Make a simple whip antenna for 5.8Ghz- linear polorized FPV video

Circular polarized antenna is the most popular antenna for FPV flying with 5.8Ghz FPV system, such as skew planar or cloverleaf antenna. Although they do allow better range and reception, there are situations where you are better off using simple linear polarized antenna, such a dipole antenna.

Linear polarized antenna is a type of FPV antenna, and it can be made very small and light weight, especially useful for weight-critical application, such as on a micro quadcopter.

In this tutorial I will show you how I make my own dipole antenna to be used on my DIY 5.8Ghz FPV combo with good result.

What’s inside a Dipole antenna

If you open a dipole antenna that comes with every 5.8Ghz video transmitter or receiver, you will find it’s just a simple Coaxial Cable inside.

linear-polarized-antenna-5.8ghz-dipole-fpv-inside-length

dipole-antenna-inside-fpv-video-transmitter-5-8ghz

But what we really need to know is the length of the exposed wire (without shielding) affects the signal frequency that it receives. If the wire was about 12.92mm then it’s tuned for 5.8Ghz, but only just a little shorter e.g. 12.70mm, and it becomes tuned to 5.9Ghz. The same theory applies to 2.4Ghz system, it’s the same such as radio receiver.

In this example below, you can see the antenna element is not very precise for 5.8Ghz, it could have been be a little longer.

dipole-antenna-fpv-video-transmitter-length-5-8ghz

Build from scratch using Coax cables

If you don’t have a ready-made linear antenna available from VTX/VRX, you can make one using coax cables.

There are many choices of coax cables:

The thicker usually means the shielding is better and it should work more effectively. But if you want something as light weight as possible while range is not the most important factor, you can consider using RX replacement cables, these have the thinnest possible shielding.

You might also need a SMA connector. But if you are soldering the antenna directly to the video transmitter than you won’t need one.

 

Calculation of wire length for antenna

If the antenna is a quarter wave monopole antenna, then its length should be a quarter of the signal wavelength.

Length (m) = c / frequency / 4

Where c is the speed of light.

The more precise wire length is, the better signal you should get ideally. If you are using Coaxial Cable, it doesn’t matter how long the cable is with shielding, all that matter is the exposed wire (the part that is without the shielding). Alternatively if you are using a wire like copper wire, just cut it to the calculated length and you have a monopole antenna :)

tx5823-5-8ghz-200mw-video-transmitter-antenna-wire-cable

Further reading: 5.8Ghz Frequency Bands For different FPV brands.

28 thoughts on “Make a simple whip antenna for 5.8Ghz- linear polorized FPV video

  1. Scott Walker

    Other than size and ease of placing on a build, what are the situations where you would want to use a small dipole instead of a CP antenna? Wondering becasue of the Emax Magnum I just purchase has the option for both. I will primarily use that quad for racing and within 200 yds (175 meters) of distance.

    Reply
    1. Jesse

      In the case of say an Echine Aurora 100 for example – crash survival being a higher priority than range, possibly. It’s why I’m here anyway.

      Reply
  2. jacobbloy

    could this work in reverse also? measure your whip antenna and the choose the best channel for your antenna?

    (299792458 / 4) / length = best Channel

    Reply
    1. Jon

      To keep the transmitter happy with the antennas impedance. A full wavelength is fine, and any 1/4 of that. But, we use 1/4 as a standard because it is short and still allows slight gain. There are other lengths that work equally as well, but need matching circuits to keep the transmitter from burning out. 1/4 is just the simplest length we use.

      Reply
  3. Serg

    What is that little cylinder metal shape is on the top of the antenna?
    What is the purpose of having it?
    Thanks

    Reply
  4. TheCric

    >>If you are using Coaxial Cable, it doesn’t matter how long the shell is, all that matter is the exposed wire (the part that is without the shielding).
    I’m not so sure of that. Coax simply help to not emit but I have forgot how it works exactly (I studied that a long long time ago but even with coaxial cable you may have to take in accounts length to open the wires from the coax to antenna where the signal is the strongest because of standing waves. Maybe this can helps for those who understand : solar.fc.ul.pt/lafspapers/coaxial.pdf , so as a rule of thumb, a cable cut at n times waves length is better than a random cut, and you have to take in account the celerity into the coax (2/3 usually). To be verified by somebody who has studied wave propagation more recently than me.

    Reply
    1. Jon

      You are mostly correct. After 35 years in broadcasting, I can definitely say the length of the shield portion is DEFINITELY important. I would just say go the other way though. Avoid any length that is resonant. Random lengths that don’t equate to a 1/4 wavelength (or 1/2, 3/4, 1 etc) won;t be a problem.

      With the short wavelengths 5.8 uses, it’s pretty easy to hit one if you just use a random length.

      Reply
  5. Jimmy

    hello.
    I fly on 5733 mhz . Speedof light is 299792458 m/s .
    299792458/5733/4= 13.073,105616605
    my wire length is 13.073 mm ??

    Reply
  6. Kirk

    Hey Oscar !!!

    Thanks for this I love the simplicity of the single copper antenna on my whoop. Yesterday I broke my vx1100 cloverleaf and cut one of the leafs down to 12.92mm and soldered it back on to the center pin of where the other one was and voila !!! reception… cant really tell but I think its better than before. don’t really expect much from the 10mw cam but it works well enough for cruzin’ around the house and I didn’t have to buy another !!!

    is 12.92 the magic number for 5.8 or is there another resource I should refer to as I fall down this rabbit hole ???

    Thanks again and Cheers !!!

    ~Kirk

    Reply
  7. Jos G.

    The Taranis and the others you mentioned are for 2.4Ghz. Oscar’s blog post is right on the money for a 5.8ghz antenna. The picture shown is a sleeved dipole not a regular dipole. The sleeved dipole has a ground plance “bullet” of the same length as the exposed wire. I’ve had really good results with a basic dipole – exposed wire around 12.9mm length. The speed of signal is slower in the wire than in the air hence the very small discrepancy in the calculated 1/4 wavelength and the length of the wire.

    So to recap – 2.4Ghz uses a long wire for 1/4 wavelength based on the frequency calculation.
    For 5.8Ghz video the exposed wire length = ~12.9mm

    Reply
  8. Michael Nørgaard

    The simple cobber antenna in your picture. What lenght is that. I find it hard to understand the way you explain “bare” wire lenght og a whip antenna. There is no bare exept at the very bottom where antenna ond two negative wires stick out. The simple splutioon with jus one cobber cable seems more easy and if it works as godd I’ll use it. Building a H8 mikro indoor surprise for the son. Like this one: youtube.com/watch?v=DREHnDY1WVk

    Reply
    1. Oscar Post author

      Please search for “length” in the article you will find the number. “bare” is basically the exposed wire without shielding.

      Reply
  9. David

    The bullet shaped piece is not plastic, it’s metal and serves as a counterpoise. It and the exposed center conductor together make a dipole antenna.

    Reply
    1. Chuck Fore

      That metal bullet shaped piece is a counterpoise for the antenna. It’s similar in operation to a ground plane on other antennas. It basically balances the antenna and makes it a certified dipole.

      Reply
  10. Pilotgeek

    The antenna is shorter on some due to the velocity factor of the coax cable. 11.71 is likely correct when tuned to match the cable.

    Reply
  11. Albert

    If I search the web for 5.8 linear antenna length I get very different results. You saying here the correct length should be 12.92 mm. On rcgroups some folks are saying 2.3 and 2.7mm. wtf? So what is the right length?

    Reply
    1. Oscar Post author

      everywhere I have seen suggests around 13mm, and I have been using that in my DIY antenna and getting decent results..
      if you measure any existing commercial antenna you should also they use this length…
      lastly, i would really appreciate it if you could join the forum: http://intoFPV.com … I don’t want to miss your comment, because I only check my blog comments only once a week, but I use the forum daily!

      Reply
  12. Casper

    I guess it’s better to calculate the wavelangth as ”Length (m) = (c / frequency ) / (4 x 0.93)”
    The velocity of electricity in copper will effect the lengt of the antenna.

    (Sry for my english)

    Reply
    1. FAERRY

      i’ve been searching everywhere for this! wouldn’t that make it slightly shorter? i’m trying to figure out why the taranis and other 3rd party replacement antennas i’ve bought are around 32.5mm instead of 31.2mm?

      Reply

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