Introduction: Yagi UHF Antenna for the 70 Cm Band (430-440 MHz) by F4HWK

Picture of Yagi UHF Antenna for the 70 Cm Band (430-440 MHz) by F4HWK

In this tutorial i show you how to build a Yagi antenna for the 70 cm band (430-440 MHz). This antenna can be used to receive the ham satellite communications with any portable UHF receiver.

Here i provide all build steps, with all dimensions you need. At the end as usual i provide SWR measurements to confirm its performances in terms of impedance matching.

You can also find some simulation results on my blog as well as the estimated gain: http://kavea.free.fr

Step 1: Reflector and Director Elements

Picture of Reflector and Director Elements

Reflector and director of this three element Yagi are made from 4 mm diameter threaded rods. The length of the reflector is 345 mm and the length of the director is 276 mm. These elements are mounted on a square PVC bar 11.5 x 11.5 mm with butterfly screws. The distance between the reflector and the director is 300 mm. Cf. photos.

Step 2: Driven Element

Picture of Driven Element

The driven element of this Yagi is a folded dipole which i made from a piece of RG316 cable. The total length of the dipole is 30 cm. In order to transform this piece of coaxial line into antenna you need to remove a bread shield in the middle of the coax, and solder braid shield to a central conductor at its both ends (cf. photo). To give some rigidity to the dipole you need to insert it into a PVC tube with a cut out window. So that you can solder a feed line to the dipole (cf. photo).

Step 3: Assembling

Picture of Assembling

The final step is to assemble all parts together. Insert the reflector and the director into a PVC bar holes, then with a hot glue attach the driven element to the PVC bar, 150 mm away from two others elements. Put some hot glue inside the PVC tube holes to ensure the impermeability of the antenna.

Step 4: Measurements

Picture of Measurements

The developed antenna has been measured with a VNA in terms of SWR and S11 parameters. The result shows that antenna has a bandwidth between 419.5 and 451.2 MHz with the respect of SWR of 2:1, which is more than enough for the 70 cm band (430-440 MHz).

F4HWK

Comments

tytower (author)2017-07-30

It is not a dipole either .

What you have is an antenna with a
reflector behind and a director in front . Do you understand how
reflectors and directors work? The signal is bounced off the director
out of phase and doubles the next cycle to the driven.

Also better to use alloy rods and avoid the corrosion. Where did you learn your antenna design principals . Woolworths?

kavea (author)tytower2017-07-31

1/ I'am an engineer in antenna design having a PhD, i understand very well how it works. Moreover, the electromagnetic simulation confirms very well the working condition of this antenna (cf. my blog).

2/ Google for "coaxial cable dipole" if you don't believe me. The electrical structure of the driven element is a folded dipole. The center break of one inch ? It's not true and depends on the cable diameter and frequency. If a want an antenna working at 5 GHz its total length will be around 1 inch so you understand that you cannot have a break of 1 inch all the time :) Moreover the impedance depends a lot on the position of the driven element compare to the director and the reflector, there is a electromagnetic coupling between these three elements which modify the S11 parameter.

3/ Google for "Yagi antenna". If you electrically connect director to reflector and to the driven element it will not work.

Read "Antenna Theory" from Balanis, it's a good book to begin with antennas.

vipin786 (author)2017-07-30

Can you suggest simple antenna for SDR I am searching them for a while but none seems to be portable and useful, other than Discone, but they need to be very large to be used.

kavea (author)vipin7862017-07-30

What frequency band you are interested in ? The term SDR is too huge, today it often means just a software receiver. For example i use an "SDR Play" which works from 1 MHz up to 1.5 GHz, and depending on the frequency band i have different antennas, because you cannot have an universal antenna which could cover all this spectrum in an efficient manner.

But generally speaking, if your SDR receiver has a 50 ohm input impedance then there is no any difference from any other 50 ohm receiver in terms of antenna requirement, you can use any antenna you want. There are no "SDR antennas".

However there are broadband antennas, like Discone, but even so, you would not be able to cover all the band down to 1 MHz..

In your case if you want ONE small antenna for all bands, i would suggest an active antenna (google mini whip). I would be a good trade off but not a perfect solution. Unfortunately small, broadband and efficient (at the same time) antennas don't exist and cannot exist.

tytower (author)2017-07-30

There is a name for that form of end driven
element which escapes me atm. Yagis feed the center conductor via a
capacitor to the driven element. The driven element is grounded to all
the other elements and the boom.

A dipole is split in the center with center conductor feeding one leg and earthed shield feeding the other side . The center break should be one inch to obtain a 50 ohm impedence

Yours has been fluked to give a useable SWR of 2:1 but ideally you want 1:1 SWR which is easily obtained with a proper yagi

tytower (author)2017-07-30

Iam of the opinion that this is not a yagi antenna . A yagi earths the other side of the driven element to the antenna which is solid conductor to the director and the reflector . I don't see a connection between those three elements in your diagram.

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