This seems counter-intuitive on the face of it. Raising a higher-frequency
antenna N feet will produce a lower take-off angle (and generally better on-air
results) than raising an identically designed lower-frequency antenna the same N
feet, because the increase in height is a greater multiple of a wavelength at
the higher frequency. Yet, I believe Rick's observations (assuming from his
description that they are on-air report-based tests, not engineering
measurements) are probably accurate. Here's my off-the-cuff explanation of why
this could be. If I'm way off, somebody please set me straight!
Raising an HF antenna that is less than a wavelength above ground will almost
always cause the take-off angle of the main lobe to drop closer to 0 degrees as
ground reflection effects are decreased. As you approach the propagating
ionospheric layer's MUF, the critical angle of a wave (i.e., the angle greater
than which the wave will not be refracted by the layer) approaches 0 degrees.
Since the F-layer MUF is mostly well within the bounds of the HF spectrum at
this point in the cycle, going from 14 MHz to 17 MHz to 21 MHz means that the
critical angle is getting smaller, and so the take-off angle required to achieve
propagation (and communication) is likewise smaller.
So you might make a lot of contacts and get good reports in the 2000-mile range
on 14 MHz using the lower antenna's higher take-off angle, but 21 MHz might not
support a propagation path that short (i.e., with a take-off angle that high),
because it is close to the MUF. Raising the antenna to get the lower take-off
angle appears to make more of a difference at the higher frequencies because the
only F2 propagation supported near the MUF requires a take-off angle closer to 0
degrees.
So, even though you don't need to raise it as much (in terms of absolute height)
to achieve the same relative take-off angle, it appears to make more of a
difference on the higher frequencies. My guess is that during a sunspot cycle
peak, where the MUF is often above 50 MHz, this effect wouldn't be observed on
the HF bands: You'd make a lot more shorter-range QSOs on 15M or 10M with the
lower antenna then, because the ionosphere would be supporting propagation at a
higher critical angle.
Please feel free to punch holes in this reasoning. I'm just starting to learn
this stuff, and I could use the instruction. :-)
73
Bill / W5WVO
Rick Karlquist wrote:
> I did a bunch of A/B testing of inverted vee's at 30 and 60
> ft on the same mast. The increase in height helped on the
> average of a few dB on 20 (not a huge difference) but helped
> substantially as the frequency went up, such as on 15 meters.
>
> Assuming you could extrapolate the results to Yagis (and I
> don't see why not), then it gives you an idea of when it
> is worth cranking up.
>
> I also notice that operating mobile, I do a lot better on
> 20 meters than the higher bands. This seems to be in line
> with the inverted vee tests.
>
> Rick N6RK
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
> "Wireless Weather Stations", and lot's more. Call Toll Free,
> 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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