Yes, I think that is exactly the case. I've both modeled and used
(Field Day) center fed non-resonant dipoles as well as off-center fed
dipoles, and the impedance effects represent the major difference (and
in several instances, the major benefit) relative to a typical center
fed resonant antenna. As you go up in frequency such that the wire gets
much above a full wavelength the pattern gets pretty squirrely no matter
where you feed the antenna, but staying away from the center usually
avoids the very high impedance such as the situation you mention where a
center fed 40m dipole is driven on 20m.
In the case of a 40m dipole, by the way .... if you feed it on 40m at a
point 25% or 30 % of the distance from one end it still radiates with a
normal broadside pattern but it has a moderately higher impedance that
requires a tuner or some other matching scheme. If you feed it at that
same point on 20m, the impedance is not that far from a workable match
to 50 ohms (certainly manageable with the tuner in most transceivers),
but the antenna is now a full wave with high angle lobes off the ends.
I used this same effect on 160m and 80m for a low dipole that presented
a pretty low impedance on 160m .... by choosing the feedpoint properly
I could raise the feedpoint impedance on 160m and still have a decent
impedance on 80m. I'm sure that low dipole had a lot of ground loss,
but it was all I could put up at the time.
You're right about the feedline radiation issue with any off-center feed
arrangement, and some sort of balun should really be used. Since the
impedances go up the more off-center the feedpoint, that can be a small
complication, but in my opinion it is manageable.
All things considered, I'd probably go with a non-resonant, center-fed
(approximately) antenna and a good tuner as the most practical multiband
solution since the balance is inherently better.
Jim Lux wrote:
> Here's an interesting question.
> Between a off center fed and a center fed dipole of the same length
> (e.g. something like the CW80s), and, assuming one has a reasonably low
> loss tuner to deal with the reactive component and match issues, what's
> the tradeoff between the two...
> Seems like as far as radiation goes, the radiating part is the same
> (except for incidental radiation off the feedline, but the chokes
> theoretically prevent that), so, going off center just changes the range
> of impedances one has to match over the various bands. As I recall,
> going off center makes all the impedances in the several hundred ohms
> magnitude range, so one uses a 4:1 or 9:1 transformer.
> But, maybe the only real advantage is that it avoids some really
> obnoxious lengths to match (e.g. trying to feed a 40m center fed dipole
> on 20m leads to a VERY high impedance) by essentially moving the match
> to somewhere more convenient.
> If this is the case, then one can also do a similar thing by just having
> a non-resonant length.. in the center fed case, make the dipole resonant
> at say, 8 MHz, so it's not an exact full wavelength on 14 MHz. Then
> deal with the reactance at the tuner.
> Jim, w6rmk
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