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Transmission line for 915MHZ Antenna

Purchased several RFM69HCW Radios, 915MHZ and I have to say I have no pressing issues getting them to communicate.

But now I want to mount my 3 inch wire a few inches (or more) away from the board. The obvious issue is that this would lengthen the antenna.

Did some research - What I need is called a transmission line. One site said to use a bifilar transmission line as it's simpler than coax.

This is certainly not "simple" to me. I think it might be just a twisted pair but I'm not sure.

Do you have any guidance? A link to "bifilar transmission lines for dummies" perhaps? Or a quick and dirty solution to go with my 3 inch wire antenna?

TomHumbleAndHandsome

Posts: 59
Joined: Mon Feb 15, 2021 6:38 pm

Re: Transmission line for 915MHZ Antenna

A transmission line represents the parasitic inductance and capacitance of wire, and their effect on signals moving along the wire. They come into play when one of the assumptions for lower-speed singals stops being valid: the wavelength of the fastest signal in the circuit is no longer at least 10x bigger than the longest conductive path in the circuit.

A lumped-element transmission line looks like this:

lumped-transmission-line.jpg (25.94 KiB) Viewed 164 times

And in a high-frequency transmission line the inductors and capacitors are continuous all the way along the wire.

A 'bifilar' transmission line is two pieces of wire running parallel to each other. The inductance is natural for any current moving through the wire, and the capacitance is proportional to the size of the wires and the distance between them. Twisting the wires together at a fixed number of turns per inch increaes the inductance, making the natural resonant frequency of the line lower. Above about 100mHz, any twist creates too much inductance, so the wires just run straight and in parallel.

Beyond that, very little about transmission lines has an easy version, and all the math is hairy. That's why PCB design software like Altium has high-ticket RF simulation packages.. they're worth every penny to someone who has to do RF design full time. The tools necessary to measure RF signals are also pricey because their internal hardware and wiring has to be even faster than the signals they measure.

Without those simulations and tools, anything you do will be guesswork. I'd suggest using a couple pieces of 24-gauge phone wire, pulling them between two pair of pliers to straighten and work harden them, putting them about 1cm apart, and sandwiching them between two layers of tape. I have absolutely no idea what the RF characteristics will be, but would expect the inductance and capacitance to be reasonably low.

To make the antenna, put a 90 degree bend in the end of each wire so the free ends head off in opposite directions, perpendicular to the parallel sections, and in line with each other. Make those sections 1/4 wavelength long to get a balanced half-wave antenna with a built-in transmission line. That should at least reduce the impedance-matching issues between the antenna part and the transmission-line part:

antenna-and-transmission-line.jpg (84.57 KiB) Viewed 164 times

Posts: 63921
Joined: Thu Feb 11, 2010 2:51 pm

Re: Transmission line for 915MHZ Antenna

Mike, thanks. I like the idea of pulling two wires with pliers etc. Real DIY!

I just discovered this product on your site:

900Mhz Antenna Kit - For LoPy, LoRa, etc Product ID: 3340
along with a SMT uFL connector?

Is the adapter cable a transmission line? That would be plenty long enough to reach outside of an enclosure.

TomHumbleAndHandsome

Posts: 59
Joined: Mon Feb 15, 2021 6:38 pm

Re: Transmission line for 915MHZ Antenna

Yep. Coaxial cable is a transmission line.

The size and composition of the insulation between the center conductor and the shield make it a distributed capacitor that interacts with the parasitic inductance of the center conductor itself. They're tuned for specific impedances: usually 50 or 72 Ohms.