The gain at a 90 degree take off angle isn't a very reliable indicator of
NVIS performance! 40-75 degrees is more useful.
Having done very controlled testing of different antennas and different
heights on 80 and 40 meters over 3 years my operating experience is:
1. Anything less than 1/8 wave high has increased ground loss and is less
than optimum.
2. Actual performance of antennas at heights of 1/8 to 1/4 wave vary a few
dB, depending upon ground conditions, distance of path, SSN, time of day,
season, etc. and isn't worth arguing about.
3. 1/8 wave NVIS antennas will pickup less interference from distant
stations and storms and have less problem with multipath distortion using
Pactor 3 digital modes.
4. I love EZNEC and HFTA and RadioMobile modeling programs. But sometimes
field tests don't agree with the predicted performance in specific real
world instances. RF propagation is a complex process with a lot of
variables. Always take some time to verify your modeling with realistic,
controlled testing. And realize that the next time or place you test it
may yield different results!
Steve, W3AHL
----- Original Message -----
From: "K4SAV" <RadioIR@charter.net>
To: <towertalk@contesting.com>
Sent: Tuesday, August 25, 2009 6:05 PM
Subject: Re: [TowerTalk] Folded dipole
> Maximum gain at 90 degrees elevation for a 40 meter dipole is at 27 ft
> above average ground. Anything between 23 and 33 ft will be within 0.1
> dB of that. It doesn't vary a lot with ground quality, although EZNEC
> thinks that 15 ft is the max gain point when over salt water. That may
> be a simulation error (I don't know). Some people may think max NVIS
> gain is achieved at a lower height because of a NEC simulation using a
> Mininec ground. With that ground, EZNEC will give you a gain of 19.8
> dBi for a 40 meter dipole 1 foot above ground (due to massive simulation
> error).
>
> I don't think there is any justification for intentionally putting a 40
> meter dipole lower than about 23 ft. Originally there was a reason, or
> so the story goes. It doesn't apply to amateur radio however. As you
> put the antenna at a lower height, the low angle lobes get attenuated
> more than the high angle lobe gets attenuated. So when you don't want
> to be heard by the enemy at great distances, you put your antenna very
> low, then crank up the power until you can just barely maintain
> communications with the close friendly troops. Actually cranking down
> the power is almost as much help as lowering the antenna. If you go
> from 25 ft to 3 ft, the gain at 10 degrees elevation decreases by 12.4
> dB while the gain at 90 degrees elevation decreases by 10.4 dB (over
> average ground).
>
> Maybe if you don't want to be heard by those unfriendly DX stations,
> this would be a good idea.
>
> Jerry, K4SAV
>
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