As part of a continuing thread off-reflector with N4KG, I did some modeling,
responding to a question about a 3/2 wave mode antenna and the probability of a
six point pattern, as would be obtained using a 40 meter dipole on 15.
The antenna was a parasitic 3/2 wave wire spaced 0.2 wl from a wire driven
element which was varied from 1/2 wave to 3/2 wave.
The current patterns on the *parasitic* were familiar to anyone who has modeled
closed or open cell devices as found on F12 C31, etc. The current bunched near
the center of the 3/2 wave and did not at all display the familiar 3/2 wl
triple hump with phase reversal at the ends. The length of the parasitic was
not critical, and varied only in minor ways as the length was varied from 3/2
to 2/2.
The pattern showed the 6 point shape ONLY when the *DE* was extended to a full
3/2 (1.5). If the length of the DE was reduced even as little as to 1.3 wl, the
familiar bipolar pattern reemerged. Some minor gain over a dipole was possible
with certain combinations, along with slight F/B, and significant F/S.
The model used is *quite* simple, and contains none of the traps and issues
which cause models to fail. It reinforces the idea that a shorty 40 (without
linear loading, that's an exercise for another day) will work on 17 meters, at
very least as well as a rotatable dipole.
The sudden and substantial pattern change between 1.5 and 1.3 wl DE may
indicate some criticalities which if uncovered, could explain some of the
outwardly contradictory anecdotal material seen on the reflector. Personally,
it also raises a suspicion that the 402CD's complex element may have more than
one secondary resonance in the teens, possibly with a modal trigger point in
the ham bands, capable of being flipped by variance in feed, mounting, etc.
Anyone have specs on the 402CD's loading coil?
>
> From: Dave Hachadorian <k6ll@juno.com>
To: <towertalk@contesting.com>
> Date: 2001/04/25 Wed AM 09:01:25 EDT
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] C31XR / CC 402CD Question
>
> On Mon, 23 Apr 2001 10:56:38 -0600 n4kg@juno.com writes:
>
> > Do you see 6 lobes on 17M on your 40-2CD when you rotate it?
> > If so, then it is acting as 3/2 WL current fed Long Wire.
> >
> > If you only see 2 lobes (broadside to the DE), then it is
> > acting as an isolated (as opposed to trapped) DIPOLE.
> >
> > de Tom N4KG
>
> I did a quick experiment yesterday with another ham a few miles away.
> The 40-2CD has only two broad lobes, broadside to the elements(6dB
> BW 120 degrees)on 18 Mhz. There is a sharp
> null, only a few degrees wide, of about 18dB off the side. The
> front and back lobes are identical in amplitude and shape. SWR
> was 2.4 at 18.1 Mhz. Pattern amplitudes were measured with a step
> attenuator.
>
> Dave Hachadorian, K6LL
> Yuma, AZ
> K6LL@juno.com
>
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