An Introductory Note About DIY Antennas

Making a DIY antenna is lot of fun and satisfies creativity.
It also looks very simple and easy.
However, in order not to end up in frustration, there are certain facts which should be known by all attempting to make a DIY antenna.

Antenna Parameters
Antenna’s following characteristics affect its performance:

  1. Gain
  2. SWR / Impedance
  3. Radiation Pattern

Gain:
Considering the limit of line-of-sight reception a maximum of 300 to 350 nm, Plane flying at maximum height of 45,000 ft (FL450), and 200 watts or so power pumped into aircrafts antenna, a receiving antenna with Gain around 6 dBi in horizontal direction is very good to receive almost all the aircrafts up to 350 nm.

SWR / Impedance:
Since the Receivers and Coax have impedance either 50 ohms or 75 ohms, an antenna with its impedance matching with receiver/coax impedance will perform good. If antenna impedance is within the 50 to 75 ohms range, the SWR is 1.5 or less, which is a decent and acceptable value.
SWR higher than 1.5 results in reduced performance. SWR in excess of 2 will give poor performance. Higher the SWR, poorer the performance

Radiation Pattern:
A good radiation pattern should have maximum gain in horizontal direction (far away aircrafts), and low gain in directions 45 to 90 degree (nearby and overhead aircrafts). The ideal radiation pattern is like ∞ (lazy eight).

AVAILABLE ANTENNA DESIGNS
There are a lot of antenna designs available which meet above criteria.
However almost all high gain antennas sch as Coaxial Collinear, Wire Collinear with Coils, Wire Collinears with stubs, Stripline antennas etc, require following to give good performance:

  1. High accuracy of dimensions.
  2. Have impedance mismatch with receiver / have high SWR, and require additional components to bring down SWR
  3. Require test equipment and technical know-how to tune to designed frequency, and to reduce SWR to 1.5 or less

Obviously, these are unsuitable for DIY antenna making by a normal ADS-B hobbyist, who does not have any test equipment and any technical know-how.

ANTENNA DESIGNS SUITABLE FOR A NORMAL HOBBYIST
The only antennas which are** (1) reasonably tolerant to dimensional errors** and (2) have impedance in range 75 to 50 ohms (SWR 1.5 or less) are listed below. These are therefore suitable for making a DIY antenna by an average hobbyist without any test equipment and without any technical know-how.

  1. The 1/2 wavelength dipole (2 limbs, each 1/4 wavelength)
  2. The 1/4 wavelength monopole with ground plane (Spider & Cantenna)
  3. The stock whip mag-mount antenna (supplied with DVB-T dongle), cut to size and placed on a metal object such as a food can.

These easy antennas have one disadvantage. These have low gain (1.5 to 2 dBi).
If used with very short runs of coax, these perform ok. With long runs of Coax, these require amplifier. With introduction of FlightAware ProStick and ProStickPlus with integral RF-preamplifier, this deficiency is overcome. Even if a generic DVB-T (black/blue) is used, adding a satellite amplifier or a custom made LNA covers the low gain deficiency of 1/4 wavelength groundplanes.

COMMERCIAL ANTENNAS
If a hobbyist is not interested in fun and satisfaction of making a DIY antenna, and is simply interested in best performance, he can purchase a commercial antenna such as Flightaware 26 inch antenna (Gain 5.5 dBi, SWR<1.5), or high gain antennas by other manufacturers, available at ebay/amazon/respective sites.

For FlightAware antenna:
https://www.amazon.com/dp/B00WZL6WPO

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It’s actually worse than low gain. It’s a negative gain. i.e. a loss of signal strength.

The i in the acronym dBi refers to an isotropic radiator which is a theoretical antenna consisting of a single point in space, and thus does not actually exist as a physical antenna.

When compared to an isotropic radiator, a dipole has approximately 2.15 dB of gain.
Thus, an antenna that has a “gain” of 1.5 to 2 dBi, is actually 0.15 to 0.65 dB lossier than a dipole.

Manufacturers use the dBi reference because it makes the “numbers” look better.
Referencing antenna gain to a dipole (dBd) gives a closer approximation of its real-world performance.

Ref: http://www.m2inc.com/blog/dbi-vs-dbd/ and
http://www.eham.net/ehamforum/smf/index.php?topic=12148.0

Bill KR6K

This DYI antenna most likely has many problems. But I have it on a table on a north facing window 14 off the ground and lots if trees. I am getting a good bit of data at 150-200 mi and 200-250 mi. I expect to do very well when I put it on the roof.

I was fretting over what wire to buy for minimum signal loss. As you can see I have the radio connected directly to the BNC/SMA and just used a USB cable to the Pi. I bought a 30 USB cable for went I mount it on the roof. No worries about loss.

This is a screen shot of my results for two setups. The left image is from the rooftop factory made antenna, the right is a 5 element coco that I put together. I paid very close attention to measurements and assembly. The coaxial was inserted into a piece of 3/4" PVC with a 68mm piece of of metal conduit around the bottom of the PVC. This antenna is sitting on a window ledge facing NW. I have built several and this one seems to be the best performer so far.

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Really looks like that it has an obstacle in the northwest direction, despite being on the rooftop.
What model of antenna is it?

Apart from that, hearing about co-linear antennas that work at all is always nice :slight_smile:

Easier to show the links. This is the “window” site facing NW and this is the other omni-directional.

But you have several abandoned sites that you can delete. Here:
https://flightaware.com/adsb/stats/user/SBYTower#stats-97349
https://flightaware.com/adsb/stats/user/SBYTower#stats-86821
https://flightaware.com/adsb/stats/user/SBYTower#stats-96890

These two screen shots appear to have been taken at the same time with the same aircraft in view.

I do the same and it shows exactly how each antenna is working in real time whereas the link to the stats page just shows historical counts over roughly the same time.

My screenshots. The P1 is the top screenshot and is connected to the CoCo. Pi3 is the second and is connected to the FA antenna.


The two antennas on the right are ADS-B. The black one to the North is a Flightaware antenna and the long white one is the home made CoCo. Both use V3 dongles and amp/filters and the coax is RG6 Quad.

Really easy to see which is the better antenna. Note the CoCo is about 1.5M higher.

I agree with @wiedehopf that there seems to be something wrong with the antenna on the roof. It is seeing the same to the North East but all other directions are worse.

The CoCo sitting inside seems much better than the antenna on the roof. I would be mounting the CoCo on the roof as soon as I could weather proof it then go about seeing if you can make another one at least as good if not better.

S.

Thank You for mentioning that I could delete the abandoned sites. I was able to delete one of the PiAware sites but not the FlightFeeder. Nice setup you have.

I did a similar setup years back on UHF FRS band… I guess the very nearby FA antenna is acting like a Yagi antenna reflector towards your CoCo antenna. I realised the homebrew CoCo is very subsitable to near by metal object which reflect the signals to one direction too strong… just sharing my experience and observation…

@abcd567, how do you use your N1201SA analyzer when creating / tuning a DIY ADS-B antenna? I just received my N1201SA and would like to evaluate my 1090 and 978 1/4 wave ground plane antennas.

I have not used it for over an year, so dont remember well, but what I remember is:

  • Turning ON/OFF is by simultaneously pressing Power (OK) and CTRL buttons and hold them pressed for few seconds

  • M button toggles the display between Numeric (current values) and Graph (for SWR vs Freq plotting).

  • CTRL + M button brings in settings

Thanks - I think I figured it out. Had to shorten the center wire versus theoretical length to get the VSWR in the 1.5 range, probably due to the insulation on the wire impacting the velocity factor.

I’ll test out the tuned antennas tomorrow.

Picked up a plane 167nm away versus my previous max of 115nm :sunglasses:

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What length did you cut to?

@kenf3
Waiting anxiously for your answer.
Question: Did you shorten only whip, or whip & radials both? If only whip, then what was length of radials?

Latest simulation results showed 64mm (whip & radials), which I disregarded thinking it is because simulation can easily have upto +/- 5% error.

QuickSpider-5

Do you always tune for minimum VSWR, or does gain also play a role?

In 1/4 WL ground-plane, gain varies very little with few mm change in length. So basically I tune based on SWR.

Anyway the “Vector Impedance Analyzer N1201SA” I have mesures only Impedance and SWR. It cannot measure gain.

QuickSpider-6

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55mm & 65mm …

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I tweaked the 1090 antenna down to a 54mm insulated whip and tuned the 978 antenna to a 56mm whip. Both use 65mm insulated radials.

image

image

UPDATE …

When I installed both antennas into my project and checked their VSWR again, they both got worse (VSWR 3.6 & 2.6 and R~21 ohms for both).

Should I re-tune or leave them as-is???

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Bad SWR because the antennas are very close.

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