Built my first antenna and doubled my coverage


5/8 wavelength spider results in
Lower Gain
Higher SWR
Poor Radiation Pattern

Please see below 3 simulations
1 of 3: whip 1/4 wavelength, radials 1/4 wavelength
2 of 3: whip 5/8 wavelength, radials 1/4 wavelength
3 of 3: whip 5/8 wavelength, radials 5/8 wavelength

Simulation 1 of 3: Whip 1/4 wavelength, Radials 1/4 wavelength

Simulation 2 of 3: Whip 5/8 wavelength, Radials 1/4 wavelength

Simulation 3 of 3: Whip 5/8 wavelength, Radials 5/8 wavelength


The simulations posted above were done using software 4NEC2.
I have now done the simulations again, this time using software MMANA-GAL.

The Gain in the two simulations are quiet different. 4NEC2 shows reduction in gain, while MMANA-GAL shows increase in Gain.

The SWR in two simulations tally, and both show a very high SWR.

MMANA-GAL Simulation 1 of 3: Whip 1/4 wavelength, Radials 1/4 wavelength

MMANA-GAL Simulation 2 of 3: Whip 5/8 wavelength, Radials 1/4 wavelength

MMANA-GAL Simulation 3 of 3: Whip 5/8 wavelength, Radials 5/8 wavelength


Very interesting - thanks for the analysis.

I am wondering if it’s worthwhile adjusting the length of the telescopic antenna that came with the Nooelec USB SDR to be either 1/4 wave or 5/8 wave (or as close to it as feasible) instead of just leaving it fully extended.


Can’t hurt to try… You may learn something!



I put it at 5/8 wave (or as close to it as you can’t really see the feedpoint in the telescopic antenna but I assume it’s close to level with where the coax exits the antenna).

Has any modeled the FlightAware antenna that is currently on Amazon ?


Sorry, I dont have details of Flightaware antenna (design, dimensions), so I cannot model it.

**Suggestions: **

(1) Place the magnetic base of telescopic antenna over the lid or bottom of a large food can made of iron. This will help in 2 ways:

(a) The magnet in antenna base will cling to the can and hold antenna in position.

(b) Antenna’s 1 inch dia base magnet provides the groundplane (the braid of coax is connected to it). This is too small a groundplane. When magnet clings to the food can, the food can will enlarge the groundplane and reception will greatly increase.

(2) After purchasing DVB-T dongle, the very first thing you should purchase is a pigtail, with MCX male pin at one end (for dvb-t) and F-female connector at other end to connect regular antenna coax (RG6, RG58 etc).

The MCX male connector of pigtail is delicate, and sometimes breaks.Better order 2 pigtails, one in use, one spare, to be immediately available if mcx connector of one gets damaged. These are not costly ($3.80 + free shipping from China)

http://thumbs3.ebaystatic.com/d/l225/m/mdrImDH82MNX8tYr1OYk0sA.jpg http://m.ebay.com/itm/F-female-nut-bulkhead-to-MCX-male-RG316-cable-jumper-pigtail-15cm-/320847773379

http://thumbs4.ebaystatic.com/d/l225/m/mnn821kGZ8JMM1I-6zITnuw.jpg http://m.ebay.com/itm/RG174-6inch-RF-pigtail-F-TV-female-jack-pin-bulkhead-to-MCX-male-straight-Cable-/251399601339

**(3) Please also see this post:




This is post #701 for this thread started by JPolch in November. I’d like to take a second to express my appreciation of the broad range of topics and information that have been posted. I have learned much and avoided many ****-ups. Thanks to all the posters.
joel posting from sites 704 - 10333 :mrgreen:


The difference from moving an antenna from inside to outside is huge. At least double the message rate.

My double glazed windows definitely prevent a lot of EM signals getting in.

Simple cantenna initially with about 1m coax inside to 6m coax (2 x 3m premade with F connectors linked) outside. Only set it up because I now have a spare RPi and RTL-SDR dongle to play with.

Plus an old bit of meccano :smiley:

Though it’ll probably go rusty too quickly.

Not the best picture because it was getting dark.



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.



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.




Your situation is something like this:


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 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): **

(2) Name “Cantenna” chosen by me (Aug 2014) - Originally I called it “1/2 λ sleeve dipole”:


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!



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.