# Feeding a PCB dipole with single ended RF signal

As suggested how do you feed a Dipole antenna with single ended RF IC. I found tons of examples single antenna Line feeding a differential RF port on IC but not other way round.

What I want to do is feed a PCB DIPOLE antenna with singled ended feed from the IC and match the impedance. I have considered using a 50:75ohm Balun in reverse as per the below image but not sure this will work. If I can use a discrete component only solution that would be great as well. Any reference or advise would be great.

Freq 868Mhz.

IC RF line 50 ohm impedance. Dipole antenna needs to be fed with 71,3ohm impedance. Each leg of the Dipole I have as 82mm in lenght (164mm overall) https://www.mouser.com/ds/2/611/JTI_Balun-0900BL15C050_2005-06-263841.pdf EDIT: Using this circuit design with either a 50:50 ohm, or a 50:75 ohm Balun and matching accordingly ?

EDIT: To be as discrete as possible my current dipole feed circuit looks as follows, the device will be used to match the 50 ohm micro strip to the 73 ohm Dipole feed. any suggestions

• What's a "0900BL15C050E"? At least you should add link to the datasheet. – Curd Mar 1 '18 at 11:26
• mouser.com/ds/2/611/JTI_Balun-0900BL15C050_2005-06-263841.pdf – Rustie0125 Mar 1 '18 at 11:32
• It should work, but I am sure there are better (lower insertion loss, output impedance closer to your value) alternatives. The device you linked has 50 Ohm differential output impedance. – Lior Bilia Mar 1 '18 at 11:59
• Correct, But it seems 50 to 75 Balun with center freq 868 is not very common. i wanted to us this device but can not seem to find stock anywhere johansontechnology.com/datasheets/baluns/1350BL15B0075.pdf – Rustie0125 Mar 1 '18 at 12:02
• @Johann van Niekerk: ... so why didn't you draw it that way – Curd Mar 1 '18 at 12:55

First, make a stubline/series trans line matching circuit to match your 50Ω to 71Ω:

The lengths you need result from reading the ratio of $\lambda$ from a smith chart.

After that, you could implement a Rat-Race Coupler in PCB technology, matched to your 71Ω source:

In the naming above, you'd feed in your unipolar signal at P1, and attach your dipole to P2 and P4. (omit P3, or terminate it to ground with 71Ω – if your dipole is well-matched, the power leaving P3 should be 0.)

To give you a feeling of the sizes involved:

a $\frac\lambda4$ piece of microstrip line is 45mm long on 1.6mm thick FR4 (assuming an Er=4.7 and 35µm copper), so you your rat race ring would need to have a circumference of ca 275mm, corresponding to a diameter of 87.5 mm. That's not really small – but if you have PCB to spare (or can elegantly route this around your whole circuit!), that's a nice way of going at this.

If you need to save space, you need to be smarter about the rat race: You might want to replace the transmission line $\frac34\lambda$ element with an actual RC (lowpass!) that is equivalent in phase shift, for example.

Another option would be to go for a Wilkinson power divider (which is only $\frac13$ of the size), followed by a $\frac\lambda2$ phase shifting network on one side only.

• Hi Marcus Thank you for the information, it is not a approach I have considered but unfortunate space limitation makes my option very limited. The other issue that is not addresses is the Dipole dual feed line converted into single feed to IC? – Rustie0125 Mar 1 '18 at 13:45
• I Have updated the post with my current circuit. the idea is to get a 50:50 or 50:75 Balun and then adjust to make match closer ? – Rustie0125 Mar 1 '18 at 13:55
• The PCB recommendations I gave are Baluns. That's what they do. And I also gave advice how to replace the lengthy transmission lines with small components. I don't understand what else you're asking for – if you're asking for "what single device can I buy", then that's off-topic as it's a shopping question. I explained how you'd typically solve that problem. You asked for discrete component solutions, I gave you exactly that! – Marcus Müller Mar 1 '18 at 20:24
• Marcus you have answered how to match the network in multiple ways yes, thank you for that, I am asking as a component level how to physically connect a single endled RF line to a dipole antenna ( two feed points) is a balun required if indeed will the above circuit work? or do i connect one leg of the Dipole to ground and match the other leg as per normal? Im trying to find the simplest cost effective way without loosing some of the 13db available to me. Apologies if im unclear – Rustie0125 Mar 2 '18 at 4:14
• Ah, OK. Connecting one leg of the dipole to ground won't work. You need something to function as a balun, because that is exactly what you want. – Marcus Müller Mar 2 '18 at 7:32