At 02:14 AM 2008-12-03, jim Jarvis wrote:
>The 2 lambda loop (20m element operating at 10m) would have a very
>high impedance, on the order of 2k ohms. The 1 lambda loop (10m
>element on 10m)
>would be around 100 ohms. 100/2000 = 1/20 = 5%. So, your side
>nulls would be
>filled by around 5%. Not such a big deal, if you can accept the f/s
>ratio of maybe 13dB
>Note, it's not the f/b of the array -- that's established by the
>parasitic element.
>
>What has me confused is the opposite case... what happens when the
>10m element is operated at 14 mHz, in parallel with the 20m
>element? The system of
>tying the elements together for feeding is being used, so it must
>work. What's
>happening? The suggestion has been made that the 1/2 lambda loop
>can be considered a quarter wave
>parallel transmission line, which is shorted at the far end. The
>far end short is thus
>transformed to an open at the feed point.
Jim,
My models show that a 2 wl square loop will have a feed Z of around
270-300 ohms. This is the case for a single loop and also for a
concentric loop around a 1 wl loop with both fed in phase from equal
voltage sources. (One of the nice things about harmonic operation of
quad loops is that the impedance is moderate on all harmonics,
compared to dipoles that have very high impedances on even harmonics.)
A 0.5 wl loop has a very high impedance, usually well over 10K ohms
at (anti)resonance for HF wire loops. Your comparison to a shorted
1/4 wave line is appropriate.
Note that in most cases, a loop cut for 20m will be resonant a little
below 10m. Also, a 10m loop will be antiresonant a little below the
20m band. Parallel operation on either band will result in a
relative phase shift between the currents in the two loops due to the
reactance present.
The free-space pattern of 20m and 10m loops fed in phase at the
bottom of each loop is quite interesting on 10m. The elevation
pattern is broadly down toward the ground, with about a 4 dB F/B
(bottom/top?) ratio. The side nulls of the free-space azimuth
pattern on the horizon are only about -7 dB. The current ratio at
the feedpoint is about 2.3 to 1.
On 20m, the current ratio is about 8 to 1, with the max current in
the smaller loop at the top, away from the feedpoint. The azimuth
pattern is pretty clean, with F/S of about 23 dB and only a slight
downward skew of the elevation pattern (with bottom feed).
73, Terry N6RY
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