is a small sample) switched to some
other option due to poor performance. I can't say why
from my own experience, whether it was because
vertical polarization works better on 160m, or the low
impedance at the feedpoint led to higher losses
in the feedline (or possibly the tuner, depending on the
line length), or the use of ladder line with stranded
copper-clad steel conductors (which increases losses
on the lower bands).
Note that the original MOR-Gain dipole dates from the time
of tube rigs, which often could match a somewhat high SWR
(more so for higher impedances than lower ones). So if the
coax feed was close to an odd multiple of 1/4 wavelength
on the lower band, the low impedance (~12 ohms) would be
transformed up a couple hundred ohms, which many rigs
could match. (The old Johnson Ranger, for example, was
specified to match 40 - 600 ohms just by adjusting the
output stage, with no external tuner needed.) So while
the antenna performance hasn't changes, the expectations
of what SWR is acceptable has.
Any time you bend a dipole antenna, it requires a longer
conductor for resonance. Sometimes the difference is not
enough to worry about. But for this particular configuration,
there is a lot of coupling between the parallel wires, and
that changes the required conductor length. For an antenna
to cover 80m and 40m, for example, the current distribution
is very different, which is why the second resonance is on
just twice the base frequency rather than 3 times (the
traditional 40m / 15m combination) for a straight dipole.
Whether this is the best approach for your antenna depends
on a lot of factors. It's convenient that it can cover both
40m and 80m with one antenna (even with coax feed),
although you probably will still need a tuner in the shack,
as the SWR bandwidth is very limited on 80m. But you
probably will run into issues with bandwidth and low
feedpoint impedance with many other types of shortened
antennas as well.
Author
Topic: Wire element spacing used on the COBRA HF wire antenna (Read 299 times)