There are two approaches to "coil loaded only" dual band dipoles
(also works on a vertical).
1) Use a coil with enough inductance that it acts like an RF choke
on the higher band.
2) adjust the location of the coil and the inductance so it resonates
the entire length of the antenna on both bands.
The first approach is easier and typically results in shorter antennas
- much like putting a mobile resonator on top of a 40 meter vertical
(think 4BTV -> 5BTV) - but efficiency and bandwidth is much lower on
the lower band.
The second approach is more difficult. Antenna lengths are typically
between full size for the two bands but efficiency is generally higher
on the lower band while the higher band may show a slight bit of "gain."
I don't know of any commercially made "dipoles" that use the second
approach but there was a PC based program floating around that claimed
to be able to calculate such antennas - of arbitrary length. Some of
the old antenna handbooks used to contain a graphical solution for the
coil loaded dual band dipole based on a QST article in the 1960's if I
remember correctly.
73,
... Joe, W4TV
On 2014-12-17 12:50 PM, Jim Brown wrote:
On Wed,12/17/2014 9:13 AM, Hans Hammarquist via TowerTalk wrote:
The solution for this is to place the capacitor outside the coil
instead of inside.
There's another solution, a very good one, that's been around for a
while. W9INN (SK) used it in many of the antennas he sold; Barry at
Hypower Antenna Company has been using it for at least 10 years. Barry
has a 2x1 3-call, but I always forget it. :) We've worked several times
in contests.
The solution is to add an inductor that is NOT a trap to each leg of a
dipole, carefully tweaking the design so that the antenna resonates on
two or more bands. I used an 80/40 version of this design in Chicago to
fit 80 into about 100 ft between supports, have used a 160/80 version
with a 40M fan element at three different QTHs with good results. The
only negative is that it's usually impractical to get it high enough to
have a good vertical pattern on 160M.
I've measured the traps for both of these antennas with a Vector Network
Analyzer, and they are NOT functioning as traps -- their stray C is
small enough that they resonate FAR above the bands they cover. I've
also plugged the loading coils into NEC and confirmed the design.
The 80/40 version has the loading coil approximately a quarter wave from
the feedpoint, and there's a relatively short section perhaps a 1/10
wavelength after the coil. The 160/80 version is built the same way.
The SWR bandwidth of these antennas is roughly comparable to a half wave
dipole on the higher band, and significantly narrower on the lower band.
It is VERY sensitive to the length of the short section on the other
side of the coils, so is very tricky to tweak. My experience, as well as
predictions of the NEC model, are that it radiates as well as any other
dipole at the same height.
BTW -- I told Larry about this yesterday in private email.
73, Jim K9YC
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