Andy wrote: Yes, the antenna gain goes down so there is less signal.
But there is also less noise and interference, and in many cases the
noise drops faster when the antenna is lowered, such that the S/N
improves. Depends on the source of the noise, of course......
You have to get really specific to build a case for a low dipole
improving the signal to noise ratio. If you start with a 40 meter dipole
at 50 to 70 feet, and if you are listening to a signal arriving from a
high angle and the noise source is arriving at a low angle, then you
would have a better signal to noise ratio on a lower antenna. However
the converse of that gives you a poorer signal to noise ratio.
Since most band noise in the summer comes from thunderstorms, and by far
the strongest of those are fairly close, that noise source will be
arriving at high angles, and the low angle noise sources will be
insignificant by comparison. In that case, a low dipole (35 feet or
less) provides no improvement in signal to noise ratio no matter what
kind of signal you are listening to.
Jerry, K4SAV
Andy wrote:
> Here are a few reasons I've seen for using a lower NVIS antenna:
>
> Yes, the antenna gain goes down so there is less signal. But there is
> also less noise and interference, and in many cases the noise drops
> faster when the antenna is lowered, such that the S/N improves.
> Depends on the source of the noise, of course. So as a receiving
> antenna for high elevation angle signals it becomes better.
>
> Depending on ground and the water table in your area, the effective
> conducting layer may be some distance down, and then the antenna
> should be lower to be the same height above it.
>
> With these rotten sunspot conditions, you can't play NVIS much higher
> than 80 meters, if even that at night. Up a quarter wavelength on 80
> or 160 is not practical for most of us.
>
> Low works well for field expedient antennas. Even though they might
> not be optimal, they are good enough for (non-)government work.
>
> Some NVIS antennas add a low reflector on or a little above the ground
> surface, like a yagi. As I understand it, the optimum spacing from a
> reflector wire IS less than a quarter wavelength when that reflector
> is inductive (cut ~5% longer) ... as opposed to a large or infinite
> ground plane where 1/4 wavelength gives highest gain.
>
> I've rarely (if ever) seen hiding your signal from the enemy as a
> reason, nor simulation results. Most folks who recommend lower
> antennas do so based on experience ... either that they found the
> lower antenna worked better, or that it is good enough and the extra
> height isn't necessary.
>
> Andy
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