Newbie antenna question. I don't deal with RF stuff, at all.
I want to listen to air traffic communications that sit between 118 and
135MHz. The rubber ducky indoors isn't cutting it.
It looks like two easy to make antennas to make would be the quarter wave with 4 prongs as a ground plane and one sticking up, all about 23" or or so. It seems no balun is needed, just ground the downward prongs and connect to the coax shield.
The other is a dipole, but a balun is suggested for that. Do I need to even bother with that strange whip of coax the length of each antenna segment if I'm not transmitting?
Why would I pick one design over the other? Any good stories about the black arts of antenna design and selection?
Mount rubber ducky on ground plane, the run coax to it outside... it's probably the foil-backed sheet rock in your house that's killing the signal. ...Jim Thompson
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| James E.Thompson | mens |
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I love to cook with wine. Sometimes I even put it in the food.
There's no foil in these old walls, but there's plenty of interference from lights, switching power supplies and other junk that's always on. I do need something more sensitive to pickup the signal from the folks at the airport. Planes miles away and up and no problem to receive. I see tons of info about each antenna type, but nothing on why you'd select one over the other in the real world.
Newbie antenna question. I don't deal with RF stuff, at all.
I want to listen to air traffic communications that sit between 118 and
135MHz. The rubber ducky indoors isn't cutting it.
It looks like two easy to make antennas to make would be the quarter wave with 4 prongs as a ground plane and one sticking up, all about 23" or or so. It seems no balun is needed, just ground the downward prongs and connect to the coax shield.
The other is a dipole, but a balun is suggested for that. Do I need to even bother with that strange whip of coax the length of each antenna segment if I'm not transmitting?
Why would I pick one design over the other? Any good stories about the black arts of antenna design and selection? =================================================================
If you want to build something, go ahead, but I'd first dig up a set of TV rabbit ears, preferably one with an impedance matching or tuning knob. Your frequency range is right between the VHF lo (below 88 MHz) and VHF hi (above
174 MHz) bands, and they are broadband so should cover your target nicely. Stay with an old passive set, not one of the fancy electronically amplified ones that might have bandstop filters where you don't want them.
Do you want to listen to airplanes in flight, on the ground at the airport, or both? How far away is the airport.
Generally, that's good enough. However, it doesn't have much gain. That's probably not an issue if you're listening to airplanes in flight, but might be an issue if you want to listen to them during landing or on the ground, where you need an antenna with a low angle of radiation and some more gain.
No. Without a balun, your coax cable will become part of the antenna system. If the signal is picked up by both the antenna and the coax cable, there might be some cancellation. There may also be some losses if your coax cable parallels some metal structures, such as a mounting pipe. In general, a balun is easy enough to build, and for receive only is quite small. However, the overall benefits will be minor and the balun can probably be ignored.
Also, the vertical dipole picks up very little signal directly overhead, which will be a problem. Here's a lousy animation of a simple vertical dipole pattern at various elevations above ground. Notice the total lack of signal above.
That's easy. The uglier the antenna, the better it works. If your home made antenna looks like a gorgeous work of art, it will probably work rather badly.
The real reason is that different antennas have different antenna patterns and gains. For example, if you build a complex antenna, that has plenty of gain near the horizon to hear the airplanes on the ground, the antenna pattern will look like a flattened donut or a pancake. You'll hear the airport just fine, but directly overhead, you won't hear anything. In other words, too much gain is a bad idea for your application.
I would recommend the simple ground plane antenna mounted on the tallest pole you can find, using either RG-58c/u coax, or what I prefer, RG-6/u CATV coax and F to BNC adapters. I think height is more important than gain in your situation. However, I would need to know more about your location and distances involved before I can offer a more specific suggestions.
Sure, but they're secret. Also, I prefer to practice the "green arts", which is where I convert antenna designs into the green stuff.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Distance from me to O'Hare is about 15 miles. Not interested in ground communications, doubt there's any way to even pick up those. I can probably get an antenna up at about 30 feet off the ground. Not really too interesting in being the neighborhood lightning pole though- there's not a good way to get a real ground connection anyways. Even an ugly contraption inside would probably help a bit, just not sure which one to start with.
For the other poster- I no longer have any tunable TV antennas either. Did come across some sort of channel 3/4 modulator for a video game system or computer though. That was a laugh.
It depends on where 0'Hare has its transmitters. The NAV stuff is usually somewhere near the field. The COM radios are often on hilltops or tall buildings. Maybe you'll get half the conversation. Since you don't care about ground comm, then any antenna with a tolerable hemispherical pattern should work. The common discone is probably the best for that, with a pattern pointing mostly up. Very little gain, but lots of bandwidth. You could probably do as well with a common ground plane antenna, except directly overhead, which will probably not be a problem.
If you need to use substantial amounts of coax cable (i.e. over
100ft), you might consider a mast mounted preamplifier to compensate for the cable losses. I couldn't find one specifically for aircraft band, but it seems easy enough to design an build one. Something like this: except for aircraft band.
Incidentally, what are you using for a receiver? Most scanners are fairly deaf for AIR-AM reception. If you're having reception problems, it might be the scanner/receiver/whatever that's the problem. You could do the RF measurements, but it's easier to borrow a known working scanner/receiver from a friend and try it at your location.
Ok. Maybe not so tall.
We don't have much lightning on the left coast, so I can't comment on best practices.
The secrets part is easy. An NDA usually covers the details. It's the green stuff that's sometimes a problem.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I think Steve's approach may be the easiest way to construct an air band antenna, and it will work about as well as any you can build. (I might have suggested using an SO-239 connector instead of the BNC.)
I am using a Ham Radio mobile antenna in my attic. It has a magnetic mount, and uses a couple of strips of aluminum foil for the ground plane. The magnet sticks to the bottom of a large coffee can, with the strips of aluminum arranged in an "X" sandwiched between the magnet and the can.
My scanner has some air band frequencies included, and I often hear aircraft talking to an airport about 40 miles away. I can not hear the ground side. On rare occasion, I have heard aircraft testing their radios on the 121.5 MHz emergency frequency.
Depends how much gain you need. A folded dipole without a balun will work over a fairly wide range provided you don't want to transmit.
There is a cute DAB radio wavelength design made entirely by peeling back the insulation off a length of coax with a small coil to match it.
Whatever you make will probably be better off outside away from local in house electrical interference and any foil backed insulation.
Whichever one you have the right bits for in a junk box for will be fine. Gaining height usually helps improve range to control towers.
If you wanted to transmit then matching really matters but you won't harm an input stage with a mismatched aerial it just won't be quite as good as it might be with a correct matching network.
Agreed. I forgot to include the "not". Please note that I corrected myself in : "You could probably do as well with a common ground plane antenna, except directly overhead, which will probably not be a problem."
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
This approach is simple and effective. Make the radials about 5=10% longer, and droop them about 15 degrees for an excellent match to 50 ohm coax. #12 wire would be better
As you thicken the elements, you'd need to shorten them (slightly) to maintain the same resonant frequency.
Thicker elements will result in a somewhat flatter SWR-vs-frequency curve, which is usually a good thing.
There will be slight variations in the antenna's radiation pattern as you add radials. The more radials (the closer the radial set is to a solid cone) the more symmetrical the pattern will be.
Two radials, 180 degrees apart, is enough to give a pattern which is close enough to circular as makes no difference in most applications. Add more if you like; they won't hurt but I suspect that you won't notice a performance difference in practice.
The thicker the elements, the wider the bandwidth. Extra points for "rounding" the ends, which also improves bandwidth.
With one ground radial, the horizontal pattern will look much like a mutilated dipole. In fact, it is a dipole with one element somewhat askew. Two elements makes the omnidirectinal horizontal radiation pattern look like a slightly squashed circle. This is not too horrible if you want a little more gain on one direction or a little less in another. However, the amounts are tiny and not worth the effort. Three elements and above begin to look more like a classic dipole, with a good circular horizontal radiation pattern.
There are cases where I've used 4 or more horizontal radials. In general, those are where I'm trying to reduce ground reflections, such as in a doppler direction finder. In effect, the horizontal radials act as a shield, blocking any RF that is coming from below the horizon, most of which is ground bounce. The effectiveness of this shielding is dependent on the number of radials, so for this problem, more radials are better. However, they have no effect on the performance of the antenna.
4 drooping radials are fine for what you're doing.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
A 1/4-wave with a ground plane should work plenty good. Rigging a vertically polarized dipole would be a pain -- if you wanted to go there a J-pole antenna would probably be easier.
A 5/8 whip would have more gain for transmitters that are on a level with the antenna, but would have less gain for airplanes at high altitudes -- which is why I'm suggestion the ground plane antenna.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Any time you put a metal thingy up in the air, you become a lightning attractor. If yours is lower than the other nearby conductive objects (power poles, etc) you might be a bit better off. Maybe. Safest thing to do is disconnect your antenna and toss the lead-in outside.
You will obviously need some coax. Get it long enough so that you can waste a few feet if my suggestion proves inadequate for you:
Strip the outer jacket and shield of the coax about 22" from one end. Connect the other end to your radio. Put it up in the air vertically. You can try different locations if you wish. You can try horizontally.
If you find it suitable, then you should weatherproof the stripped end.
If this does not work, then cut off the stripped portion and attach a ground plane antenna as others have suggested.
Looking "cool" isn't going to help. In RF, the uglier the antenna, the better it works. Also, the stranger the antenna, the stranger it works.
You might get some useful ideas for building aviation band antennas from there: Unless you have an antenna analyzer, or network analyzer, I suggest you stay away from anything more complicated than a dipole or ground plane.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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