If you could get it above chimney height it’ll make at least as much difference again. (might need management approval)
Spray it with paint.
Also apply silicone or similar sealant over and around the F-Connector (in which the whip is inserted). This will prevent rain water & moisture from entering the connector & the coax cable.
I fully agree.
Try 1/4 wave also, to find out which length works better for you.
[quote=“abcd567”]
I’ve currently got an amplified 8-leg spider at the rear of the house on a length of PVC pipe, but unfortunately it’s not above the roof line.
I definitely want to get the amplified spider on the chimney but I don’t currently have any ladders long enough to get up there.
Thanks abcd567 very much for the tips.
I am currently on business travel but looking at my stats for the last 3 days having the antenna extended to approx 5/8 wave has provided the best Mon/Tue/Wed that I have had over the last couple of weeks. I will check back further in my logs when I get home to confirm.
Ordered some parts and hopefully they will arrive in time for me to try building an antenna on the weekend.
SIMULATION OF 2 DIY ANTENNAS
Simulation 1 of 2: SPIDER (8 Legged )
Gain = 1.7 dBi
SWR (75 ohms) = 1.3
Simulation 2 of 2: CANTENNA
**Gain = 1.9 dBi
SWR (75 ohms) = 1.5
**
The Birth of Cantenna
The Spider has existed for a long time, frequently used by amateur radio operators.
A close relative of Cantenna, the “Coaxial Dipole” has also existed for a long time, but was not so popular as the Spider. It was mainly used in rubber ducky antennas of Hand-held radios. It has a very narrow diameter sleeve, which was made of a copper tube or pipe, slightly bigger in diameter than coax. It did not have the performance as good as it’s newly discovered wide-bodied relative, the Cantenna.
The Cantenna was born only about an year ago, as a result of series of my experiments on CoCo. In June 2014, I added a decoupling sleeve, made of Pepsi can cut to 69mm, at feed point of my CoCo. This experiment was not successful, possibly as the sleeve was too large in diameter for decoupling purposes.
Instead of discarding the sleeve (Pepsi can cut to 69mm), I decided to use it to make another type of antenna, the sleeved dipole, and it proved very successful. It’s performance was much better than “Coaxial Dipole”. This was birth of Cantenna.
After 2 months I renamed sleeved dipole to “Cantenna”.
**(1) Cantenna Invented by me - The Very First Cantenna ever made (June 2014): **
http://forum.flightradar24.com/threads/3831-best-antenna?p=53094&viewfull=1#post53094
(2) Name “Cantenna” chosen by me (Aug 2014) - Originally I called it “1/2 λ sleeve dipole”:
http://forum.flightradar24.com/threads/3831-best-antenna?p=55872&viewfull=1#post55872
I’m guessing you realise the term Cantenna has been used for years certainly for WiFi antennas?
I remember making one about 10 years ago with a Pringles can!
[quote=“antthomas”]
You are right, as far as the name is concerned.
(1) I discovered this duplicate use of name “Cantenna” several months after I coined this name independently without knowing that it is already being used for another antenna. In fact I have recently found that the name cantenna was used for a product “dummy load” as far back as 1961 to 1991. Obviously both me and the designer of WiFi horn/waveguide antenna have used this name without knowing that it has been already used.
(2) The principle & design of the cantenna you are referring to is completely different from the principle & design of cantenna I made. The only thing in common is that both use a can, and hence both adopted the same name. The cantenna you are referring to uses the can as a horn/reflector/waveguide, the actual antenna being a monopole or dipole located inside the can. My cantenna uses can as groundplane and a coaxial sleeve, and the whip outside the can acts as radiating element.
Below is a table showing SWR vs Transferred voltage & power.
In case of a Transmitting antenna, it is the power (watts) radiated which is important. The radiated power is directly proportional to power transferred to antenna. Hence it is the power (watts) transferred to antenna which is important & is the criteria for transmitting antennas.
In case of a Receiving Antenna (our case), it is the voltage transferred to receiver or amplifier which is the criteria. This is because the front end of Receiver or Satellite Amplifier (to which antenna is connected) is a pre-amplifier which is a voltage/current amplifier. Hence for ADS-B Receiving purposes, it is the Voltage transferred (yellow highlighted columns of the table below) which is important and applies.
Please see the photo below. The Satellite amplifier’s output is rated in microVolts, not in microWatts.
Question to abcd567 - I am trying to build an 8 element CoCo antenna for 978 MHz. I bought Remee RG6/U cable for the reason that this was the only cable with known velocity of 82% (i.e. 0.82 velocity factor) I can find. Based on the formulas I saw so far, I calculated the CoCo Remee cable element length for 978Mhz to be 125.77 mm. Yet, after I build the antenna and put it on trail I got inferior results comparing to 4 leg spider (76.69 mm “arm” length). I will try to play with the impedance matching, but I am not sure how you calculated the “44 mm twin wire transmission line for impedance matching for 1090MHz” and how to adjust that length for 978MHz. Could you shed some light on the calculation?
disappointing…
I’d be tempted to slide the elements apart a little (1mm at a time) to see if it improves, if it goes the wrong way cut them down - say - 3mm then try adjusting again.
… if it all goes wrong, you have the spider that works better anyway.
I definitely will try your suggestions 
I built my Cantenna using a Bush’s 8.3oz baked beans can that is just over 70mm long (1mm beyond the required 69mm 1/4 wavelength) that had a pull off end which saved cutting and exposing sharp edges etc.
Ir’s been running now for 3 days and is actually working better on my windowsill 5 feet off the ground than the FA antenna mounted in the same location, but have not been running it long enough for it to be a good comparison due to daily variations.
I hope to get the FA antenna outside and up on a mount this weekend.
I find it odd that a Cantenna would work better than the FA antenna but perhaps it’s not a good performer lower to the ground due to it’s gain pattern (I don’t think anybody has modeled it yet to compare plots).
We also have the complication in Florida that the interior of our attics is lined with metal foil to reflect the heat and keep the house cool which will certainly block and or distort the receive range anyway.
The reason may be as follows:
Cantenna is low gain (1.9 dBi) but has excellent SWR/impedance match. Radiation Resistance is 50 to 75 ohms, and Reactance nearly low, a close to ideal situation. It is therefore a very good performer for SHORT runs of coax where there is extremely little attenuation.
On longer run of Coax (high attenuation) and NO amplifier, a high gain antenna like FA or a good CoCo will perform much better due to their high gain which will compensate for attenuation by long run of coax.
On longer run of Coax (high attenuation) but WITH amplifier, the high gain antenna will still perform better, but the difference between Cantenna & a high gain antenna will will be very much reduce due to amplifier’s high gain & automatic gain control.
Another factor may be the radiation pattern of the two antennas. In a particular location, more aircraft fly at a certain height range. An antenna which has radiation pattern matching that range of height will perform better than the other antenna having maximum resposnce for a different height levels. This phenomenon is totally location specific.
Another factor, as you already mentioned may very likely be the height of antenna above ground, affecting it’s characteristics, the FA antenna being a better performer at several meters above ground, while Cantenna gives good performane closer to the ground. When you move both FA Antenna & Cantenna to roof, the effect of ground height will be more clear.
Thanks Peter. A very good practical tip. I will try it with my not-so-good CoCo.
(1) The twin-wire was a failed experiment as I discovered it in repeat test few days after posting the twin-wire as successful design 
. Please see this post, (read upto end of post) http://forum.planefinder.net/posts/1692/.
(2) The length of twin-wire was calculated based on the CoCo impedance from simulation, and using Smith chart to determine the length. See this post, http://forum.planefinder.net/posts/1317/
(3) The reason for failure, as I discovered recently, was that I did not insert a capacitor, wrongly thinking its effect negligible.
(4) See a recent method I designed & tested successfully, inserting a length of coax, 75% to 90% of Coco’s 1/2 wavelength element + a small capacitor, 1.5 to 3.5 pF, between CoCo & Feed Coax:
(5) I am now considering using twin-wire with capacitor. The twin-wire has advantage, over a piece of coax, of being adjustable continously by pushing in/out, while piece of coax requires making many pieces of different lengths and trying these pieces one by one. However since the twin-wire’s characteristic impedance is 211 ohms, while piece of coax’s characteristic impedance is 75 ohms (same as feeder & receiver impedance), this will require calculations & manupulations. I will come back with this method if I succeed.
(6) I am going to try a variable trimmer capacitor, adjustable from 2pF to 5pF, instead of trying different fixed value ceramic capacitors. This will help to use the optimum value of capacitor.
