High SWR On Coaxial Cable—Not A Good Idea
History: Amateur radio operators before WWII used ladder line to feed their antennas. Transmitters incorporated link coupling to feed ladder line which provided a very efficient transmission system. With this system they were able to feed one wire antenna on multiple bands and did not concern themselves with making the antenna resonant nor did they get overly concerned about SWR because losses were minimal. Their primary concern was to be able to “load” their transmitter into the feed line and they could not use sharp bends in the ladder line nor could they place the ladder line underground or near metallic objects.
After WWII coaxial cable became available as surplus and the amateur community adopted it’s use in a big way because it could be buried, bent and close or on metallic objects. This made using feedline much easier and convenient. The negative feature of coax cable is the loss incurred primarily due to the small diameter of the center conductor. Many of us to this day have chosen to ignore these losses and compromise our transmission systems. High loss is not a significant issue if the cable run is short. If the run is long, the losses can be significant. High quality (and expensive) coax is manufactured that boasts air dielectric in lieu of PTFE or foam. This type of coax is not flexible and therefore limited in application but is low loss but not feasible for the average amateur radio operator.
SWR Loss: In broadcasting we operate on one frequency and therefore can operate an antenna that presents a 50 ohm non-reactive load to the coax and thus an SWR of 1:1 or close to that. With this type of installation the feedline losses are those of the line only.
We who operate within the amateur radio community wish to operate on a wide range of frequencies and with limited resources wish to use one wire antenna. This is problematic if using coaxial cable. Some have suggested using some “magic” length of coax and maybe an impedance matching transformer between the antenna and feedline. This all is a compromise at best because the mis-match between the antenna and coax varies depending on the frequency of operation. Also, the longer the coax run (and higher the loss), the better the SWR “looks” which here again the operator is fooling himself/herself. So in reality, total feedline loss is the summation of specified feedline loss and SWR loss. The higher the SWR the higher the SWR Loss!
Moderately long coax runs can display loss as high as 3dB even when properly terminated. This means that with 100 watts from the transmitter 50 watts will be dilivered to the antenna. Most amateur transceivers can tolerate an SWR of 2:1 but any higher than this and the transceiver starts into fold-back to protect the output stage of the transmitter. By feeding a wire antenna with coax and operating on frequencies from 1.8 Mhz to 29 Mhz, the SWR can easily exceed 10:1. By adding the coax loss and SWR loss together, one can easily show a loss of 6dB (that yields 25% of the power to the antenna). Also, this kind of loss means 75% of the power is being dissipated in your feedline system which means if you are running legal limit you are stressing your transmission system big time.
Solution: Install very inexpensive ladder line which shows losses within a few tenths of a dB! Feedline loss is low and high SWR losses are negligible! The antenna/feedline system is comprised of a random length wire antenna (not necessarily resonant) fed with ladder line directly. Because the transceiver uses an unbalanced (coaxial) output, a transitional box called an antenna tuner is required. The antenna tuner will need to feed a balanced feedline (ladder line) and the tuner may be located near the transceiver if the ladder line can be gracefully fed into the radio room.
If the antenna is located a significant distance from the radio room and perhaps you wish to install the feedline under ground or attached to metallic supports, there is still a solution by using ladder line. You may use ladder line from the antenna feedpoint down to the antenna tuner mounted in a weather proof box below the antenna and from the tuner—coax back to the radio room. With this type of installation, one may also use an L-Network tuner to feed a single wire to the antenna (like a Windom or longwire). This system is also applicable to shunt feeding your tower with a slant wire.
By remotely locating the antenna tuner presents another problem-how can it be tuned from the radio room. Some amateurs use an automatic tuner remotely located but that presents the problem of power and control cable. Many amateurs prefer to build their own antenna tuners with great satisfaction but here again controlling them remotely has been a big drawback with this approach. This problem stimulated the development of StepperTune. StepperTune is capable of controlling two stepper motors remotely so one can fabricate a tuner and add stepper motors for control or install stepper motors in an already fabricated tuner. StepperTune memorizes the location of each stepper so it is capable of adjusting both steppers to preset positions for automatically acquiring low SWR for a particular operating frequency (8 memories for both steppers). StepperTune also is comprised of 8 memories used as a “lookup table” to automatically acquire stepper preset positions based on transceiver band information. StepperTune also can “fine adjust” the steppers for low SWR automatically.
StepperTune Interconnect Cable: One Cat 5 cable (8 conductors) is required to control your tuner with StepperTune. 200 ft of Cat 5 is possible using a 13.8 volts DC supply. If one wishes to use a longer run of Cat 5 simply increase the supply voltage up to 24 volts DC. The other option for a longer run would be to use larger gauge conductors in the control cable.
Stepper motors may not be over driven with excessive current. If overdriven on a regular basis the permanent magnet field can be de-magnetized rendering the motor useless. StepperTune has a current control function allowing the operator to “load in” a target operating current and StepperTune will acquire that current level and memorize it for normal operation.
Each stepper (bipolar) has four conductors (two windings) which are fed by StepperTune. Each winding receives both polarities which yields eight different drive situations from the H-driver within StepperTune. If one encounters a stepper problem it can be a formidable diagnostic problem. StepperTune is capable of running diagnostics on the H-driver, interconnecting cable and stepper.
Ladder Line Source: I prefer “True Ladder Line” by W7FG for three main reasons:
- Less wind loading and thus less movement during windy conditions.
- Less loss and less SWR change during rain conditions.
- 600 ohm vs 450 ohm for less loss with higher feed point impedances.
Here is the link to W7FG True Ladder Line: http://www.trueladderline.com/w7fg-design-600-ohm-open-wire-feedline/