My previous outing with my trusty old Hendricks PFR-3 portable field radio (see QRP on Deck) was with a Random Wire antenna. The big advantage of the random wire is that it is non-resonant on any band and can be tuned on any band. I prefer to use resonant antennas that can be paired with radios that (unlike the Hendricks PFR-3) don’t have an internal tuner. My favourite resonant antenna is the End-Fed-Half-Wave (EFHW).
The PFR-3 does have an internal manual tuner. It takes a little skill and patience to use, but it does actually work quite well. If I am shivering in the cold while freezing raindrops keep falling on my head, my preference is to put up an antenna that needs no tuning. That is where the EFHW excels.
First, a note about the EFHW. It is a half wave on only one, single band. It may also work on harmonically related bands; for example, an EFHW cut for 80 meters will often have fairly low SWR on the 40m, 20m, 15m and 10m bands and may be usable without a tuner on all those bands. In practice, I have found that harmonically related bands sometimes require a little “touch-up” tuning for best SWR.
The “end-effect” discussed here in a previous post creates a small problem getting an EFHW to be resonant on multiple bands. There are two easy ways to overcome this; either use a linked antenna, or separate interchangeable conductors for each band. A linked antenna comprises an overall wire length cut for the lowest band of interest, but which can be separated into resonant sections for each band joined by clips, or some other arrangement.
Unless it is intended to operate on more than band during an operating session (a portable, temporary, outdoor session) it may be more convenient to simply use the correct wire length for the band of interest. Even a linked antenna wire will have to be pulled down to change bands since the links may be out of reach from the ground.
So, in the true spirit of QRP, I put together a simplistic, basic, lightweight EFHW antenna for the 20-meter band. It comprises 31 feet of wire and a very small 49:1 transformer. My Norcal SWR/power meter showed an SWR of 1.5:1 and an output power of 3.8 watts from the radio.
Sand Grenades
I launched one end of a long length of mason’s twine tied to a small balloon filled with sand (I jokingly call it a “sand grenade”) into a tree and hauled up the far end of the wire to a height of around 20 feet. The other end of the wire was connected to the miniature 49:1 transformer (to transform the very high impedance of an EFHW down closer to 50 ohms). A few feet of RG-174 coax connected the antenna to the PFR-3 radio. I would like to replace that RG-174 with low loss RG-316 if I can find a source for a cable with 50-ohm BNC connectors on each end.
Hendricks PFR-3 Modifications
I have made a couple of minor modifications to my PFR-3. First, I don’t like the original power connector. I removed it and re-routed the wiring to an Anderson Powerpole connector closer to where the wires join the PCB. This necessitated cutting a small aperture in the metal case. I also removed the internal battery holder. It was designed for “AA” type cells none of which hold a charge long enough for my needs. Changing internally mounted batteries involves removing four tiny screws and that is certainly not something I want to do out in the field.
I built a pair of 3S1P (three cells in series) LiIon battery packs using scrounged 18650 cells. I haven’t yet installed a BMS (Battery Management System) in these packs so I monitor the voltage carefully to make sure they don’t get excessively discharged. When the charge is depleted I recharge the cells individually on a commercial LiIon charger. Eventually I will order a 3-5Ah Bioenno LiFePO4 battery to replace these packs.
Each pack fits into a rugged Hammond aluminum enclosure and the two packs side-by-side exactly match the dimensions of the PFR-3 case. The two packs are bolted together and the radio sits on top secured by industrial strength Velcro. It makes a nice, convenient package that is easy to use in a rapid deployment situation in the field.
And the Results?
This was another “Backyard-On-The-Air” trial designed primarily to check out the antenna and weed out any gremlins in the setup. I found two gremlins. First, one of the Powerpole connectors wasn’t locked into place properly and caused an intermittent power connection. That was an easy fix. The second gremlin was a little more pesky; an old audio cable with 3.5mm plugs on each end was making my keying even more erratic that it usually is. I replaced the cable with a name brand, high quality cable with gold-plated plugs and the problem went away.
Before packing everything away ready for a real field deployment I responded to a couple of POTA activators. One, in Indiana, gave me a 559 report and the other, in Missouri, gave me a 529. They were both a solid 599 at my backyard station in Southern Ontario. Mission accomplished; one of my QRP rapid deployment field rigs is ready for service.
Ham Radio Outside the Box 