Limited vertical space and no ground - can any low emission angle antenna be set-up?
I live in 9 stage building, on the last stage. The total area of roof is about 12x18 meters, made from reinforced concrete and coated with some rubber-like material. Since our area is very windy, there are certain legal restrictions for putting vertical objects on the rooftop; The object's height and location should be selected in a such way, that in case of fail, it won't fall down from the roof. Basically, this means that, either I can install one, 6 meter high pole in the center of the roof, or lower ones - near to the edges. Also the problem is, that I have no grounding available on the roof, and even if I install it, the level of the downwards wire will itself become quite prominent antenna element on certain frequencies. So, my choice is restricted to yagis and dipoles, and if we consider low allowed elevation, 80 and 40 meters become basically not available for me. 20 meters band is quite overloaded here with local stations, so almost no field for DX. On 17 meters situation is much better - DXs from Canada or USA can be heard regularly (Now I'm using sloped T2FD, it's good on rx, but almost no use on tx). 2 element yagi for 17 meters, at 6 meter height shows acceptable elevation angle, but it's definetly hard to build and tune for newbie. So I thought that maybe I can modify classic dipole to low height situation. I've downloaded MMANA-GAL, spent several hours studying it, and certain result was achieved, you can see drawing and source .maa files in attachment.
This antenna has SWR less than 1.5 at across complete 17 meters band, and is mounted only 2 meters above the ground (roof). While it's main emission energy is at quite high position, 8db at 49.9 degrees, it's still way better than classical dipole. which, at this installation height, fires directly to the sky, with it's 90 degree angle. If re-calculated, this antenna can be used on 20 meters band also, but bandwidth is very narrow, which is not good
Now the main question is, how realistic MMANA-GAL calculations are? I've selected proper characteristics of the ground in the options, according to program manual, and also considered all other requirements. So how do you think, is 7.7db gain and 50 elevation angle achievable with this antenna design?
Also, may be there are other antenna designs, that are "flat", and does not require ground?
Can you lay radial wires on the roof, or screen or mesh on the roof?
What is under the tarred paper or water barrier of the roof, is it metal??
Yes I can lay radials or place metal net there, but, aside me, local cable tv and internet company guys also have access to the roof, so I'm afraid if they step on it while I'll be transmitting, they may got hurt?
There's two 9 stage buildings nearby, at distances of 25 and 45 meters. In theory, it's possible to hung a dipole from my building to one of these, but there are a lot of trees around, and also, center feeding will be hard - need another support wire for coax. There's another, 16 stage building, to roof of which I have unrestricted access (friend lives there), but the problem is, it's quite far, about 135 meters, so wire should be very carefully selected and be of heavy-duty type.
I'm attaching picture of the another building for reference. It's 45 meters away from mine.
Your QTH looks like mine - check out my profile page for more info.
One of the biggest mistakes we make is jumping to conclusions. Your roof may not look like ground to a dipole on 40 meters. And in your environment you may find 80 is too noisy for anything but semi local work anyway... Don't talk yourself out of something because someone said it won't work.
Check out Jackite fiberblass poles. Do some geometry or trig on the dimensions for an inverted V dipole or doublet.
I've successfully worked Europe and South America from the midwest US on 100 watts with only a 60 foot long doublet inverted V with the apex at 30 feet, and the ends near ground level...
You'd be surprised what will work. Don't sell yourself short.
Last edited by AE2CS; 09-10-2010 at 06:39 AM.
I like the idea of sending a line to the tree and string a dipole, or hide an inverted V in there if you can. That's what I'd do. I like wire antennas because of the price.
'CS just beat me to the inverted-V idea; I was doing the trigonometry!
At 6m centre height a 17m inverted-V (ca. 8.5m/leg) with a 90 degree apex angle could be run in almost any direction; it might just fit across the 12m dimension of the roof.
my doublet is 30m long and the center is attached just below the roof line of my apartment building (21 meters or so high). From there, the two 15m legs run out at something quite less than a 90 degree angle... Each leg runs perhaps 5-10m to a tree, then over the branches and down vertically suspended by weights.
I feed it with 300 ohm twinlead TV line. I use a small tuner with 4:1 balun and I can tune it from 80m to 6m.
I worked Hawaii this morning. I worked Slovenia last night. I'm probably not the strongest signal, but I get through
Some day I'll have an 89 foot tower and monoband yagis. But for now, this is ok...
I have not modeled the antenna and honestly I wouldn't know how. There are just too many variables... The trees, my building, fire escapes, misc urban infrastructure... not to mention the George Washington Bridge, which is nearly a stone's throw away...
What I can tell you is that it works...
What I can also tell you is that I have found very often that the results I end up with, and the *actual* problems I end up solving are very, very different (usually) than the problems I *thought* I had, or the ones somebody else told me I would have...
You will ultimately have to experiment and see what works for you. The reason I commented was because your location looked so similar to mine.
Well, I already tried Inverted V. using fiberglass fishing rod. Sure it works, but I want something with better gain.
There is no way to have gain without being directional. That means some directions would be no good, other directions better.
Originally Posted by 4L4AGU
From Georgia you have populated DX in almost every direction.
A second problem is noise from the building.
You might want to run a longwire from the roof to another building, and a vertical on the roof. Then between the two antenna use the best one. You could lay a ground system on the roof, either screen or wire, and use it as a counterpoise. The biggest problem would be stealing the wire or people falling on it, not electrical shock problem danger.
Yes I know that directionality will "kill" certain directions for me, but it's OK for me. Below is the approximate list of countries I've logged during last year (only RX, my T2FD is good for only local, 20-30km contacts).
14 & 18 mhz.
Really rockin' and always good signal:
1. Romania & moldova
2. Serbia, Croatia, and nearby countries
3. Saudi arabia/israel
4. Russia 4, 5, 6, 9 regions
6. Spain and Portugal
more rare, but still prominent:
Germany & poland, sometimes - austria
Baltic countries and northen europe (except finland, never recorded any contact). most active is Lithuania
UK and Ireland, USA & Canada (northen states of USA) , France
Very very rare (1 or 2 contacts logged):
India, Mozambique, south african republic
Ukraine, belorus, russia 0, 1, 2, 3 regions, sometimes I hear local rig chewers chat from Italy.
Russia and Ukraine mostly, only once heard Israel.
If you mark all these countries on the map, some kind of "beam" clearly can be seen. As I know, T2FD is not directional, but this directivity is may be caused by my geographical location - I'm surrounding with mountains from all sides, but they are significally lower in north-west direction, so this may be the reason. It's really strange to me, to not hear local hams, like ones from Turkey or Armenia or Azerbaijan (recently I've logged contact between Canada and Armenia. Canada was heard 59+20, while I can't hear Armenia at all).
Considering all above, I already have some "directivity" and it's OK for me.
Last edited by 4L4AGU; 09-10-2010 at 06:03 PM.