Brought to you by PCS Electronics, www.pcs-electronics.com
So what is this swr (vswr) everyone talks about?
SWR is a meas re of how well two devices are impedance matched to each other. Typical radio/TV transmission eq ipment
is designed for 50 ohm load impedance, so we s ally se 50 ohm cables and b ild or b y antennas that are specified for 50
ohm. While most cables have flat impedance over freq ency (they meas re 50 ohm at all freq encies yo are likely to se)
the same is not tr e of the antennas.
A 1.0:1 VSWR is a perfect match. That means the load impedance is exactly 50 ohms. A 2.0:1 VSWR is obtained when the
load impedance is either 25 ohms or 100 ohms.
Beca se most transmitters will deliver f ll power with a load VSWR of p to 2:1, this val e is s ally considered the limit for
acceptable operation. Many prefer to keep their VSWR below that however, b t for all practical p rposes, it is nnecessary
to spend time or money trying to get m ch below a VSWR of 1.5:1. The benefits will be hard to meas re and even harder to
notice.
On the other hand, coaxial cable losses increase rapidly, for a given freq ency of operation, when the antenna VSWR
exceeds 2:1. This can even, in some extreme cases, res lt in the coaxial cable b rning, even when r nning 100 W. Using a
higher grade of cable will definitely improve things, b t even high q ality coaxial cable becomes very lossy when VSWR
exceeds 3.0:1 at higher HF freq encies (or VHF and higher).
Coaxial cable
Coaxial cable is an electrical cable consisting of a ro nd, ins lated cond cting wire s rro nded by a ro nd, cond cting
sheath, s ally s rro nded by a final ins lating layer. The cable is designed to carry a high-freq ency or broadband signal,
s ally at radio freq encies. Coaxial Cabling is a two cond ctor closed transmission medi m that is often sed for the
transmission of RF energy. It yields excellent performance at high freq encies and s perior EMI control/shielding when
compared to other types of copper cabling. Coaxial cabling is commonly fo nd in broadcast and networking systems. Most
coaxial cables have a characteristic impedance of either 50 or 75 ohms. The RF ind stry ses standard type-names for coaxial
cables. The U.S military ses the RG-# or RG-#/U format (probably for "radio grade, niversal", b t other interpretations
exist).
The common RG-58 from Radio Shack is NOT the best yo can do and can eat a lot of yo r effective power o t! Use it
only for short r ns. BELDEN makes terrific coaxial cable in vario s q alities and with very low loss (meas red in
dB’s…decibels). 3 dB loss = 1/4 of yo r signal strength - either lost or gained. Watch o t for the correct impedance; RG58,
RG213, H-500, H-2000 and H-155 have 50 Ohms, RG-59 and RG-6 have 75 Ohms. Most antennas and transmitters
incl ding o rs are 50 ohm. Check o r website for good coax. Don't b y more than yo need to make the long r n to yo r
antenna and don't make p a few "j mpers" to go between yo r exciter, VSWR meter and yo r antenna as all yo 'll do is
create higher SWR and more line losses. H-155 or H-200 are good choices! RG-142 with Teflon is recommended for wiring
inside cabinets, for bal ns, Wilkinson co plers and everywhere where resistance to heat is req ired as ins lation won’t melt
d ring soldering or operation.
Connectors
Connectors come between coaxial cable and yo r amplifier, exciter or antenn. There are many standard VHF RF connector,
s ch as BNC, N, 7/16, SO239 and others. Use good q ality connectors as cheap types se cheap plastic instead of Teflon.
The good ones are s ally easily recognized by higher prices. Another reliable method is a test with soldering iron; Teflon
won’t melt while plastic will.
Mains power supply and mains power cable
Yo ’ll notice the mains cable is not incl ded, b t can be obtained in any radio/comp ter/hardware shop at the cost of abo t
1 US$. It is the type sed in yo r PC for mains power. Since these cables vary from co ntry to co ntry and we had tro ble
getting the exact type locally we decided against incl ding them, especially since finding them is so easy locally.
Signal exciter for your amplifier
Make s re the driving exciter/signal so rce is of power level the amplifier is designed for. Over-driving and nder-driving
both res lts in poor performance or even damage.
