General Notes on Portable Antennas
The same sorts of antenna designs will work for portable antennas as for any other application. The main differences between portable antennas and more permanent ones are:
- mechanical construction, to make it easier to pack, install, and store
- size and weight may be limiting factors for transportation. This is particularly true for antennas that need to be carried on foot.
- portable antenna supports need to be smaller and lighter, or already existing on the site
- set-up and take-down time may be more important
- adaptability, the ability to change it for different situations, may be very important, especially if you are using field supports.
The first step in choosing a portable antenna is to identify which of these factors are important in your application, and what your limits are.
Size and weight may be less of an issue when traveling in an RV / motor home / caravan, especially if you can lay long items on the roof, but become more critical when they have to fit in a backpack, on a bicycle or horse, or in carry-on or checked baggage on a commercial airline. For example, the boom for my TA-33jr yagi is a single piece 3.5m (12 feet), and I can only carry it in my van with one end sticking out the window; A different boom that would come apart in two sections would fit inside. Weight is not just a matter of transport, but set-up as well: are you strong enough to set it up by yourself if necessary? (I remember some emergency communications kits that ended up too heavy for one person to put it in a vehicle.)
Set-up and take-down time requirements vary depending on whether you plan to stay in one place for week or two, or just a couple hours on a SOTA peak before you have to head back down the trail. Quick setup times may dictate simpler antennas, possibly at the expense of efficiency. Set-up time also depends greatly on practice, as we have observed over the years setting up a tri-band yagi beam for Field Day.
Antenna Supports either need to be already available on site, or brought with you. If your site has trees, that may be all you need (although it can be difficult to put up an antenna if the trees are too dense). Otherwise, you have to bring your own, and that may limit the height you can achieve.
Let’s look at these in more detail as they relate to portable operation.
Mechanical Construction of portable antennas generally involves using more flexible wire, as it is less prone to kinking and easier to unwind (and roll up when you are done). The wire gauge will need to fit within your allowable weight limits. With such small wire, the plastic insulation adds considerably to the wire strength. At the same time, to the extent that portable antennas tend to be temporary, insulation breakdown due to sunlight generally is not a problem. Most of my portable antennas use relatively small ( 1mm / AWG #18 to 0.4mm / AWG #26) stranded, insulated copper wire of unknown types, and strength has never been a problem.
Along with smaller wire, smaller coax cable also saves space and weight, as well as reducing stress on marginal supports. At 100W, RG-58 coax ( 5mm diameter) is quite adequate, at least to get from the antenna to the ground. For a long run, you can splice in a heavier cable along the ground. For backpacking, RG-174 or RG-316 ( 2.5mm diameter ) or similar cables can be used, depending on your trade-offs between weight and performance.
Insulators and other parts of the antenna can be made relatively small and light as well. Synthetic (plastic) ropes generally are good enough insulators that they can be tied directly to the wires, at least at the 100W power level.
Small ropes will reduce the bulk somewhat as well. 3mm / 1/8 inch ropes are generally adequate as far as strength: the main limiting factor is the ease of tying / untying / untangling them, a factor that appears to change with age and eyesight. I find I prefer somewhat larger ropes now than I used 40 years ago! The use of carabiners, clips, and other rope aids can speed set-up and take-down, especially for those who have never become familiar with tying knots.
Of particular importance to maximizing operating time is reducing the time spent untangling ropes and wires. I have a standard winding method that is very fast, and greatly reduces tangles. Having some sort of winder or frame, and winding ropes and wires to minimize twisting, makes a big difference, especially by about the third time you set up an antenna and the ropes and wires are no longer in the same neat arrangement that they were when you left home. Practice helps a lot in all that, too.
Approaches to antennas
Hams take several different approaches to portable antennas:
- pre-cut coax-fed antennas (such as dipoles) that do not need a tuner
- doublets fed with twinlead to a tuner
- end-fed wires, sometimes connected directly to the transmitter (or to a tuner at the radio)
- self-supporting vertical antennas, often with tuned loading coils
My experience with portable dipoles and similar antennas is that, once tuned, they generally don’t require readjustment in the field, unless the set-up is severely compromised (like running the antenna along a wire fence). That depends somewhat on what SWR your rig will accept, of course. My approach was to tune up the antenna initially at a lower height than I expected to use it in the field. That tended to improve performance when the installation not optimum. An exception may be 80m (and 160m), where the antenna is so close to the ground (in wavelengths) that the tuning may shift more, and the operating bandwidth (especially for shortened antennas can be quite narrow. In that case, the ability to make field adjustments is handy. Otherwise, one can often get by without a tuner, or even an SWR meter if your rig doesn’t have one.
Doublets can make an excellent multi-band antenna, although they generally require the use of a balanced tuner, which adds weight and bulk to the station, along with tuning time if it is a manual tuner. Note that some tuners can have high internal losses, which will vary depending on the impedance and frequency. This is a good thing to test ahead of time. Tuner losses are easier to detect at high power than at QRP, because the coil (the usual source of loss) will heat up faster. You need to test it on all bands with the antenna you plan to use.
Be cautious about substituting speaker cable or other small wire for twinlead when using a doublet: the losses in such cables, where the insulation is not designed for RF use, can increase quickly with frequency, especially above 10 MHz or so, and at high SWR. It is worthwhile testing the losses in your feedline ahead of time.
End-fed wires come in several flavors. Those longer than about 3/8 wavelength can be reasonably efficient with a limited ground system, and, if long enough, can provide useful gain towards the ends of the wire. Short wires with a wide-band matching transformer and coax to a tuner at the rig can have comparatively high losses, which need to be considered against the ease of use (along with possible losses in the tuner). End-fed half-wave antennas (or multiples of a half wave) have a high feedpoint impedance, but can be quite effective with an efficient matching network. (Note that many auto-tuners won’t match the high impedance.) For monoband use, an L network tuner works well, with minimum losses, and can be pre-adjusted so no field tuning is needed. Wideband transformers are popular these days for multiband operation, but a small manual tuner will also work. (Check your transformer for losses – many popular designs on the internet can dissipate half the applied power in the ferrite core.)
As the wire gets longer, the arrangement needs more consideration. Long wires (over 3/4 wavelength or so) work best when horizontal, which requires an additional support (unless the ground is sloping). A sloping wire reduces the average height above ground, and the pattern may be less effective on the higher harmonics (although it can be very effective over salt water). I’ve had reasonably good results running a wire up and over a tree branch, then out to another tree, which is a bit more work to set up but seems to work better for multiband operation than a sloping wire with the same maximum height.
There are many commercial versions of portable vertical antennas that can sit on a small tripod and disassemble into a convenient kit. Unfortunately, some of them don’t explain the importance of ground radials for proper operation and tuning, and radiation efficiency. They may be a good choice when operating space is relatively limited, but by the time you add a pair of elevated quarter wave radials, you have used as much space as a dipole would require anyway. They can work well, especially over soil with good conductivity, or by salt water, but practice ahead of time to make sure it tunes easily with the radial system you plan to use.
When using shortened antennas, particular less than half of full size, the SWR bandwidth can be narrow, and field tuning may be required, especially if the antenna configuration varies a lot. In that case, make sure that the antenna is easy to raise and lower, as it can take several sets of adjustments to get it set just right. (An effective balun will make the process much easier as well.) When circumstances permit, a pre-cut wire dipole or quarter wave wire may be faster to install than some of the commercial antennas that use loading coils, besides being lighter and costing less.
A further note about setup time
I have a heard a number of hams complain that a particular type of antenna is too complicated or takes to long to set up compared to their favorite type. In most cases, it seemed to me that the ham had never actually set up such an antenna, and certainly hadn’t done it enough to do it easily. If set-up time is particularly important, then practice ahead of time, and also pay attention to how you pack up the antenna so it is easy to set up the next time rather than wasting time untangling wires. If the antenna has a loading coil or a telescoping whip, set it up several times in different configurations to find the best setting, and then mark it so it is easier to return to the same setting next time.
How long does it take to set up a portable antenna? The only time I’ve measured it in the field it took me half an hour to string up a set of dipoles for the pre-WARC HF bands while camping in VK7: in a new location, in the dark, by myself, while holding a fish in one hand. Usually it is much faster: we did a dry run of how to install a 40/20m dipole for Field Day by hanging it from a tree branch. The guy in charge turned around to answer a question, and when he turned back we were just finishing tying off the ends of the wires. If it took more than about 3 minutes it was because I was explaining all the steps as we put it up.
It takes about 15 minutes max to raise my TA-33jr triband HF yagi to 9m / 32 feet on a sectional mast, with just 2 or 3 experienced people (once the antenna has been assembled). Of course, it will depend a lot on what supports you are using: hanging a dipole from a convenient tree branch can be very quick, assuming you are prepared (and practiced) at getting a rope over the branch. Trying to install a mast on a rocky mountain top may be more difficult if you can’t pound stakes for guy ropes and is nothing to attach it to.
Whatever antenna you choose, keep in mind the conditions where you expect to set it up in your planning, to make sure your assumptions are appropriate. Then be prepared with alternatives when the conditions don’t work out as you expect! (It is quite different guying a mast on a flat lawn where stakes can be driven anywhere, compared to doing so on a pile of rocks.) So an important ingredient is creativity, being able to look at a situation and figure out different ways it might be installed, rather than trying to do it the same way you always have when the situation calls for a different approach.