Dipole antenna, balun and impedance matching

[mersault]

Hello,

I'm trying to make a dipole antenna for UHF (433 MHz). I have a 50 Ohm unbalanced output so I need to balance it and do an impedance matching to get the best performance possible. I bought some Minicircuits' ADT1.5-1 (https://www.minicircuits.com/pdfs/ADT1.5-1.pdf) to do the job.
I made a PCB, soldered the transformer and the legs of the dipole obtaining something like this.

Lengths of the legs were calculated for the frequency given (about 16.5 cm)

What I obtained in the VNA was this
Log

SWR


As you can see the antenna is not doing a good performance on the frequency that I want

What can be wrong?
I tried reducing the legs and the only result that I got was an almost perfect antenna for 975 MHz with legs of 8.4 cm each one.

Should I need to put a matching circuit (capacitors and inductances) after or before the balun?
In that case I should measure antenna impedance. I'm thinking to do something similar to this (page 3, top)
**broken link removed**

Anothe option is to make a lattice balun but I'm wondering how much power that balun can handle

I'm trying to avoid a balun made with coaxial cable because I'm also looking for impedance matching. The best balun for a dipole antenna could be a 1:1 that means a SWR of (at least) 1:1.5, but, if there's no other option I think that I choose it.

As a general question, How can I test the antenna to see if it is well balanced? Can I do that with a VNA, Spectrum analyzer?

Any ideas or suggestion to improve my antenna will be welcome!

Many thanks.

Reagrds
M.

 

[jiripolivka]

Where was your dipole located during VNA test? Close object do affect dipole (antenna) impedance.
Antenna balancing does not show explicitly on impedance but on the radiation pattern.
To me it looks like you need to read some basics on real antenna testing.
Try ARRL Radio Amateur's Handbook, or ARRL Antenna Book for best hints.

 

[Joel-Tang]

the balun has a return loss of about 11dB at 400MHz so you can't really expect to know if your antenna lengths are correct by measuring S11. Even so, the 10dB of return loss is quite good considering that more than 90% of the power is being transfered.
I hope this helps.

Cheers,

Joel

https://ca.linkedin.com/in/joelrichard/

 

[EdmundZheng]

Try to fine tuning of the length of antenna.

 

[biff44]

he he he...11 dB is about all you are going to get unless you want to trim the antenna a little shorter and add some impedance matching elements. You should be happy with that.

A ferrite ring around the coax cable right at the board would probably help a little too.

 

[rf1008]

I think your dipole performance measured is good and could be accepted according to your balun performace.

Of course you may debug the impedance matching at balun input,maybe a little improvement will be there.

 

[mersault]

Hello guys,

Many many thanks for your answers. I will try to answer and comment the responses that you gave me

Where was your dipole located during VNA test? Close object do affect dipole (antenna) impedance.
Antenna balancing does not show explicitly on impedance but on the radiation pattern.
To me it looks like you need to read some basics on real antenna testing.
Try ARRL Radio Amateur's Handbook, or ARRL Antenna Book for best hints.

I tested the dipole above a table, hanging from the table (about 60 cm from the floor) and also I used a special box that I made.
The box is covered by RF absrober for UHF band (**broken link removed**)
In all the cases the results were the same

the balun has a return loss of about 11dB at 400MHz so you can't really expect to know if your antenna lengths are correct by measuring S11. Even so, the 10dB of return loss is quite good considering that more than 90% of the power is being transfered.
I hope this helps.

Cheers,

Joel

https://ca.linkedin.com/in/joelrichard/

If I can measure the performance using S11 (due to the return loss), what should I use?

- - - Updated - - -

Try to fine tuning of the length of antenna.

I cut the antenna, in small steps (1mm), and the performance didn't get better at 440.
I olny obtained an almost perfect antenna for 975 MHZ when the length of each dipole was 8.4 cm

he he he...11 dB is about all you are going to get unless you want to trim the antenna a little shorter and add some impedance matching elements. You should be happy with that.

A ferrite ring around the coax cable right at the board would probably help a little too.

So, you say [coaxial] -- [ferrite] -- [balun] -- [matching circuit] -- [dipole]?

It is suposed that the balun has an impedance ratio made for matching a dipole (1.5:1 or 50-to-75 Ohms)


Thanks again guys for all your help and time.