Hi Bud,
Thanks for comments.
I used sea water in the model to correspond to your copper plate
example. I also tried other less conductive mediums for the roof as a
sanity check. What changes is the strength of the high angle stuff.
No surprise that a dipole 5 ft above a conductive surface would be a
train wreck both as a radiator and as a match.
I just looked at an 80M dipole 20 and 30 ft above that sea water roof.
Feedpoint Z about 15 ohms at 20 ft, about 30 ohms at 30 ft. Narudi's
shack is on the roof, so a feedline will be short enough that a 3:1
mismatch on 80M should not be an issue with RG8. Obviously, a different
conclusion if his shack was on the first floor, with 400 ft of coax
running to it. If you change the roof medium to less conductive,
feedpoint Z is about 28 ohms for an 80M dipole 20 ft above the roof.
I understand the goal of your 5 ft high antenna in the example for a
domestic contest. Narudi is in Jakarta, and his objective is CQWW.
I still think the major issue will be RX noise.
73, Jim K9YC
On Mon,8/10/2015 5:08 AM, W2RU - Bud Hippisley wrote:
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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