As you may already know, low loss 75 Ohm hardline is freely available from your local cable TV service provider, but the question is, can it be used in a 50 Ohm antenna/transmitter system? That is a good question.
Before you attempt to try it, be sure to use cable that already has a PVC covering. If your hardline is an aluminum color or has an exposed metal covering, you may want to avoid it, unless your runs will be buried or somehow protected. Also, try to get non-hydroscopic foam dieletric, as this is more resistant to moisture. It will feel and look like hard foam and will be glued to the copper center conductor. Stay away from anything that uses air as the dielectic because it has a tendency to draw in water.
30 meters of new 75 Ohm hardline has only about 3.77 dB loss at 2 GHz. Dropping this into a known 50 Ohm antenna system, with a matched 75 Ohm transmitter will result in an additonal 0.18 dB loss (1.5:1 SWR). If a 50 Ohm transmitter is used, the worst case impedance that could be presented would be 112.5 Ohms, giving you a loss of 0.68 dB (2.25:1 SWR). Total the losses like this:
Examples Cable loss (dB) + Mismatch loss (dB) + Connector loss (dB) = Overall cable loss (dB) 50 Ohm transmitter with 75 Ohm hardline to a 50 Ohm antenna system (0.5 dB overall connector loss) 3.77 + 0.68 + 0.50 = 4.95 dB overall cable loss
Compared to 30 meters of Times Microwave LMR-400 (0.5 dB overall connector loss 6.69 + 0.0 + 0.50 = 7.19 dB overall cable lossHowever, if you were to trim your transmission line to a multiple of a one-half wavelength (after taking into account the velocity factor, 80 - 87% typically), then your transmitter should be matched and you would only add in 0.18 dB as the mismatch loss. This is because reguardless of the characteristic impedance of the transmission line, the impedance of the load (antenna) is repeated every one half wavelength along the entire line. This little half wavelength trick will work on any impedance transmission line. Be sure that your antenna is tuned to the proper impedance before you start though. A little known fact is that commercial antennas are actually factory tuned to 60 Ohms. This allows a person to drop the antenna into a 50 or 75 Ohm system with minimum fuss.
An alternate method of matching involves connecting the two different impedance lines together with an one-quarter wavelength matching section. This matching section's impedance is equal to the square root of product of the two impedances being matched. So, to match 75 to 50 Ohms, you would use a quarter wave length (of the frequency being matched) of 61.2 Ohm coax. Since 61.2 Ohm coax probably doesn't exist, use a stripline etched on a printed circuit board. Through the magic of sine waves, odd multiple harmonics can also be matched using just one matching section. A quarter wave section cut for 146 MHz will also match 438 MHz, a 816 MHz section will match 2.45 GHz. Since these types of matching sections are frequency dependant they will generally only match a small band of frequencies.
In any case, the use of 75 Ohm hardline may work for your application. It never hurts to try.
Proper hardline connectors must be used to avoid any more signal loss. Commercial connectors are available from Amphenol, and Cablecon. Here are some ideas for homebrew hardline connectors.
Perl Equations
# $SWR = Standing Wave Ratio
# $Zo = Impedance of the transmission line
# $Rl = Impedance of the load
# Assumes the loads are resistive only!
$SWR = $Zo / $Rl;
# $p = Reflection coeffient
# $mm_loss = Mismatch loss in dB
# $rtrnloss = Return loss in dB
$p = ($SWR - 1) / ($SWR + 1);
$mm_loss = -4.34295 log(1 - ($p ** 2));
$rtrn_loss = -8.68589 * log($p);
# $length = One electrical wavelength (tip to tip in meters)
# $frq = Frequency in MHz
# $vf = Velocity of propagation factor of the transmission line
$length = (300 / $frq) * $vf;
# $match = Impedance needed for a quarter wavelength matching section
# $Za = Impedance of line A
# $Zb = Impedance of line B
$match = sqrt($Za * $Zb);
SWR Facts by W8DMR
- A low SWR indication does not necessarily mean that everything is operating properly. It does indicate that the antenna system, consisting of the transmitter, feedline, and antenna are very close to being properly impedance matched. Changing frequency without some change in SWR is usually cause for concern.
- Reflected power does not flow back into the transmitter causing overheating and/or inflicting damage. Mismatched impedance essentially detunes the amplifier allowing damage to occur.
- No transmission line must be any specific length if a transmatch (antenna tuner) is available. Varying the length of the feedline doesn't change the SWR, but it does change the impedance presented to the feedline tuner connection.
- At frequencies below about 35 MHz, when using open wire (low loss) feedline, signal levels due to SWRs as high as about 6:1 will be essentially the same as signals produced by a near perfect 1:1 SWR.
- Neither the antenna or the feedline must be self-resonant to operate properly. Nearly any feedline and associated antenna may be resonated by using an antenna tuner (transmatch).
- Using a transmatch to resonate an antenna feedline system does not change the antenna or feedline impedance. It provides the required inductance/capacitance to resonate the mismatched antenna system, via reactance canceling, also referred to as conjugate impedance matching.
- Most losses in an antenna system occur in the acompanying transmission line. The Ohmic losses due to very small diameter conductors are an obvious exception. Nearly all antennas are quite efficient radiators.
- For low-loss transmission line, a SWR meter placed anywhere in the line will read the same value. At the antenna, at the transmitter, or somewhere inbetween yields the same reading.
Why Must The Impedance Be Matched?
To help explain this, lets use an analogy with a battery. This battery with be capable of supplying 5 volts DC and will have an internal resistance of 5 Ohms. The battery's internal resistance must be taken account of in all of our calculations. It's this internal resistance that causes the voltage to drop when the current demand grows. This will decrease the actual available voltage from the battery and will limit the amount of current it's capable of sourcing.
The battery is basically consuming a part of its own energy. This internally consumed energy is not available as power to the load.
Table of energy available for various loads
Load Total Internal Available Available Power Resistance Resistance Current Voltage Drop Voltage For The Load
0 5 1.000 5.000 0.000 0.000 1 6 0.833 4.167 0.833 0.694 2 7 0.714 3.571 1.429 1.020 3 8 0.625 3.125 1.875 1.172 4 9 0.556 2.778 2.222 1.235 5 10 0.500 2.500 2.500 1.250 <-- ** 6 11 0.455 2.273 2.727 1.240 7 12 0.417 2.083 2.917 1.215 8 13 0.385 1.923 3.077 1.183 9 14 0.357 1.786 3.214 1.148 10 15 0.333 1.667 3.333 1.111
** = Maximum power transfer when load and internal resistance are equal Voltage = Current x Resistance Power = Current x VoltageWhen the load resistance is the same as the battery's internal resistance the power transfer is at its maximum. This will be true for all electrical circuits and antennas.
CommScope P3 CA/JCA Specifications
Physical Dimensions (mm) P3 500 (1/2") P3 750 (3/4") Nominal Center Conductor Diameter 2.77 4.24 Nominal Diameter Over Dielectric 11.43 17.22 Nominal Diameter Over Outer Conductor 12.70 19.05 Nominal Outer Conductor Thickness 0.64 0.91 Polyethylene Jacket Versions (mm) Nominal Diameter Over Jacket 14.22 20.83 Nominal Jacket Wall Thickness 0.76 0.90 Minimum Bending Radius (cm) No jacket, standard 16.50 22.90 No jacket, bonded 10.20 17.80 Jacketed, standard 15.20 20.30 Jacketed, bonded 8.90 15.20 Maximum Pulling Tension (kg) 136 306 Electrical Characteristics Capacitance 50 +/- 3.0 nF/km Impedance 75 +/- 2 Ohms Velocity of Propagation 87% Maximum DC Resistance (20 degrees C, Ohms/km) Copper Clad Inner Conductor 4.40 1.87 Outer Conductor 1.24 0.68 Solid Copper Inner Conductor 2.72 1.21 Outer Conductor 1.24 0.62 Attenuation (@ 20 degrees C, dB/100 meters) P3 500 P3 750 Frequency (MHz) Nominal Maximum Nominal Maximum ~~~~~~~~~~~~~~~ ~~~~~~~ ~~~~~~~ ~~~~~~~ ~~~~~~~ 5 0.52 0.52 0.33 0.36 30 1.25 1.31 0.82 0.85 50 1.64 1.71 1.08 1.15 100 2.39 2.46 1.57 1.71 150 2.82 2.95 1.87 2.03 200 3.31 3.38 2.23 2.33 250 3.77 3.94 2.53 2.66 300 4.13 4.30 2.79 2.92 350 4.46 4.69 2.99 3.18 400 4.82 5.05 3.25 3.44 450 5.12 5.35 3.44 3.67 500 5.41 5.67 3.64 3.87 550 5.74 5.97 3.90 4.07 600 6.00 6.27 4.05 4.30 750 6.69 7.09 4.54 4.86 865 7.22 7.68 4.89 5.28 1000 7.91 8.27 5.33 5.71 2000 12.57 12.97 9.99 10.39
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