# Receiving distance with rinky dink SDR Antenna?

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#### humblegeo

##### Member
I'm thinking about trying to build one of the 2 wire folded dipole antennas but I'm confused on the measurement for the materials. Lets say I wanted to build one for the aircraft band mid frequency 127 mhz. How long of a piece of copper-aluminum-brass rod/wire would I need? I found this formula:
L ft = 467/F mhz so 467 divided by 127 = 3.677 feet would that be right and then start bending it around to form the typical folded dipole shape. Or is there a lot more to it than that. The 1/8" brass rods I was able to find in the local hw store "Lowes" are only 3 feet long.

#### humblegeo

##### Member
Or I could just build a simple dipole using the suggested TV balun with two rods connected to it. I guess each rod would need to be 1.838 feet for 127 MHz? This seems much longer/bigger than the little antenna I built for 1090 MHz ......for the air band? It was only something like 138 mm?

Appreciate it
Larry G

#### Dog

##### Member
The antenna size has to do with the frequency wavelength. 127 mhz and 1090 mhz are a long ways from each other they are very different lengths.

A 5 mhz halfwave dipole would be 93.6 feet in length or 1123.2 inches. 127 would be 3.685 feet or 44.220 inches. 800 mhz = .585 feet or 7,020 inches, 1090 mhz = .429 feet or 5.152 inches, 2.4 ghz (wifi) is .195 feet or 2.340 inches, etc.

You can do a Google search for antenna calculator and get several online calculators and a few downloadable apps. There are also mobile apps if you use a smartphone.

But to answer your question on 127 mhz. 3.685 feet or 44.220 inches is a halfwave so each side of the dipole would be half that.

The main element on a quarter wave ground plane would be 22.110 inches and the radials would be 23.216 inches.

Regarding the brass rods. You can use any copper wire. I used 12 gauge on my 1090 ground plane. Straightened it by hand.

If you are wanting to listen to air band research "Off Center Fed Dipole"

There is also a "shortie" version that each element is half the size. (This is what I build to listen to airband.)

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#### humblegeo

##### Member
Trying to understand this: so if your using the sdr with RTL 1090 program to listen to aircraft band then you need to use the little antenna for 1090 MHz because that's the frequency that RTL1090 operates on, but if your using your scanner like my PRO-668 to listen to the air band then you would want to use the dipole antenna for 127 mhz.

#### Dog

##### Member
Trying to understand this: so if your using the sdr with RTL 1090 program to listen to aircraft band then you need to use the little antenna for 1090 MHz because that's the frequency that RTL1090 operates on, but if your using your scanner like my PRO-668 to listen to the air band then you would want to use the dipole antenna for 127 mhz.
Correct. 1090 mhz isn't any aircraft band though. ADS-B are transponders mounted in an airplane to report positions to ground receivers for safety purposes mainly.

ADS-B with a SDR dongle is pretty specialize. Using the little antenna made for that frequency is better. Also to make it more confusing the higher the number or the frequency 1090 vs 127 the better cable and stuff comes into play because of losses.

Antennas like the "Off Center Fed Dipole" are considered to be "wide band" antennas.

Or for example a "discone" antenna are typically used with scanner because they are "wide band".

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#### humblegeo

##### Member
I can see how building and experimenting with small antennas can be addicting especially when you start digging in and learning the science and theory behind it, like a lot of you guys have done. I appreciate the education. It gets pretty deep, I realize that now. I'm sure folks can get an engineering degree on nothing but Antenna design.

Off Center Fed Dipole" for "wide band" sounds pretty darn good too. Especially for my pro668 where I would like to get better performance over a greater frequency range. I have the little RS 800MHz antenna and the RS telescopic center weighted one and they work ok but I would like to get better performance.

#### M105

##### Member
I'm thinking about trying to build one of the 2 wire folded dipole antennas but I'm confused on the measurement for the materials. Lets say I wanted to build one for the aircraft band mid frequency 127 mhz. How long of a piece of copper-aluminum-brass rod/wire would I need? I found this formula:
L ft = 467/F mhz so 467 divided by 127 = 3.677 feet would that be right and then start bending it around to form the typical folded dipole shape. Or is there a lot more to it than that. The 1/8" brass rods I was able to find in the local hw store "Lowes" are only 3 feet long.
The calculator I used for the folded dipole is: Martin E. Meserve - K7MEM - Folded Dipole Design

The folded dipole design makes a close match to the 300 ohm impedance of the 300 to 75 ohm transformer. A simple dipole is closer to the 75 ohm impedance of the coax feed line and requires no transformer. The folded dipole offers a few advantages in my opinion. The elements are effectively larger in area so offer a little wider bandwidth on the intended band. The antenna is DC grounded. In other words if you put an ohm meter across the antenna leads it will indicate a short circuit. This hurts nothing at RF frequencies but tends to make the antenna less prone to voltage spikes from lightning and static charges. The antenna is "quieter" due in part to the DC grounding as well as reduced response to out of band signals. It tends to shunt RF that is not near or a harmonic of its design frequency.

All antennas are compromises in one way or another so different applications require different approaches. What makes an antenna like the folded dipole good for something like monitoring one band also make it not so good as a multi band antenna. One of the neat things about the little TV transformers is that they slip over an F connector so changing antenna elements is easy. I have both 455 and 790 MHz antennas that I interchange on a single mounting bracket.

##### Member
The OCFD is a great project, doesn't necessarily require that you make it from the actual copper piping and PVC T-joint meaning you can make one from wire and a balun/transformer just as easily (the copper pipe version has a bit more bandwidth I suppose), but the downside to the OCFD is that it's not particularly good at the high end UHF stuff over 700 MHz. Yes, it receives them, and I use my OCFD most of the time in my own RTL-based monitoring setup aka it's my "primary" antenna that stays connected the majority of the time but I still use my RS 800 and Diamond RH77CA as well when needed like if I grab my laptop and go out for a bit aka "mobile" I suppose would be the proper term.

The wire version of the OCFD could be considered mobile as well since it's just wire and can be rolled up in a few seconds flat, and mounted on a wall or tree (yeah, a tree) with a few pushpins as required but for the OCFD one thing that actually is somewhat critical is the feedline: it really does need to be kept at a perpendicular mount for at least about 18" coming of the T-joint/balun/transformer so it won't affect the reception.

But the one fact still remains: the OCFD, while being considered a wideband antenna, will not do nearly as well at 700 MHz and higher than the RS 800 will or even what I recommended earlier which is making 1/4 wave ground planes from wiring/coat hangers and SO-239 chassis mounts cut/tuned to the specific band center frequencies you want. For something like one antenna that can get the 772 MHz to 861 MHz coverage you could cut one for about 817 MHz (the center of that frequency swath) and it'll receive equally well for both that newer 700 MHz and the well establish 800 MHz public safety bands (and there's some commercial stuff in the 800 MHz range too). It'll actually get the 935-940 MHz commercial stuff too fairly well - it works better than the OCFD for those higher frequencies, so it's something to consider if you have stuff up there in those ranges to monitor.

The lower the frequency, the longer the antenna is the most basic fact with respect to antenna technology. Technically a very long antenna theoretically is capable of receiving anything at and above what it's length dictates as it's design wavelength, but obviously if you can acquire/build/buy/etc an antenna tuned aka cut to a specific length for a specific frequency then that antenna will outperform (in almost all situations) anything that's considered to be "wideband" regardless of design.

But yeah, building your own simplistic antennas can be a fun and yes addictive thing at some points because it's so easy to make them and they work very well. I'm not knocking professional commercial antennas for any particular reasons other than the cost because it's not like there's a brand new antenna design that suddenly gets better reception than anything that's ever come before: all the antennas you'll find on the market today are based on well established basic fundamental antenna theory so, when I realized I could build my own from similar and sometimes the same parts and save myself a few hundred bucks in the process well, that's when I decided that's what I'd do when I find myself needing a specific antenna whenever that happens.

Commercial antenna manufacturers exist because agencies like law enforcement, businesses, government, military, etc, can't really "make their own" in such mass quantities they usually require so that's their purpose and I get that, but as a consumer and a monitoring hobbyist going on 40+ years now whenever I see a few pieces of aluminum or metal or steel cut to specific lengths in specific forms aka antennas and I see the price some of them charge it just bugs me, it really does, considering what antennas are and how simple they can be to make. Sure I'd love to have a really nice discone like the popular AT-197 that's used for MilAir (225-400 MHz) comms but considering the price of that beautiful beast is \$1900 USD (and it sells for \$250+ used on eBay when it rarely appears and it doesn't last long) it's not something I could ever afford so, I have to look at it and think "Maybe I could build something similar..." and so that project remains on the back burner, as the saying goes.

#### humblegeo

##### Member
I am definitely planning to build the 1/4 wave ground plane antennas that you suggested. I just have to get some parts in so I can start on it. The right connectors and copper wire, rods, tubing etc etc. One thing I do think a person sort of needs is a signal strength meter of some sort otherwise how do you really know if the antenna you built is in the right physical position, location, you need a way to tweak things and see the result on some sort of a meter. RTL1090 program has sort of a signal strength meter that helps when your trying to improve reception...well scanner have a signal strength meter of sorts I guess with the bar graph but it's pretty crude.

#### Dog

##### Member
I am definitely planning to build the 1/4 wave ground plane antennas that you suggested. I just have to get some parts in so I can start on it. The right connectors and copper wire, rods, tubing etc etc. One thing I do think a person sort of needs is a signal strength meter of some sort otherwise how do you really know if the antenna you built is in the right physical position, location, you need a way to tweak things and see the result on some sort of a meter. RTL1090 program has sort of a signal strength meter that helps when your trying to improve reception...well scanner have a signal strength meter of sorts I guess with the bar graph but it's pretty crude.
That is one thing that I like about using VRS. The polar plots are amazing at telling you how your antennas are working. In a few hours to a day you can learn a lot.

Not every antenna of the same design is going to be the same however it is a starting point to learn how an antenna acts. An example if one antenna picks up more below 9,999 feet than another at the same height you know one has a lower take off angle than another. OR one antenna gets a lot more distance than another but everything is above 30,000 feet, etc.

##### Member
As I stated above, all antennas are based on the basic theories of how antennas are supposed to work so, as long as you follow the basics - and the 1/4 wave ground plane is about as simple as it gets aside from a pure monopole aka whip or even telescopic whip you can adjust (it has just the one element) - and the main vertical element is cut to the appropriate frequency (as well as the ground/radial elements being the same length or just a tiny bit longer) it's going to work.

And yes, you can use the signal strength meter in most any modern scanner to get an idea of how it affects things but since you'll be using some SDR hardware that can provide even more accuracy in terms of seeing the signal strength visually graphed in the spectrum. Yes you can run all sorts of antenna plotters and other software to determine all sorts of neat info (and there's nothing wrong with that since it's more learning which is always a good thing) but it's not an absolute necessity to make use of the hardware you have or the antennas you're making.

If you can hear stuff better with one antenna you build than one that came with the hardware then yep, you're on the right path.

#### humblegeo

##### Member
Work in progress. 817 MHz. I used Romex which someone had suggested earlier and a great idea, 12/2 which also has a ground so it's really like 3 solid copper conductors and the outer insulation on the other two conductors strips off pretty easy. 15ft of it for \$10 or you could say 45 feet of material. That will make a bunch of small antenna.

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#### SCPD

##### QRT
Oh I trashed my rinky dink antennas made my own for the first set of R820T's Now I have the NESDR Smart rtlsdr dongle w/ 3 GOOD antennas. The smallest one was spec'ed for 433mhz ISM band but it works well for indoor ADS-B reception in my experience. I have logged over 2100 unique aircraft into my personal Basestation.sqb in which VRS database writer plug in does for me. I started it about 4 days ago.

The antenna is sitting on a large tin coffee can in my window sill. Nooelec redesigned the NESDR Smart dongle to take 50 ohms. It came with a strong magnetic mount base with RG-58 6ft of coax. Lots better than the rinky dinky thingy.