# Low mass, low frequency transmission line

I'm interested in a low frequency (10 MHz - 100 MHz) radio receiver array, made up of crossed dipole antennas. I'm working on a design project mainly focused on the mechanical engineering side of making a low mass array printed on thin polyimide sheets for deployability. So, as I am not too familiar with antenna theory, I was hoping for some guidance.

First are the antennas. The goal is to keep everything small and light. The idea was to use thin copper or aluminum ribbons to make crossed dipoles on the polyimide sheets. The dipoles will be about 0.3m long to be $$\ll \lambda$$ at all wavelength.

Then there's the transmission lines. The maximum length of a transmission line for my setup is $$\sim$$ 30 m. I haven't been able to learn much about transmission lines in this frequency range. Specifically, I'm interested in what the dimensions have to be (length vs. $$\lambda$$, diameter or width, separation between lines) and what materials are good. Could it be as simple as more conductive wire?

Will microstrips lines be applicable here? That would be ideal, since they are thin and very light and could actually be printed on the film. Could microstrips even be used for the antennas themselves?

You would not want to use microstrip for the antennas. Microstrip is a kind of transmission line, and transmission lines by design limit their electromagnetic fields to a small area in and around the transmission line. This means they do not radiate or receive, which is what allows them to transmit a signal from one end to the other without much loss or interference. The antennas could instead be made of simple, single conductors printed on the sheet.

But for bringing the signal from the antennas to whatever processing unit you have, microstrip could work, as well as any number of other printed transmission lines. The particular geometry of the transmission line, and the electric and magnetic properties of the material its printed on will determine its characteristic impedance. Since you control the design of this system on both ends, there is a wide range of impedances that could work.

The small dipoles will necessitate some kind of preamplifier at each antenna, which could likely be made quite small and printed on the sheet as well.

• Thank you! Could you briefly explain preamplifiers (dumbed down for a scientist posing as a software engineer posing as a mechanical engineer posing as a electrical engineer)? – Spuds Apr 30 at 15:25
• @Spuds en.wikipedia.org/wiki/Preamplifier. Although this is speaking about audio, the definition also applies to RF. – Mike Waters Apr 30 at 15:58
• @Spuds A concrete example of a preamplifier that might be applicable with a little adaptation to your project: w0qe.com/Projects/active_monopole_amplifier.html – Phil Frost - W8II May 1 at 23:58
• This is awesome, thanks! – Spuds May 1 at 23:59

Based on the little information you've given us, you might be able to use off-the-shelf Ethernet cable, like Category 5 (aka Cat 5) cable. Owing to its ubiquity, it is probably the most inexpensive option available. Each cable comprises four (4) 100-$$\Omega$$ unshielded twisted pair (UTP) transmission lines rated to 100-MHz. The loss at 100MHz is 21.6dB per 90m, so your 30m application would sustain about 7.2dB at the high end of your frequency range. Should you require shielding between pairs, shielded twisted pair (STP) cable is also available.

If you need more bandwidth, Cat 6, 6a, 7 and 8 provide up to 2GHz performance. These cables also include crosstalk specifications and may include shielding of individual pairs.

• If the answer has solved your problem, please click the checkmark icon on the left to mark it accepted. This helps keep track of which problems are still unsolved. – Brian K1LI Apr 30 at 0:57