VHF antenna question

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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?



Re: VHF antenna question
On Wed, 14 Oct 2015 16:11:10 +0000 (UTC), Cydrome Leader

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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
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: VHF antenna question
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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.



Re: VHF antenna question

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.

-----
Regards,
Carl Ijames  



Re: VHF antenna question
On Wed, 14 Oct 2015 16:11:10 +0000 (UTC), Cydrome Leader

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RF is magic.

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Do you want to listen to airplanes in flight, on the ground at the
airport, or both?  How far away is the airport.

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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.

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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.
<http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/animated-v-dipole.html

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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.

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Sure, but they're secret.  Also, I prefer to practice the "green
arts", which is where I convert antenna designs into the green stuff.


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: VHF antenna question
For a 1/4 wave vertical at 125 Mhz / 50 Ohm cable  
        
Calculated Results  
Calculated Vertical Length 0.571  Meters  
Calculated Vertical Length 1.87  Feet  
Calculated Vertical Length 22.46  Inches  

Calculated Each Radial Length 0.571  Meters  
Calculated Each Radial Length 1.872  Feet  
Calculated Each Radial Length  22.46 Inches  

One vertical, four radials, make from 16 gauge sold copper wire, soldered to a panel mount BNC connector  

It is receive only so the difference in gain on 116 to 138 Mhz Airband is negligible.

If you mount it on the roof use a lightning arrester...

Steve  




Re: VHF antenna question
  snipped-for-privacy@gmail.com wrote:

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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.

Fred

Re: VHF antenna question
On Wed, 14 Oct 2015 12:05:26 -0700, sroberts6328 wrote:

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   This approach is simple and effective. Make the radials about 510%%  
longer, and droop them about 15 degrees for an excellent match to 50  
ohm coax. #12 wire would be better

Re: VHF antenna question
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I assume no harm is done by using even thicker wire if that's what I have?

Is there any difference between 3, 4 or 5 radials? I see the commercial  
units vary a bit in numbers of these but with no explanation given as to  
why.

Re: VHF antenna question

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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.

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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.





Re: VHF antenna question
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I have a bit of 10 guage solid copper that has no other immediate use.

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4 looks cool, so I'll go with that.

Re: VHF antenna question
On Thu, 15 Oct 2015 19:48:53 +0000 (UTC), Cydrome Leader

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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:
<https://www.google.com/search?q=aviation+band+ground+plane+antenna&tbm=isch
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     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.
Re: VHF antenna question
On 10/15/2015 3:48 PM, Cydrome Leader wrote:
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3 or 5 would look even cooler.  Odd numbers rock!

--  

Rick

Re: VHF antenna question
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I guess the measuring out 72 degrees per radial would add some fun. I've  
got a 200 or 300 watt soldering iron I'm really itching to use before I  
contaminate it with some really nasty flux for stainless steel. Too bad  
I'm not dealing with microwaves and can't make the whole thing out of  
plumbing.

Re: VHF antenna question
On Thu, 15 Oct 2015 19:48:53 +0000 (UTC), Cydrome Leader

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  Make it like this one...

http://tinyurl.com/np3c7hz

  Or better yet, like this one!

http://tinyurl.com/oopsayp

or maybe this one...

a fractal quad....

http://tinyurl.com/nak3zt2



Re: VHF antenna question
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wonder if there's a mobius strip antenna yet.

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bizarro.



Re: VHF antenna question
On Thu, 15 Oct 2015 22:50:07 +0000 (UTC), Cydrome Leader

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  No, but here is a bicone:

http://tinyurl.com/p2fon4c


 And a discone:

http://tinyurl.com/nrob9hq

Here is your mobius: (well...  close anyway)

http://tinyurl.com/qzydssn

Re: VHF antenna question
On 15/10/2015 19:21, Dave Platt wrote:
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Yes. I think I read somewhere that the inventor (was it George Brown?)  
back in 1920s or 30s used two radials but the customer (police?)  
preferred the look of 4 and that is what has stuck.

piglet


Re: VHF antenna question
On 10/15/2015 5:11 PM, piglet wrote:
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Don't know about the name, but read the same info. In response, I used  
1/8" copper tape on a window with a vertical and two drooping radials as  
an antenna. It worked much better than the ducky. Keeps it out of harm's  
way.

Re: VHF antenna question
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Interesting. An alarm foil antenna would be cool.

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