[TowerTalk] 40-30m dipole design

[jimlux]

On 3/17/16 12:40 PM, Jim Brown wrote:
> On Thu,3/17/2016 12:14 PM, jimlux wrote:
>> There's no big advantage to linear loading: you might as well use a
>> good low loss inductor at the feed (the "shorty 40" does this).
>
> Are you certain about this, Jim? Both change the current distribution,
> but linear loading changes it least at the center, where current is
> greatest. The inductor places maximum current in the inductor, which
> doesn't radiate.

I agree.. It's like base loading a vertical (with a top hat perhaps) vs 
loading it half way up.  BUT, a 30m dipole isn't that much shorter than 
a resonant 40m dipole, so the current distribution is nicely modeled by 
the usual "half a sine wave from end to end".  That current distribution 
doesn't have significantly different gain (1.93 dBi vs 2.14 dBi). 
However, if a infinitely small dipole is 1.5dBi and a full size dipole 
is 2.15dBi, the gain is going to be somewhere in the middle.

I would assert that if you're running a dipole, you're not obsessing 
about the last milliBel of gain.

So then, you're just at the problem of feeding a reactive load with 
30-300j impedance, and a high Q inductor will help nicely with that. A Q 
of 100 should be easy (3 ohms out of 300), and would be 10% loss (0.5 
dB?) Maybe that's where the "1dB less gain for the short antenna" comes 
from: 1/2 dB from the pattern being slightly broader and 1/2 dB from 
loss in the matching/loading components.

You could probably tap the inductor to get a 50:30 impedance transformation.

The resulting antenna WILL be narrower band than a dipole (but again, a 
75% length antenna isn't hugely reactive... it's not like a 1 meter 
compact loop or a 7 ft whip on 40 meters, where the VSWR bandwidth is 
10s of kHz.





NEC doesn't model that very well, treating the inductor
> as a lumped element, with no change in the current distribution across
> it. NEC probably would model linear loading pretty well if the model
> accurately described the antenna.

True enough, and linear (or distributed) loading might get a more flat 
topped current profile, which will have MORE gain than a dipole (just as 
a aperture with uniform illumination has more gain than with a tapered 
illumination)



>
> 73, Jim K9YC
>
>
>
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