[TowerTalk] 40-30m dipole design

[jimlux]

On 3/17/16 2:54 PM, jimlux wrote:
> 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 just ran a NEC model (using 4NEC2 to compute the matching network).. 
the 15 meter long (30m band) dipole with a T network (91nH series, 352pF 
shunt, 7.47 uH series) has 1.75 dBi gain (vs 2.14dBi for ideal dipole).

The 2:1 bandwidth is a bit more than  300 kHz; 1.5:1 bandwidth is about 
150 kHz.

I leave it as an exercise for the reader to figure out the voltages and 
currents on the components (which was the original question)


So, in summary

full size 40m dipole: 2.15 dBi gain
75% size dipole: 1.93 dBi gain
75% size dipole with coil Q 250, cap Q 1000: 1.75 dBi gain   (about 
80-84 degree 3dB beamwidth)


the 40 m full size antenna has a swr of 1.4:1 at best and a 2: 1 BW of 
about 400 kHz (not much wider than the 75% sized antenna with a matching 
network).  And the match is better with the matching network (50 ohms vs 
72 ohms.

The beamwidth of the full size is slightly narrower (76-80 deg)