If you are using the whip antenna (which came with DVB-T), you can improve it’s performance (maximum range & number of planes) by doing two simple things shown in the sketch below.
This shows that I should cut it at 67mm or 1/4 length.
I have the exact same antenna as pictured but mine is 122mm in length and so cutting it at 67mm isn’t 1/4 length.
Should I still cut it at 67mm ?
Yes, cut it at 67mm, irrespective of it’s original length.
The antenna should be cut to 1/4 of wavelength, and NOT 1/4 of antenna’s original length.
At 1090 Mhz, the wavelength of radio wave is 275 mm. Hence 1/4 wavelength = 1/4 x 275 mm = 69 mm.
Due to end effect, the antenna should be about 2mm shorter than 1/4 wavelength, i.e 69mm - 2mm = 67mm.
Any advice on modifying the newer stock DVT-B antennas (ie. the telescoping ones)?
I imagine the first thing to do is to extend it to only the 1/4 wavelength size. Then pop it on a can/metal sheet to give it a ground plane.
At the most “collapsed” state, it’s still too long.
Where to you measure from? The screw at the pivot point? Somewhere else?
Here’s a picture.
Telescopic antennas cannot be cut to size. Try to collaps it to 67mm height. If it is still longer than 67mm, then you cannot do anything other than to use it in fully collapsed condition. Closer the fully collased length to 67mm, better the results. How much is the length of your antenna when fully collapsed?
The antenna starts from the point where black plastic base ends and nickle plated metal starts, and not at the pivot screw. Length to be measured from the start of antenna to tip of the antenna.
An alternative is to order a pigtail, F- female to MCX male, and make an easy DIY antenna optimised for 1090 Mhz. The easiest one is Cantenna which requires only an F female to female connector, cheap & commonly available at most satellite/cable tv stores and supermakets.
I’m just curious more than anything, but what if you had the pivot for the telescoping antenna at a 90 degree angle (i.e. the antenna vertical, but the magnet attached to a vertical metallic surface). Would the length then start from the pivot point?
(1) Yes, in this case antenna length starts from pivot point. The ADS-B transmission being done by Vertically Polarized radio waves, only the vertical length of antenna counts as antenna length, and horizontal parts do not contribute to antenna function.
(2) Horizontal metal surface parallel to ground provides ground plane. Vertical metallic surface acts as vetical limb of a dipole, extended not only down, but also up & side wise. I have never thought of or tried this configuration. However this does not seem to provide the advantage of either ground plane or of a dipole. Give it a try, and may be you get some improvement. Keep the antenna base stuck to the upper part of vertical metal sheet, so that almost entire metal sheet length is below the vertical part of antenna.
Another option could be to unscrew the telescopic antenna and screw in a shorter piece of copper/metal which is the correct length.
ABOUT THE DIMENSIONS
(1) Diameter of Can:
I have tried cans of various diameters upto 100 mm.
Cans less than 2⅝ inch (67mm) diameter gave poor results.
Cans with diameter 2⅝ inch (67mm) and up to 3⅞ inch (100 mm) gave nearly same result.
I did not try bigger than 3⅞ inch (100 mm) diameter can. You may try bigger dia cans to find out if “bigger is better” holds true.
(2) Height of Can:
Optimum height is ¼ wavelength, but this has lot of tolerance and somewhat higher or shorter cans don’t make much difference. You can try cans of various heights to find which one gives you best result.
(3) Height of Whip:
This is most effective in improving performance. The optimum length of whip is ¼ wavelength (69 mm for 1090 Mhz & 77 mm for 978 MHz). Due to “end effect”, the whip is made few mm shorter than ¼ wavelength, i.e. for 1090 MHz 69mm - 2mm = 67mm, and for 978 Mhz 77mm - 3mm = 74mm.
- Wavelength λ in mm = 300,000 / frequency in MHz*
last days i made a test with two raspis and standard dongle antennas along your tip ‘cutting to 67mm’.
first ran them against each other without modification to get a base result - then one with shorter antenna.
i did not test the can under antenna - but just cut off the top. improvement was about 20% - very cool
I’ve been playing with a few of the whip antennas.
What would be the difference for using the telescoping antenna at the 1/2 wavelength size? (since it can’t get to 1/4).
Is there an optimum length for the telescopic antenna?
It does not matter whether antenna is telescopic or solid wire. Optimum length is 1/4 wavelength. The 1/4 wavelength antenna has advantage of low impedance, close to coax impedance (50 or 75 ohms) and dvb-t impedancs (75 ohms).
If it is is a 1/4 whip:
- with horizontal radials, or horizontal metallic plate, impedance is 35 ohms,
- with 45 degrees slanting radials, impedance is 50 ohm,
- with 90 degrees slantig radials ( = a dipole), impedance is 75 ohms.
- with a Can, impedance is between 75 & 50 ohms depending on Can dimensions.
The 1/2 wavelength antenna, although higher in gain than 1/4 wave antenna, has impedance of several hundred ohms. This high impedance severely reduces flow of signal from antenna to receiver. As a net result 1/2 wave antenna performs worst than 1/4wave antenna. The only way to take advantage of 1/2wave antenna is to have some impedance matching arrangement between antenna & rest of the system.
Would it make any difference if the antenna was mounted horizontal or vertical? Was considering mounting on my outside metal gutter.
Mode S signals are vertically polarized. If you mount the antenna horizontally it will be almost completely deaf.