FlightAware Discussions

Which one is the best antenna?

No, they are not that bad, but certainly not 12dB either.
Observably better than a PCB Dipole, but not 3dB better.

You’re saying the performance of this stripline antenna is “underwhelming” but it’s better than the cantenna? Can you elaborate regarding message counts, range and number of planes tracked with each antenna at your location? Thanks.

(I wonder what they’re referencing the supposed 12dB of gain on, a 1/2 wave dipole, a 1/4 wave ground plane, the theoretical isotropic antenna, or something else?)

the ‘cantenna’ is a fun, rainy sunday afternoon project, but is not a performance antenna. The fact it often out-performs the PoS mag base antenna provided for tv reception only goes to show how bad the mag base is.

My comparison site has been offline since last year (I’ll bring it up when I get back to it).
To be honest, the performance simply wasn’t different enough to warrant a lot of effort

Don’t overthink it. 12dB is just a number snatched from the air.
It’s just a marketing claim they hope no one will test.

Have a play when yours arrives and make up your own mind.

There’s absolutely nothing wrong with the cantenna. It’s basically the same as the 1/4 wave ground plane antenna with sloping radials which is a tried-and-true antenna design that offers around 2.4dBi of gain. This style of antenna (the sloping radials style, not the beer can style, but electronically they’re pretty much the same thing) is in use worldwide for various commercial and amateur radio applications. It took me less than an hour to build mine, a few minutes to get up in the air as high as is possible here, and I was seeing more airplanes from farther away. They’re great easy-to-build and inexpensive starter antennas.

The mag-mount antenna with its various screw-on elements is meant to be more of a general-purpose starter antenna just so people can begin using their new SDR dongles right away. The antennas are wide-band so they don’t perform optimally at any given frequency except perhaps by accident. Antennas are tuned circuits but things like the telescopic whip on a mag-mount are more like probes sticking into the air, they’re not really even antennas. However, they’ll usually pick up something and any antenna is better than no antenna.

Regarding the stripline, the description says it has four 1/2 wave antennas, so I still have no idea how they came up with “12dB” of gain when a 1/2 wave antenna has 2dB of gain. Even if they’re phase-matched so they’re additive it would only be 8dB total. Still, that’s better than the 1/4 wave ground plane’s 2.4dB so I’ll take it.

The stripline should arrive within another week so I guess I’ll find out what it’s like then: see if it does anything at all to increase my range (doubtful, unless it sneaks out at night and dynamites neighbors’ houses and cuts down trees) and numbers of messages/planes. As I’m not feeding FlightAware I don’t have a stats page so it’ll just have to be anecdotal as VirtualRadarServer’s ad hoc report form doesn’t have message count or distance as search criteria. I suposse I could do an SQL query both before and some time after changing antennas, sorted by the two criteria, and compare the two results, and I think I’ll be too anxious to start using the new antenna to have time for SQL queries.

Quote from an old post (August 2015)

NOTE: Simulation results give only indicative figures. Reliable results are those which are obtained from actual measurement, using VNA + Directional Coupler for VSWR, and Anechoic Chamber for Gain.

That’s interesting. I thought it was a form of 1/4 wave ground plane with sloping radials, which per all the research I’ve done (ham radio sites, excerpts from the ARRL Antenna Handbook, etc.) plus ancient past experience normally has around 2.4dBi of gain. Is your gain measurement dBi (referenced to a zero-gain isotropic antenna) or dBd (referenced to a half-wave dipole?)

(1) The gain figures are in dBi

(2) It is not a measurement, it is result from simulation of the antenna models in a computer software. As I have mentioned in note, these computer simulation results are only indicative, and not accurate enough to blindly rely upon.

(3) The purpose of the old quoted post was comparison, i.e. to show that results of simulation of both the1/4 wave ground plane with sloping radials, and Cantenna are more or less same.

@jaymot

SKETCH 1 of 2: Die Koaxialantenne (The Coaxial Antenna)

 

 

SKETCH 2 of 2: Cantenna

 

Cantenna is a hybrid of ground-plane and vertical-dipole, with one leg of it acting as a balun. This balun type is known as a bazooka balun or a sleeve balun or decoupling sleeve. Please see above sketch.

When a dipole is fed by a coax, due to common-mode current on the coax shield, the coax itself becomes part of antenna and influences antenna performance. In order to eliminate/reduce influence of coax on antenna performance, the common-mode currents on coax shield should be eliminated or at least minimized.

This can be done by making the impedance to common-mode currents on the coax shield very high while keeping the feed circuit impedance to dipole unaffected. This is what is done by bazooka or sleeve balun. As a result there won’t be much common-mode current, while the feed current to dipole will be unaffected by the balun.

@abcd567 Ah! As my cousin’s uncle used to say when working on motorcycle engines, “NOW I see! NOW I understand!” The bottom of the can’s the ground plane while the sides are a decoupling sleeve. Thank you for taking the time to explain to me. Also for designing the cantenna in the first place: it got me started seeing more airplanes at a longer range vs. the mag-mount antenna.

Does the decoupling sleeve negate the need for a choke balun in the coax near the antenna then, or would you still recommend one?

You know how the bottom of fizzy drink and beer cans is rounded inward to help prevent the can from bursting due to pressure from the carbonization? I wonder how it would affect the radiation pattern and impedance antenna if that were forced outward such that the top (ex-bottom) of the can was rounded slightly upward or “exdented” rather than indented? I’m guessing it would probably impact the impedance more than the radiation pattern, possibly making it closer to a 50 ohms antenna rather than 75 ohms and perhaps giving a somewhat better VSWR when using, say, LMR400 coax?

Never thought of it. Most likely adding a choke balun wont harm, even if it does not benefit. Only by trying it one can find.

 

One can only know by trying it. I never tried it.

By the way, I did some research on isolation chokes which is what the “choke balun” is. Their purpose (along with similar ferrite-core chokes on the coax near the antenna) is to prevent a transmitter’s RF energy from being induced into then radiated by the coax shield which otherwise can act as an inductor. Otherwise you get power being radiated in unwanted directions which reduces efficiency, causes interference to local receivers like TVs, and can cause RF to leak back into the shack. They’re not really needed for receive-only antennas as the signal strength of received ADS-B and other radio messages is extremely low compared to, say, a 100w transmitter. They don’t hurt anything other than requiring longer coax so you can form the loops and the longer the coax the more loss it has, but they don’t help anything either.

I still haven’t tried “outdenting” the cantenna yet to see what happens. I’ve been using one of those Chinese-made PC board stripline antennas in the white plastic housing instead of my cantenna. I still plan to try it someday though.

Agreed, I wouldn’t choke an ADS-B aerial, there’s little point.

Winding chokes can be quite satisfying though. Here’s one of many GM3SEK chokes I’ve wound.

choke