Brian,
Be careful of your logic. The reason a rhombic works so well is
that most rhombics are 2 or more wavelengths long per leg. However,
putting in a resister makes the beast directional toward the resister,
not all bad nor inefficient. But the T2FD type antenna is only a 1/2
wave long on the lowest frequency making it more efficient as you go up
toward 10 meters.
I have been reading a lot of comments on antennas here, some are
correct and some just may be close at best. Let me reiterate something
I've said earlier and regularly. There is NO perfect antenna. Every one
is some compromise in one way or another. in my case, I need to deal
with 40 meter space with a desire to use 80 meters. So, part of my
horizontal antenna is bent down toward the ground. It fits. Do I have
the best antenna?? No. But, it does fit.
OCFD antennas are not really bad antennas if you follow the rules of
horizontal antennas. If the antenna was fed in the center, it would be
50 Ohms or nearly so at its 1/2 wave length. Above that, even multiples
of the 1/2 wave frequency, the feed impedance would be very high. So, it
was figured out that you can feed off center and could find a spot that
would not be awful at the even harmonics. A 4:1 current balun will take
of most of the impedance issue and bring it down into range of your
tuner. A good balun will have a loss of less than 1 db. If you use good,
low loss coax, losses will not be bad.
The 1/2 wave dipole, center fed dipole. fed with open wire and a
balun at the shack entrance makes a good antenna, and the losses may be
a slight amount lower than the OCFD, but it follows the rules of
horizontal antennas that are a specific height.
There is nothing in ham radio that is argued more passionately than
antennas, with maybe the differences between Windows and the Apple OS.
73,
Barry
K3NDM
------ Original Message ------
From: "Brian D. Comer" <[email protected]>
To: [email protected]
Sent: 12/10/2015 7:14:24 PM
Subject: [Elecraft] Antennas with resistor and OCFD.
I think stating any particular antenna configuration is unacceptable is
ignoring what could be an ideal antenna for a given situation. As I
understand it a rhombic uses a resistor, and sometimes works quite
well.
On 160 I have an OCFD 1/2 wavelength that is goes from one side of the
end
of a very small canyon to the other. The net result is that the
center of
the antenna is about 80 feet above ground, the ends are only 50 ft
above
ground. On flat ground this would be a NVIS antenna being so low;
however,
with the ground 100 feet to the west at the same height as the antenna
and
the ground 100 feet to the east being 300 feet below the antenna I
suspect
the pattern looks more like a beam pointing east. Considering I have
broken
two important rules, the antenna is OCFD, and is on average it is well
under
1/2 wavelength above ground it is amazing I can work the east so well.
73 Brian KF6C
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