On 30 sep, 03:39, Anonymous wrote:
Of all the HF antennas one might reasonably make out of wire or
aluminum tubing, which produces the strongest groundwave?
Ground-mounted quarter-wave vertical? Horizontal dipole at a height of
1/4 wavelength? Or what? I'm sure someone must have experimented, or
done computer modelling, to seek an answer to this question but I am
not finding any information.

--
-30-

Hello "Anonymus".

I assume that you want to maximize field strength close to ground.
At the low HF (and of course medium wave), the surface wave
contributes lots to the signal strength. These ones you can only
excite with a vertical polarized antenna. Also elevated vertical
antennas induce surface waves, so a ground connection is not mandatory
(but many times required practically because of antenna size).

As mentioned by other poster, 5/8lambda vertical fed over a good
ground network gives highest gain in horizontal plane (with a minor
side lobe).

At high frequency (and low ground conductivity), surface waves do not
play an important role. In that case height is the determining factor
(with high directivity in the horizontal plane). Polarization is of
less importance.

Best regards,

Wim
PA3DJS

Wim mentioned the high angle lobe you get with a 5/8 wave antenna. It
produces a sky wave which can, if skip conditions are present, interfere
with the ground wave signal and cause fading. This could be a very
common and serious problem at higher frequencies where the ground wave
is weaker and the sky wave stronger. You can reduce this by keeping the
radiator height to 1/2 wavelength or less. Antennas in that height range
don't have a high angle lobe, although they do produce some radiation at
high angles. Radiators near a half wavelength in height produce less
high angle radiation than shorter antennas, and they have more
horizontal gain, so I'd think they would be best. I'm sure the AM
broadcasters have other "anti-fading" techniques -- other newsgroup
contributors would know much more about them than I do.

Roy Lewallen, W7EL

On Sep 30, 5:40 pm, Roy Lewallen wrote:
Wim mentioned the high angle lobe you get with a 5/8 wave antenna. It
produces a sky wave which can, if skip conditions are present, interfere
with the ground wave signal and cause fading. This could be a very
common and serious problem at higher frequencies where the ground wave
is weaker and the sky wave stronger. You can reduce this by keeping the
radiator height to 1/2 wavelength or less. Antennas in that height range
don't have a high angle lobe, although they do produce some radiation at
high angles. Radiators near a half wavelength in height produce less
high angle radiation than shorter antennas, and they have more
horizontal gain, so I'd think they would be best. I'm sure the AM
broadcasters have other "anti-fading" techniques -- other newsgroup
contributors would know much more about them than I do.

Roy Lewallen, W7EL

One way to get more ground wave without introducing the high lobes is
to use a collinear vertical. Just how practical this is depends on
the frequency of operation and what you have to hold the antenna up.
For example, on 30MHz, you could pretty reasonably put a half-wave
above a half-wave or above a quarter wave. On 3.5MHz, that would be
much more difficult for most of us. Even on 20 meters, you might be
able to do a collinear vertical. I suppose that MW AM broadcasters
are unlikely to use a vertical collinear, but maybe one of the other
contributors with lots of broadcast antenna experience could comment
on that.

Cheers,
Tom

"K7ITM" wrote
I suppose that MW AM broadcasters are unlikely to use a
vertical collinear, but maybe one of the other contributors
with lots of broadcast antenna experience could comment
on that.
___________

Almost all US AM broadcast stations use non-sectionalized vertical
monopoles, but a few use a form of the Franklin antenna comprised
of two collinear, end-fed verticals.

The current thought on this in broadcast circles is that the additional
h-plane gain that provides over a 195-degree non-sectionalized
monopole isn't worth the initial cost and the continuing need to check
and adjust the power division and phasing between the sections of
a Franklin.

RF