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 Post subject: An Introductory Note About DIY Antennas
PostPosted: Fri Mar 10, 2017 1:13 pm 
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Joined: Sat Feb 15, 2014 3:08 am
Posts: 2020
Location: Toronto CYYZ
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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