Jim — I’m not sure you’re “missing” anything from a theoretical standpoint.
Some comments from a practical standpoint, however:
1. In my dormitory roof example, our objective was to work W/VE — most of
whom were very close to us, hence very high angle. We might well have had a
great DXing pattern; I just wasn’t “into” DXing back then.
2. In a two-medium model such as the one you’re describing below, the
relatively small inner medium (the roof of Narudi’s building directly beneath
the dipole) sets the feedpoint impedance of the dipole, which will be very low
because the dipole height above Narudi’s roof is very low when measured as a
fraction of a wavelength on 80 meters. This leads to what I call “super-gain”
models which can have the far field “boost” you describe but which are
extremely difficult to realize in practice, due to the difficulty of matching
such low impedance feedpoints without substantial feedline or antenna matching
unit losses. For brevity in my initial posting I didn’t mention that the low-Z
feedpoint on 80 and 40 made it impossible to properly match our dormitory
dipole with just the pi-network output of the 813 rig.
3. You say you used sea water as your inner medium. I’m not sure I’d equate
a rooftop with sea water — even “my” rooftop with a solid sheet of copper
flashing under the tar and gravel. I think having a dipole really close to a
rooftop tends to compress the entire dipole pattern because of losses in that
“ground” system directly beneath the dipole.
So, overall, I think the _pattern_ you got from your model is probably not too
horribly off, but the overall efficiency and gain of a real-world
implementation of that antenna system is not the greatest.
Bud, W2RU
> On Aug 10, 2015, at 1:56 48AM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> On Sat,8/8/2015 10:36 AM, W2RU - Bud Hippisley wrote:
>> but, with the usual wire sag, the feedpoint was about 5 feet above the
>> gravel. We weren’t worried, because the roof was at least 70 feet above the
>> surrounding terrain.
>
> Hmmm! Let's remind ourselves of Nuradi's situation. The roof is 110m high,
> 45m x 33 m. Corner to corner is less than a wavelength on 80M, more than a
> wavelength on 40M, but the distance to a corner from a wire strung between
> the two corners is less than a quarter wave on 80M, less than a half wave on
> 40. Assuming an ideal conductor on the roof, it's going to act as a reflector
> going upward, but the low angle pattern will be determined in the far field.
>
> I've not worked before with two ground media, so I pulled out W7EL's
> instructions for doing so. I built a very simple model attempting to roughly
> simulate Nuradi's situation. I'm running NEC2 with EZNEC Pro5. The first
> ground medium is sea water, with a radius 120 ft (it's a rectangular building
> so that's an approximation. The second medium is Very Poor: cities,
> industrial, and it's at -360 ft. Yes, I'd like to elevate the first medium
> and have the second medium at 0 ft, but EZNEC won't let me do that. I
> simulated 40M and 80 dipoles in the range of 20-30 ft. What I got was a two
> lobe vertical pattern -- a VERY strong, very narrow low angle lobe, and a
> broad upward lobe whose strength depends on the height of the dipole above
> the roof. Very low (5 ft) makes the bottom lobe VERY narrow and VERY low
> (about 2 degrees) and makes the high lobe a lot weaker. Yes, it's a poor
> antenna -- IF its low to the roof. But if it's up 20-30 ft, a horizontally
> polarized wire looks like a nice DX antenna.
>
> What am I missing?
>
> And, like I said before and several others added -- all that stuff on the
> roof is likely to be mondo noisy.
>
> 73, Jim K9YC
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