[TowerTalk] 40-30m dipole design

[jimlux]

On 3/17/16 8:29 AM, N1BUG wrote:
> While trying to figure out how to move up a notch from my inverted V
> antennas on 40 and 30 meters I came up with a crazy idea. I was looking
> at building a rotatable dipole for 40 meters, either full size or
> possibly shortened. A 70% of full size linear loaded 40 meter dipole is
> nearly a full size 30 meter dipole. Various articles claim only about 1
> dB reduction in "gain" from a full size dipole on 40.

An "infinitesimally small" dipole has a theoretical gain of 1.5dBi vs 
2.14 dBi for a half wave dipole.  1dB reduction seems larger than what 
you'd expect.


Could relays be
> used at the dipole feed point to short out the linear loading, making a
> two band antenna? It seems good in theory but if it were that easy,
> wouldn't someone have done it already? I have no idea whether RF voltage
> across the relay contacts when operating on 40 meters would be high
> enough to make this impractical.

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).

I don't think the voltage will be all that extreme. A 30 m dipole, on 
40m, has a feed point impedance of 30-300j ohms.  You can use one of the 
antenna tuner apps to figure out what hte voltage is.



W9CF has a nice simulator in Java