Antenna construction cross Yagi, VHF band, 6 elements, 2 meters long, circular polarization, 9dB gain

Useful links:
Notes on 6-Element Wide-Band 2-Meter Yagis
http://on5au.be/content/a10/vhf/32m.html
Antenna Circular Polarization
https://www.qsl.net/sv1bsx/antenna-pol/polarization.html
Yagi Uda Antenna Calculator
https://www.changpuak.ch/electronics/yagi_uda_antenna_DL6WU.php
Coaxial Power Split Calculator
https://www.changpuak.ch/electronics/coaxial_power_split.php
Calculate the extra length of coax feeding the other dipole for circular polarisation
http://dg7ybn.de/Building/xpol.htm

In the final version, after few weeks of testing (satellite reception), I passed the two coaxial cables through the back of the plastic box, like this:

This allows the cables to run easier towards the back of the antenna along the central tube.

 

 

 

A thing that I liked to do differently is this: You see the connection of three cables for phasing and matching

Don’t connect the cables like this, all running in the same direction
because they create an ugly bulge, and can’t go nicely along the central tube.
Instead, try to connect them exactly as in the diagram, with the main coax going towards the back, and the other two towards the front. For the longer cable you simply make a loop, which looks nicer.

One thing I should have shown in the video is how to connect the shield of the three cables. The copper is easy to solder, but the 75Ω coax usually has an aluminum shield. I used a crimping pin like this
     

About the connections inside the plastic box, remember this: the longer cable goes to the Horizontal antenna. Horizontal antenna is the one towards the front.

So the blue antenna is for Horizontal and it’s more to the front. Also respect the side where to attach the core and the shield. Keep the central wire of the coaxial as short as possible. The shield can be prolonged as needed (the green wire in the photo)

Of course, the antenna can be rotated as you like, there’s no Up or Down, as long as you respect the schematic.

Another thing to pay attention to: The aluminum tube has a thin layer or aluminum oxide which is electrically insulating, so before attaching the cables, use a bit of sanding paper to clean the surface. Do this before you finish the job, because after it will be very hard. I know.

Now the main coaxial cable goes to the radio. Because the losses are big with such a thin cable over long distances, I used a very thick coax that I had laying around for years. To buy such a low loss performant cable is very demanding on the budget. This is how I did it (observe the difference in thickness).

I used an N-type connector at the bottom of this cable. Here you see attached both the VHF and the UHF cables to a coaxial relay for cross band satellite operation.

Drawings…
VHF antenna design, 6 elements, 2 meters long, 9dB gain


UHF antenna design, 2 meters long, 12 elements, 12dB gain

Be aware of the cable loss!!!
I used this Antenna (Vector-Network) Analyser to measure the cable loss…

Here you see the loss of a 15m long, RG58 (50Ω, Ø4mm, ordinary) cable.
The loss is considerable in VHF – 5dB, and enormous in UHF – more than 9dB.
-9dB means that only 10% of your power is passing thru.
Almost the entire antenna gain is lost over the cable.
So a high quality, thick, expensive cable is mandatory, especially in UHF. RG58 is to be used for short connections, of less than 3 meters.

 

6 comments

  1. Hi,

    Thanks for very useful information.

    Is it possible to sell a pair of VHF & UHF satellite antenna qty 01 each ?
    Please let me know the delivery period and cost ?

  2. Hello Kumar,
    Sorry, I can’t do that.
    The materials are very cheap, but the manufacture is entirely done by hand, no automation at all, and this will be very expensive. Plus the shipment is very complicated, because of the big size.
    I hope you can find someone near you, who can do it for a more convenient deal.
    Sincerely,
    Racov

  3. Hi Racov and congratulations on your work, really fantastic.
    Please take me some doubts regarding the 432MHz antenna, Can all the elements and boom have the same diameter as the one of 144MHZ? I think there may be problems but… I ask this because my local supplier only sells 5 mts tubes and it was easier to cut. And, can the elements be screwed to the boom in the center point? And whats the full wood antenna holder size, or recomended?
    One other question, after 2 years what’s your opinion on the antennas perfomance???
    PS: you were very brave on mounting the antennas with that wind…hihihihi
    Thank you and sure i’ll buid the antennas because of the dozens i’ve seen so far seem the most reliable and it’s what i was looking for.
    Best regards,
    José – CT1BRN

    1. Hello José,
      Yes, for 432MHz antenna, you can use the same material as for 144MHz. I understand, you depend on the products available on the market.
      If you screw the elements to the center boom, instead of passing them through the middle, then the horizontal and vertical plane will be slightly separated by a distance (the boom diameter). If you ignore this detail, the polarisation will not be perfectly circular, but this is a very small problem. You can compensate this distance by shortening the phasing cable (minus the boom diameter).
      Remember, the 432MHz antenna requires more precision, because its elements are smaller and closer, and few millimeters are more important than for 144MHz. Also, the connections for the dipole are harder to make, because the reflector is very close, and there is not enough space to work comfortably. If you are patient and stay calm, you will make a wonderful antenna.
      I am very satisfied with the performances of this project.
      The wooden stick is 2.5 meters in total. Ideally, you should have more than a wavelength spacing between the antenna and the nearest obstacle, otherwise the propagation diagram is affected. I think 2 meters or even less is acceptable in reality.

  4. Hi Racov,
    Once again i have some questions… sorry 🙂
    My supplier of aluminium tubes can sell me only 5 mts tubes. I cut them in half so they fit my car and are ready to drill holes for the elements. So, i would like your opinion if i can make both antenas with 19mm round tube for the boom and 10mm for all the elements, i mean all. Both antennas with same diameter tubes. Last question, the distribution box can be 80×80 mm wateproof or bigger? For the center element made as you with wood shelve cables but i´m looking for fiberglass tube to cover it.
    This is going to be my winter project along with an elevation rotor made with a linear atuator (hope it works). So, thank you very much for your grawing and videos. Hope you acn answe so i can buy the tubes. Best regards, 73’s
    Jose – CT1BRN

    1. Yes, the tubes you have are very good.
      A larger diameter makes a better bandwidth.
      The plastic box is OK. Just don’t panic when connecting the cables, because there are 4 connections and it’s a very delicate job.
      When it’s all done, if you have an antenna analyser, you cam trim a few millimetres off the radiators, to obtain the best match (if necessary). You should measure the antenna in an very open space, far from obstacles, as high as possible.

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