How to Get Started:
Beyond FM: Feedlines & Connectors
By Jim Aguirre, W7DHC
In the past few articles, I/ve discussed various aspects of VHF, UHF and microwave equipment, frequencies and operation. This time, I/d like to look at something we sometimes overlook: the importance of good feedlines and connectors. This is an especially important area when it comes to the higher bands.
We're all familiar with the standard 'UHF' or PL-259 connector. That's the one used on virtually all HF equipment. 'Common wisdom' says this connector is no good above two meters, but this isn't true. Its design criteria is 500 MHz and can be quite effectively used on the amateur bands through 70cm.
Even so, many VHF, UHF and microwave operators (including me) prefer to use 'N' connectors on the bands above 50 MHz. My preference is based not on the frequency, but rather on the better weather sealing you get with the N. It has a rubber gasket seal at both the coax and pin ends. That said, NO connector is waterproof. They should ALL be properly protected from the elements. That may be a topic for a later column.
Frequency wise, the Type N connector (named for its creator, Paul Neill of Bell Labs) is designed for use up to 11 GHz. It provides a solid, 50-Ohm connection useable on all the amateur bands through 10 GHz. For most of us, that's sufficient. Above 10 GHz, rectangular waveguide is the preferred transmission line and flanges are the appropriate connector. That too is a topic for another column.
In recent years, a new two-piece Type N connector has appeared on the market. It is significantly easier to install than the old version and, from all I can tell in my own operations, it works every bit as well. The only drawback I find is that you may have to reduce slightly the diameter of the center conductor on some of the low-loss coaxial cable now on the market in order to fit it into the pin. This is especially true if you like to tin the center conductor before assembly like I do. Judicious use of a small file and a good measure of patience are the only tools you need to make the center conductor fit.
I still use the old-style Type N connector as well and once you get used to assembling them, they're not all that bad. They work every bit as good as the new versions – maybe even better. One benefit of the old version is that it is shorter in overall length and can sometimes solve a space problem where the longer connector might not fit.
With the growth in telecommunications activity at 800 MHz and above, a new connector has now become standard at microwave frequencies. This is the SMA "Subminiature A" developed by Bendix Scintilla Corp. specifically for use at microwave frequencies. It's designed for operation up to 18 GHz. Some of the newer amateur handhelds are using this connector to mount antennas. Quite honestly, it's not very well suited to that application as it's a rather small connector with relatively low physical strength. In most amateur microwave applications, a short SMA to Type N jumper is used from the equipment to a larger feedline such as LMR-400, Belden 9913F or coaxial 'hardline.'
Feedlines and their associated loss characteristics are very important at higher frequencies. Where the commonly used RG-58 feedline has a relatively minimal loss at HF frequencies, it can easily have a 30 dB loss in 100 feet at microwave frequencies. That would be a 1000 percent reduction! Ouch.
The 30dB loss would probably also be the least of your worries with 1000 watts into RG-58 at microwave frequencies! RG-58 can look like a dead short in some higher frequency applications. The resultant heating and high SWR may well damage your equipment.
From 900 MHz up, you need to use short coax runs and cable with low-loss characteristics. There are several good ones available, but be prepared to pay the price – they are not cheap! If you use the cheap stuff, you will really PAY THE PRICE! Really short jumpers (12 inches or less) made up of small coax with a Teflon dialectric are OK to make the transition to SMA fittings.
When shopping for coax, check the loss charts and rated power levels at the frequency you intend to use it on. Buy the best coax that you can afford. It will pay off in the long run. Absolutely avoid using RG-58, even for jumpers! Very short runs of good quality RG-8X may be acceptable up to 1.3 GHz.
Don't use anything but .405 diameter or larger low-loss coax above 1.3 GHz. For fixed stations, coaxial hardline is probably the best choice. It's expensive, but long-lasting and has very low loss compared to conventional coax.
So, that's it for this article. However, let me make a quick pitch for the new Pacific Northwest VHF Society. It only costs $10 to join and you can get acquainted with the 'movers and shakers' of VHF, UHF and microwave activity in this region. If you're even the slightest bit interested in moving up to the higher bands, this would be a wise investment.
Contact me or Lynn Burlingame (N7CFO) for details and an application, or check out the society's web pages at www.pnwvhfs.org. You can download a membership application from the web site as well.
This new group does NOT compete with general or local amateur radio organizations such as the Issaquah ARC. The focus of PNWVHFS is strictly on weak-signal (SSB/CW) VHF, UHF and microwave activity.