Portable Ham Radio Station
I believe I have finally defined a portable setup that works for me. Having tried working portable from my son-in-laws location more that once unsuccessfully, I finally succeeded recently by making a few changes in the antenna setup.
What I call portable is probably closer to a mobile equivalent. It is not a feather weight, QRP system, designed for hikers and back packers.
The Antenna
Previously I have always tried using a resonant antenna. While it was resonant at the home QTH, it was seldom right on resonance at the portable location. This should not have been a surprise because we seldom find the exact same conditions at a portable location that we find when we design and tweak the antenna. There always seems to be a limit to how much tweaking can be done under portable conditions.
Also, if you don’t have close to a 1:1 SWR, the recommended transceiver class is not going to put out full power. In my case the TS-120 delivers less than half its 100 watts when the SWR climbs up to 2:1.
Some would say 50 watts is plenty for good communications, but I have a specific schedule I like to keep with a long time hamming buddy and 50 watts into a lousy antenna will just not do the job.
So, I decided to use a non-resonant antenna. Appears I end up with a non-resonant antenna most of the time anyway. My non-resonant wire is a dipole, each leg being 44 feet long. It is fed with open wire line and matched to the transceiver with a balanced antenna tuner.
This works to get the most power out of the transceiver. It also allows coverage of multiple bands and allows a low SWR without need to worry about how the antenna is installed.
The Antenna Tuner
Since I use open wire line I need a balanced tuner. One that can efficiently match parallel balanced open wire line. Some tuners use a balun to match to balanced transmission lines. While that works, it is not the most efficient means and usually introduces a ferrite or powder iron core into the system.
The antenna tuner I use is called a Z-match. It is an impedance matching device based on two tuned circuits which form an all band tank. One circuit covering 3 to 14 mhz. The other covering 14 to 30 mhz. Input to the Z-match is 50 ohm coax. Output is a link coupling which is a balanced output. Note that this is not a single coil Z-match and none of the coil forms are ferrite or powdered iron. Also this unit uses a real meter as an SWR indicator. While LED indicators can be used and do work, having a meter to indicate SWR trend makes tuning much easier. Construction information can be found here
Since we are considering power levels at the 100 watt level, ordinary replacement broadcast type air variables will do the job well. Use large knobs and you may not need reduction drives.
The Transceiver
Solid state, SWR protected, inexpensive, old, ricebox style equipment is best. In a portable application there is always a possibility that the equipment could be damaged or lost. You don’t want that $2k big rig to end up that way. So use something that is inexpensive yet can still do the job. I use a TS-120 and also have an Atlas 210x available. Both can be powered off 12 volt battery/automobile power as well as 120 vac house power.
Some of the more recent transceivers have built-in antenna tuners but I know of none that provide a balanced output. I suppose you could equip one with a conventional balun to allow it to work into the balanced line. Just be aware of the possible inefficiencies involved.