Antenna monopole


I want to build monopole antenna with 4 groundplan (spider antenna) for adsb, but which is the best for recive 1/2 lambda monopole antenna or 1/4 lambda monopole ? And please give the explanation

Thank you


Rather than giving you a fish, may I ask what your own research suggests?


For simplicity, go 1/4. If you really want to experiment, try 5/8.



As the whip length is inreased, gain and directivity increases (advantage), as well as impedance/swr increases (disadvantage).

A 1/4 wave whip with 45 degree slanting earth plane radials, has impedance of nearly 50 ohms ans SWR less than 1.5 (advantages), but has low gain of 2 dBi and a ball like radiation pattern (disadvantage).

  1. Prepare a SO239 connector with 4 radias as in the photo.

  2. Prepare 3 whips, 1/4, 1/2, and 5/8 wavelengths.

  3. Solder 1/4 whip to SO239.

  4. Put antenna on trial. Observe for a while (better run VRS plot for couple of hours).

  5. Unsolder the 1/4 whip, and solder 1/2 whip, and repeat step 4.

  6. Unsolder the 1/2 whip, and solder 5/8 whip, and repeat step 4.

I have not done simulation of Monopole Spider for varying whip lengths, but have done simulation of a Dipole, which is given below. A monopole with ground plane radials will behave in a similar fashion (for monopole, use 2x whip length = dipole length)


Now I have done simulation of 4-legged Spider with variable length Whip. Here are the results:

Click on the image to see larger size
Click once more to see full size


hi abcd567

would appreciate if you could run one more test for cantena and spider for 75ohm cable
(where I live, can’t find 50ohm, so I am using 75ohm sat cable).
which software produces these graphs ?

thanks/best regards
Evangel / 59070


Ok, I will run simulation for Spider and Cantenna for 75 ohm, and post the results.

By the way I never used a 50 ohm cable, as it is not readily available in local stores, and is costly when ordered on line. I use 75 ohms RG6 cable which is used in Satellite Dish and Cable TV, and is readily available everywhere. In fact I purchased RG6 from a Dollar Store where I got a 75 ft coil for Canadian $6.99 :smile: .

Actually the use of 50 ohm originated in early days of Wireless when some of the wireless equipment manufacturers adopted it. Gradually 50 ohm became the de-facto standard on HF and VHF, although it has no advantage in being 50 ohms. It could have been 30 ohms or 60 ohms as well and would perform equally good.

When the Cell Phone were invented, these also being wireless system, found it most convenient to adopt an already existing system and standard of 50 ohms. However when Satellite TV started, it was in GHz range, and they found a 75 ohm system better suited for this application, and used 75 ohms RG6. Their LNAs and LNBs are also rated for 75 ohms. The DVB-T being designed as a Digital TV Receiver, has an input impedance of 75 ohms. The Pro Stick has a front-end amplifier chip, most likely intended for Cell Phone use, and has an impedance of 50 ohms.

The cable impedance 50 or 75 ohm does NOT affect Radiation Pattern or Gain of antenna. It only affects SWR, which can be translated into dB loss due to mismatch. For each inter-connection between 75 ohm and 50 ohm, the mismatch loss is only 0.177 dB, This is negligible if you have only one or two 50>>75 or 75>>50 transitions.

I use 4nec2 simulation software.



thanks a lot abcd. very educational…as.usual :grinning:

Evangel / 59070


The performance difference between 50 and 75 Ohms is minimal. The lower price and availabilty of RG-6, makes using it a no brainer. This is the cable I use for my hamradio antennas as well. Actually, at the ADS-B frequency, RG-6 is better.


The SO-239, while a tried and true connector, is definitely not a good performer at 1GHz.
Here’s some numbers that show how bad it is at half that frequency.

Click on the graphs to see the results of testing at 1GHz.

For every connector in the line, an additional 0.1 to 0.2 dB is lost. (perhaps more, depending on the type of connector, and how much care was taken when it was attached to the cable - (user-built cable assembly))

Here’s an explanation of how the 50Ω impedance was chosen.


Bill KR6K


Yes, you are right, but still SO239/PL259 is the most popular flanged panel mount connector used for a ground plane antenna. It has its root in VHF (30–300 MHz, 10 to 1 meter) communications when Amateur Radio hobbyist started using it to make their DIY ground-plane antennas.

Some adsb hobbyists have realized this and used N_connector (panel mount type) instead of SO239. However I have not seen anyone reporting more than slight improvement in reception when switching from SO239 to N-Connector. May be N-connector gives substantial improvement in Transmitting scenario due to reduced power loss.


True. Even if the system is “All 50 ohms” or “All 75 ohms”, there are connectors which introduce loss. The loss in run of coax is the largest component of all the losses. The summ total of these losses is around say 2 to 4 dB. Two 75/50 transitions will add 2x0.177 =0.35 dB loss which is very small compared to total loss of 3 or 4 dB.

That goes to show that 50 ohms was found most efficient on 4 MHz, which is at the lower end of HF band (3–30 MHz, 100 to 10 meters).

Free-to-air TV NEVER USED 50 ohm system. Initially TV was only transmitted on VHF (channels 2 to 6 on 45 to 90 MHz ,and channels 7 to 13 on 175 to 215 MHz). The TV System designer did NOT use the already established 50 ohms. They opted for 300 ohm system. TV those days used a 300 ohms flat twin-lead from roof-top TV antenna to the TV set. Later, TV started using UHF band (Channels 14 to 83, 470 Mhz to 890 MHz). This resulted in 300 ohms twin-lead system replaced by 75 ohm coaxial cable.

Satellite Dish TV (1GHz and above) also adopted 75 ohms system, being better suited at GHz frequencies.

All above leads to one logical conclusion:
Since ADSB is also in GHz frequency band, it is natural and beneficial to use 75 ohms system which is well established for this frequency range.


How about this one which is built on ZERO :slight_smile: Connector.

QUICK SPIDER - No Soldering, No Connector

Click on the image to see larger size


Quite true.

But, the “UHF” connector was developed at a time when what is now called VHF was called UHF.

If “UHF” connectors really did perform well at 1GHz, secodary surveillance radars would use them. But they use SMA and N connectors. I maintain a Digital Airfield Surveillance Radar and a Pulsed Doppler Weather Radar, so I’ve first-hand experience with this. (part of the DASR is a secondary radar that operates at 1030/1090 MHz)

I’ve been an electronics tech for 44 years, and a ham for 50. In that span of time, I’ve never seen the “UHF” connector used at anything above 300 MHz except for ham radio gear.

Even Amphenol says the “UHF” connector is rated to 300 MHz, and the impedance is non-constant.

My point was simply that there are better choices than the “UHF” connector. If nothing else, a properly assembeled N connecter is waterproof. None of the UHF connectors are.

As you said, many folks use the UHF connectors and are happy with the results, which fits in with
“use-what-you-have” and the whole homebrew concept - and in the end, that’s what really matters.


thank you very much for helping :slight_smile:



Below are simulation results for Spider and Cantenna for impedance 75 ohms.
Please note that

  1. The Gain & Impedance are same for 50 ohm & 75 ohm.
  2. SWR is different for 75 & 50 ohms


Click on image to see larger size.



Click on image to see larger size.


thanks abcd567 appreciate it :smile: