L-Band receive antenna (960 MHz to 1215 MHz)

Discuss commercial and home made antennas.
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Joined: Mon Jan 30, 2017 12:59 am

Re: Guidelines for choosing coaxcable for use >1000 MHz

Post by jmcc » Tue Feb 07, 2017 4:05 pm

snn47 wrote:I would not call/consider any coax that has the diameter of the 50 Ohm RG-174 type coax a good quality thin cable for use at 1 GHz.
Thin cable is often used in short lengths for patch cables with test equipment and some pigtails where a device with a small connector has to be interfaced to a larger diameter cable with a larger connector. It is not, as I said, suitable for long runs. Many of these patch cables and pigtails are typically no longer than 30 centimetres. With good quality cable, the specifications in the data sheet are reliable enough so that it is possible to calculate the losses per length and get some idea of the insertion loss of the cable and its connectors. With poor quality cable, there are no such guarantees. PTFE, and other more expensive options, are better than polyethylene dielectric cable. This higher specification cable is more expensive but again it is not generally used for long runs.

The quality of cable used with mag-mounts can vary considerably and even with the two metre runs, it is pushing it with RG174 cable, even if it is relatively good quality. Nooelec seems to have changed from thin RG174 to RG58 on their mag-mount antennas. This does help with 1090 MHz reception. The problem is that most of the cheap mag-mounts use thin cable and while it is fine for broadcast and other strong signals, the losses will make a difference on weaker signals. From what I've read, most of the SDR dongles intended for Digital Terrestrial TV usage have an input impedance of 75R rather than 50R. With strong DTT signals, the losses caused by the small mismatch will not generally be noticed.

Unfortunately, most people's introduction to antennas with SDRs is the cheap mag-mount and concepts such as insertion loss, cable loss and link budgets are often unknown or ignored in the rush to get it all working.

Your post is a good introduction to cable choice for >1GHz. It might be worth adding a part about choosing connectors for the system. The SMA connector is a nice connector for microwave engineering when one has the proper crimping tools but it does not stand up to as much mechanical abuse as an N connector. However most people will be familiar with BNC and F connectors and, to a lesser extent, N-Connectors and TNC connectors. They tend to be a bit more robust and it might be better for people new to the hobby to use a more mechanically robust connectors for their antenna experiements.


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