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HF QRP pedestrian mobile

pedestrian mobile at the beach carrying a magnetic loop

An interesting facet of HF operating is pedestrian mobile. Even though modern equipment and batteries make it easier than it used to be, it's still a minority pursuit. In fact for many stations you'll be their first pedestrian mobile contact. Significant curiousity will be aroused and contacts will often turn out more interesting than the standard signal report and weather.

Like operating with very low power, you are testing the limits of propagation, antennas and the guy at the other end. This should increase your appreciation of these and provide knowledge that may be useful even when transmitting from home.

Pedestrian mobile has different styles, depending on whether you're hand-carrying the entire station while walking or its weight is pushed or pulled on a trolley. A trolley allows more batteries, higher power and bigger antennas while the hand-carried approach involves more severe compromises. Apart from the occasional use of a trailing radial, I've only done hand-carried pedestrian mobile and will concentrate on this here.

 

1. Location

Parks, river banks, beaches or mountains are all good spots for pedestrian mobile. A suburban street isn't so good due to noise from power lines, cars and homes. There's also the safety aspect of being distracted by the radio when around traffic or knocking out some poor skateboarder with your loop.

Most of my operating has been from nearby beaches, with especially good results if the water is between you and the other stations. If there's a pier handy, walk down it; signals may lift further.

I've had many contacts (mostly on 40 metres) started near home and continued until after reaching the water's edge. Signal reports seem to improve within about 30 - 50 metres of the water. I'll sometimes then wade ankle to knee deep to see if immersion further strengthens signals.

This test normally produces mixed results. Difference is most if using a whip and a radial coupled to or in contact with the water and least if using a magnetic loop or hand-held whip without a counterpoise. Read later for details of an antenna that maximises salt water benefits.

Don't worry if you're not near a beach; I've had excellent results from parks as well. Magnetic loops in particular give good performance. Their directionality is another advantage; the loop can be turned to null out interfering noises on receive.

 

2. Bands and times

Most HF home and portable operators can assume that their antenna for most bands will be about as good as a half wave dipole. The main cases where antennas depart substantially from this are 160 and 80 metres (where short verticals with few radials, trapped dipoles or other compromises are used) and the higher HF bands where beams are widely used.

Pedestrian mobile is quite different due to the constraints on antenna size and weight. A small hand-carried whip or loop may be 20 dB inferior to a dipole on the lower HF band and much nearer to a dipole on a higher band.

It follows then that other things being equal there's a bias towards the higher HF bands for the pedestrian mobile operator. Of course things are not equal. In exchange for their smaller antenna dimensions, higher HF bands like 6 and 10 metres are very seasonal and there are many times when no strong, workable station can be heard. Here activity might be more 'grab and go'. That is you monitor DX clusters, loggers or beacons and as soon as promising signals appear you're out the door (hoping you remembered to charge the battery).

I haven't done a lot of pedestrian mobile on 12, 15 and 17 metres. However they promise to be excellent bands in parts of the world where you have large numbers of amateurs within a single hop. Where this is not the case, such as in much of VK, the ease and reliability of contacts is less.

20 metres offers the most consistent DX propagation. There is a fair chance you'll work DX in the morning or late afternoon. However its sometimes competitive nature can make contacts hard work.

30 metres, in countries where SSB is allowed, is also worth considering, though only as part of a multiband station. During the day its propagation is similar to 40 metres except that maximum distances are greater and reliability is less for short haul contacts. Lower absorbption and more efficient antennas can mean better signal reports as well.

40 metres is extremely good for QRP portable work with wire antennas but losses can mount up with small compromise antennas. Nevertheless a carefully built loop or whip can produce rewarding results and make 40 metres the easiest and most reliable band to operate pedestrian mobile. This is in a large part due to the number of QRP SOTA and portable stations, who being in quiet locations will almost certainly hear you if you can hear them. 500 - 1000km contacts are a daily occurrence for the 40m pedestrian mobile operator.

My own pedestrian mobile activities have been a mix. 40 and 20 metres have provided the most contacts throughout the year, despite the latter's crowding. There has been a smattering of contacts on 17, 15, 12, 10 and 6 metres, with the latter two particularly good for summer sporadic-E. Contacts are possible on 80 metres but reports have always been marginal. None have yet been made on 160 metres.

The availability of small HF/VHF/UHF transceivers has meant that it is the antenna rather than the radio that determines what bands you can use. 7 to 50 MHz is a wide range. You will need some sort of tuning and switching for a whip to cover such a range. Whereas two magnetic loops will probably be required.

 

3. Antennas

90% of the effort to do with pedestrian mobile concerns the antenna. Two main types are commonly used; some sort of vertical and the magnetic loop. Their size and weight (and to a large extent, their efficiency) is limited by the audacity and physical strength of the operator.

The key rule with all antennas discussed here is that there is no such thing as an antenna that's compact, efficient and wide bandwidth; only two of these are possible. Due to the size requirements, this means that good compact HF pedestrian mobile antennas will have a narrow bandwidth and require readjustment with significant frequency shifts.

Verticals

a telescopic 
squid pole possibly useful as a pedestrian mobile vertical antenna a telescopic vertical antenna for pedestrian mobile

Suitable verticals for pedestrian mobile come in different shapes and sizes. Options include:

* A wire up a telescopic squid pole. Possibly the simplest vertical, especially if you're wanting something that's full size. An 8 or 9 metre pole is unwieldy and needs two hands to keep it stable. Its 1 metre retracted length is also inconvenient for a small backpack. I now prefer one of those 5.4 metre telescopic poles now cheaply available through eBay. With wire taped to it this allows a full quarter wavelength on 14 MHz. A small L-match antenna coupler gives operation on all bands from 10 - 50 MHz.

* A loaded vertical. This is the above but with a centre loading coil to permit operation on lower frequency bands. The loading coil, wound on a short section of plastic pipe threaded onto the pole, also has a switch to short it for use on the higher bands when it is not required. Here you rely on loading to allow resonance with a short antenna. Top loading (preferably also with a capacitance hat) is reputed to give best performance but is mechanically awkward with lightweight telescopic poles. I've found centre loading mechanically easier yet still good performing. A 5 metre centre loaded whip is about three times longer than your average mobile whip and will be much more efficient.

* A modified 27 MHz CB whip. This is ideal for 28 MHz where the whip will work if the top is trimmed. Or it can be rewound, possibly with taps added, for 21 and/or 24 MHz.

* A telescopic vertical. This is like the proprietary 'Miracle Whip' and countless homebrew versions such as pictured here. It's just a simple telescopic whip with an antenna coupling unit at the base. Major advantages are simplicity, compactness, lightness and multiband capability. Unfortunately all this costs efficiency; the antenna will receive well enough but almost all contacts will be hard going.

All vertical antennas described above require a counterpoise or radial system to work properly. This can be even harder than the antenna to arrange, especially if you wish to transmit while walking. Your options are basically (a) not bother and suffer poor performance (again like the 'Miracle' Whip), (b) have a trailing wire counterpoise, (c) carry a short rigid counterpoise, possibly helically wound and tuned, or (d) wear a contact ring around your ankle and operate while walking in salt water.

The trailing counterpoise is the simplest effective option. Unfortunately it can catch on rocks, bushes or seaweed. In populated areas it is prone to being chased by dogs or children. Its characteristics also vary with the ground you walk over. Hence you will have to readjust the L-match with different ground conditions and there may be some locations where it is not possible to get a match.

A rigid counterpoise comprising of a short helical element at right angles to the vertical element was described by VK3AM (for maritime mobile use) many years ago. This is worth considering, particularly if you mostly use one band.

My most recent (and now favourite) arrangement is a seawater contact ring held to the ankle with a thick rubber band. This is obviously only good if you operate from the coast or a saltwater bay and are willing to wade while transmitting. However it is more convenient than a trailing counterpoise and does not require frequent antenna coupler readjusting. Especially on 7 and 10 MHz performance has also been gratifying - provided you stay in the water.

If none of these appeal, you could always just operate portable (and not pedestrian mobile) and hook up to a fence or balcony railing as your ground. Or use a magnetic loop as described next.

 

 

Magnetic loops

a portable magnetic loop a portable magnetic loop

Loops are bulky in two rather than one dimension but free the operator from needing a counterpoise. But just like small verticals they involve electrical and mechanical compromises, again more severe if pedestrian mobile.

The cardinal rule when building magnetic loops is to keep resistive losses down. The thinnest hook-up wire will work fine for a dipole or end-fed, but efficient small loops require a low resistance thick conductor all the way around. That means heavy and thick copper tubing.

The variable capacitor, used to bring the loop into resonance, also must have low loss. Though expensive, bulky and heavy, vacuum variable types are typically recommended, especially if building a multiband loop.

Finally, though magnetic loops can cover a 2 or 3 to 1 frequency range, efficiency falls sharply at the lower end of the range. Avoid this by making the loop as big as you can on your favoured bands, even if coverage of others is sacrificed. For example a diameter of about 1 or 1.2 metres for 14 MHz and double that for 7 MHz.

Such a high quality loop will mean low bandwidth. A remotely controlled stepper motor and geared reduction for the variable capactitor will allow adjusment from the transceiver and makes the loop more usable.

Doing all the above will produce a fine magnetic loop that will outperform just about anything else you could squeeze into a small flat or unit.

The only problem is what happens when taken pedestrian mobile. For a start it may be too heavy. Its bulk will be unsociably unwieldy if carried along a footpath. And it might not fit into all cars or on public transport if you were to take it somewhere.

Mechanical and safety problems might also arise. Carrying the loop may cause the main element to flex and place stress on the variable capacitor connections (which will have been soldered directly to it). An extremely sharp loop may be detuned by the operator. The high voltages of same need to be kept out of harm's way.

When all these factors are considered, a magnetic loop that's great for fixed use will be almost unsuitable for pedestrian mobile. The challenge then is to look at compromises that improve portability without adding too unacceptable a loss. This is easier said than done; even one short-cut can reduce signals by 6dB or 75%.

To make a magnetic loop suitable for pedestrian mobile use, you will almost certainly have to make one or more of these compromises:

* Using a larger loop but making it lighter and easier to pull apart. There may be some efficiency gains from making the loop larger. However these are more than outweighed by the use of less conductive materials instead of the recommended copper tubing. For instance six aluminium strips could be screwed together to provide a hexagon. A thinner but continuous conductor, such as RG213 coax cable (both braid and inner connected) or even an electrical extension lead could also be used.

Loops made this way are super-light and can be packed up small. I've had some great contacts on them. However the sacrifice in signal strength is substantial. Extension lead is inferior to coax cable which is inferior to copper tubing. And aluminium strips can't be reliable or efficient with all those bolted, oxidised connections.

* A different type of tuning capacitor. A vacuum variable makes the antenna too heavy to be carried by hand so this is one compromise you'll almost certainly make. Alternatives include valve radio air spaced types (for the lower HF bands) or a beehive trimmer (for the higher HF bands). Try for a two gang capacitor and connect both gangs to the loop, leaving the frame unconnected. This reduces low-end frequency coverage but removes frame-wiper resistance, which contributes to loss. Some builders have even made their own capacitor, but frequency range may be less than with a commercial unit.

* Flexible leads for the tuning capacitor. Unless movement in the main copper loop can be eliminated, the loop will be fragile when carried due to the risk of its ends coming adrift from the tuning capacitor's contacts. A way to get around this is to mount the ends of the copper onto timber for rigidity and have short flexible jumpers to the tuning capacitor. This relieves mechanical stress on the tuning capacitor's terminals. Unfortunately this adds resistance and will compromise performance. Consequently the jumper should be as short and thick as possible (eg 5cm of RG213 coax cable, braid and inner connected).

* Keeping the copper tubing but make the loop smaller than optimum. Here you're retaining the low resistance of copper but accepting a smaller loop than required for peak performance. An example is using a 1 metre diameter loop on 7 MHz, which is much easier to transport and carry than something twice as big. It will also cover more bands, possibly up to 28 MHz. Note though that it will need a higher value variable capacitor to bring to resonance.

* One band only. Another approach is to avoid measures that compromise performance but accept less frequency coverage, possibly only a single band. It's easier to make a variable capacitor that tunes over a narrow range only. And tuning will be a lot easier so you won't need mechanical reduction drives or pullies.

One promising solution, built but not extensively tested by the author, is to use 25 x 3mm aluminium strip. These are available in 3 metre lengths from local hardware stores for under 20 dollars. A single length is cut into 2.4 and 0.6 metre sections, for the main and coupling loop respectively. The coupling loop is formed into a square with 15 centimetre sides. The main loop is formed into a square with 50 centimetre sides. However instead of connections for a capacitor, both top ends of the large loop are bent 90 degrees downward to be parallel to one another. Facing sides are wrapped in strong tape. Adjusting the spacing of these varies the capacitance and thus the resonant frequency. Such a loop will reasonate between 14 and 28 MHz though tuning will be touchy. Despite the use of aluminium rather than copper it is potentially efficient due to the lack of thin leads to a lossy variable capacitor.

Pedestrian mobile is a game in which it's impossible to be a purist. The only alternative to being willing to make compromises is an antenna so large or heavy that it never leaves the house. I've settled for two loops. A 90cm loop provides excellent performance between 7 and 30 MHz while a smaller 40 cm loop performs well on 28 and 50 MHz. The latter generally gets more use over summer during the Sporadic E season on those two bands.

 

 

4. Equipment

The very compact Yaesu FT-817 is the obvious choice for HF pedestrian mobile. The 10-watt Icom-703 can also be used but is bulkier. The FT-897 contains space for an internal battery pack. Others use ex-military radios. These are solidly built but VFO knob style tuning may not be available.

Yaesu FT-817

Those running higher power have the choice of an Icom-706, Yaesu FT-857 or older Kenwood TS-50. The main problem with these is their higher receive current usage and the heavier bank of batteries required.

Compared to the choice of antenna and power source, the selection of transceiver is the least critical decision for the pedestrian mobile operator.

 

 

5. Carrying the station

Unless you use the FT-817's internal batteries, you are going to need some sort of bag. As well it provides weather protection and holds accessories like pen, paper and earphones.

I use an over-the-shoulder bag obtained from a discount store. Find one that snugly fits your transceiver and battery with several compartments. I melted a small hole through two (with a soldering iron!) to take the power cable. If your bag doesn't have compartments, or if you need extra rigidity, a partition can be made from plastic or cardboard covered with plastic wrap.

Another option, better with heavier loads, is a backpack. The larger backpacks have a frame that may be useful to mount an antenna, near-ear speaker and microphone clip. A difficulty with them is inability to change frequency unless there's some sort of remote control arrangement, for instance buttons on the microphone. 1 or even 5 kHz steps is mostly but not always sufficient to tune in stations on the band. Some backpack pedestrian mobile operators base their operating around skeds and even use ex-military radios (often geared to spot frequencies more than tuning around) but I find continually tuning around (for stations about to sign or calling CQ) key to maximising contacts.

Bag used to carry pedestrian mobile station

When designing this aspect of your station consider the location of your rig's power, antenna, microphone and possibly earphone socket. Avoid stress on these to prevent damage to your transceiver's hard-to-replace sockets.

 

6. Power sources

I use an external 12 volt battery pack made from nickel metal hydride batteries. It was originally an ex-medical equipment 24 volt pack that had reached its replacement deadline and been discarded. It still had plenty of life for amateur use and via a local hamfest became available at a bargain price. Its flat shape (pictured below) make it an good fit for the FT-817.

There are also other battery choices for pedestrian mobile. Sealed lead acid batteries are OK but they're a little heavy to carry or their shape can be inconvenient. And cheaper types can have a disappointing lifespan. Another option, untried by me, are the very light lithium polymer batteries. A 11.1 volt pack should be suitable for the FT-817. They can deliver high currents so can damage themselves, the equipment they're connected to or your equipment bag if not handled correctly.

battery for pedestrian mobile battery for pedestrian mobile in pencil case for protection

Early FT-817s sometimes had an accessory 9.6 volt 600mAH internal battery pack available. This capacity was too small to be useful and the pack was a waste of money. Later versions have higher capacity packs which may be better. Alternatively you can get AA NiMH batteries of up to 2500mAH. Consider these if you must use internal batteries, but I prefer an external pack with somewhat higher capacity.

Whatever battery type chosen, between 4 and 7 amp hour capacity will be ample for a casual 2 to 3 hour outing.

If wearing a backpack and the sun's behind you, a solar panel may be useful as a top up charger. This suits those going for a long walk. With panels it's a trade-off between flexibility and efficiency; lighter flexible panels are more bulky and deliver less current than smaller and heavier rigid panels. I've never used solar as I'm rarely out for more than 3 hours at a time.

All the above remarks apply for 5 watts. A power increase to about 20 watts provides a 6dB gain, which may be handy for DXing under marginal condition. Small batteries may still work for this level (provided their internal resistance is low) but talk time is greatly reduced. Still this may be an adequate trade-off for brief DX contacts. A further step to 80 watts gives another 6dB lift and the power requirements are heavier still and you'll probably need a trolley for the batteries.

 

 

Disclosure: I receive a small commission from items purchased through links on this site.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

 

7. Operating and results

Hints given for QRP operating (elsewhere on this website) apply even more strongly for pedestrian mobile as you're doubly handicapped; low power and inefficient antenna. Working in your favour is the often good receive location and the interest attracted on air when others know you're pedestrian mobile.

Contacts have been made around Australia and overseas such as the USA, Antarctica and Europe while pedestrian mobile on most bands between 7 and 50 MHz. Some demonstrations appear below.

 

8. Video demonstrations of pedestrian mobile

Magnetic loops

Verticals

 

9. Conclusion

This article has provided some pointers on HF pedestrian mobile. Provided you have the transceiver it's not a lot of effort to build a suitable antenna. The station can start as being on a single popular band, with more and better antennas added as time and inclination allows.

Of the options discussed, my favourite for use by the sea is a backpack-mounted 5 metre vertical wire with switchable centre loading (for 7 MHz) and an ankle contact ring immersed in salt water. For walking on land I prefer a 90cm magnetic loop. Both of these choices can cover most if not all bands from 7 to 50 MHz.

For further ideas and demonstrations of pedestrian mobile, please visit my YouTube site.

 

 

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