Article Body


Manager - AB7RG
Manager Notes

Back Seat Shack -- Going Portable

Created by Rick Butz on 2023-05-30


Back Seat Shack -- Going Portable


Going portable to do a POTA or activate a grid is much easier and cheaper than you probably think!  If you’ve never done it, you should get out of the shack and give it a try.  It’s an experience every ham should have.  Warning: you may get hooked!


Sadly, my days of portable operations are over and done.  But I still recommend “car portable” for an easy start.  Yes, picnic benches, wire antennas in trees, and plastic sheeting to protect the rig from rain can be lots of fun.  But for simplicity, convenience, and all-around ease of operation, the Back Seat Shack can’t be beat!  Caution - please do a close-to-home shakedown cruise before you launch into the wilderness.  You’ll be glad you did.  The little gremlins are easier fixed at home than miles away.


Since you are on-air, you already have the most expensive items for car portable.  Most current HF rigs run on 13.8 volts DC so your current home station transceiver will most likely be suitable (provided it isn’t a boat anchor!).  If you use a laptop for rig control or logging, or are into digital modes, you’re all set in the “big equipment”.  If you don’t use a computer…well, you are all set too!


Assuming you have the “big equipment,” all that’s left is a suitable antenna setup and a power source.  Let’s talk about antennas first.  The quickest, easiest, and cheapest answer is a hamstick on a mag mount in the center of your vehicle roof.  You’re not going to be driving around with this setup on the roof, so you don’t need guy lines or to put out radials.  I’ve never been confronted by anyone about permissions or permits and doubt you will be - unless you’re parked in a No Parking Zone.  You are totally self-contained and there is nothing to cause concern. 


Is a hamstick the equivalent of a full-size wire dipole up in the trees?  No, but it will do a great job on 40m or up where most of our activity happens.  I have used this setup to work Europe, Alaska, Hawaii, South America, and even Oceana from all over the CONUS - all with 35W PSK.  If the bands are open, USA, Mexico, and Canada are a breeze in all modes.


Positioning is important with a mag mount antenna.  Center of the roof is where it should be.  That gets max metal centered under your antenna for a counterpoise and gives the best omnidirectional pattern.  Don’t worry about all the grounding and bonding of body parts you hear about for mobile ops.  It will work just fine without all that.  And since you are parked with the engine off, you aren’t generating QRM from your car’s systems.


I use a 5” Hustler MBM mag mount (about $35 these days) and the hamsticks screw directly into it.  As an aside, Lakeview Co. originally made HamSticks but they are no longer in productions.  The equivalent is a MFJ HamTenna (MFJ-16xxT HF Stick where the xx equals the band) and while some don’t like MFJ products, I have never had a problem with their HamTennas.  And most everyone still calls the HamTenna a hamstick!


One issue that will have to be addressed is Common Mode Current on your coax.  Since the HamTenna is an unbalanced antenna, it will try to use the coax for the other side.  The easy solution is a 1:1 UnUn.  This can take the form of a home brew “Ugly Balun” (turns of coax on an air core – lots of plans on the web) but I chose to buy a commercial choke from Balun Designs – Model 1110 – about $65.  It is good to 300 watts.  I cut the mag mount coax at about 12” and attached a PL259.  That lets me put the choke on the roof right next to the antenna and then run a short coax into the back seat of the car.  And the choke solved my RF into the computer problem as well!


I carry a 40m, a 20m, and a 15m HamTenna for my portable ops. With those 3 bands, I can always find activity.  Tuning a HamTenna can be a bit tricky.  Tune with it in place on the roof, with the choke in line.  It is easiest with an antenna analyzer like an MFJ 259b, or a Rig Expert.  But the smart money these days use one of the new NanoVNAs – much, much cheaper than the old antenna analyzers – and a must for every ham!  You tune the HamTenna by lengthening or shortening the whip portion.  Remember: “to lower the frequency, lengthen; to raise the frequency, shorten.”  I had trouble getting the 40m stick tuned in the low end of the band.  The whip wasn’t long enough.  Since I wanted it resonate (so to not need an antenna tuner), I soldered a piece of solid copper wire to an alligator clip, attached the clip to the end of the whip, and trimmed it to resonance.  I could have used my rig’s ATU, but wanted to radiate as much RF as I could.


So, we have the “big equipment” and the antenna system in place.  We only need a power source.  The obvious answer is to use the car battery.  But there are some issues doing that.  First, when parked and operating, you probably won’t be running the engine.  That creates the potential to run the battery down and being unable to start the car when you are ready to go.  That’s not a desirable situation when you may be miles from help or a jump to get you started.  There are commercially available battery isolators that solve that problem.  I chose a different approach that overcame another issue in powering from the car battery. 


The issue is your HF rig will probably (at least some of the time) draw more amperage than your car wiring is designed to handle.  That means either installing direct wiring from the battery (WITH PROPER FUSING!) or replacing fuses with higher amp fuses.  DON’T DO THAT!  Fuses are designed to protect you and your car.  If you replace a fuse with higher amp fuse, you risk an electrical fire in your car’s wiring!


My answer was to use a separate battery for the radio.  Initially I used a deep cycle marine battery set in the rear seat passenger’s footwell.  They are heavy, but since you are “car portable” who cares?  The downside was that they only lasted about 3 years before they were junk.  Then I heard about Lithium Iron Phosphate batteries.  I bought a Bioenno Power 12Ah battery (about $125 as of this writing) and it will run my Elecraft K3 at 35W for about 7 hours 50-50 cycle before it needs recharging.  These batteries require a special charger, also available from Bioenno (about $25).  Note:  Bioenno Power has a handy sizing chart for Ham Radio on their FAQ page.


Interestingly, I found my limiting factor for off-the-grid portable was my laptop battery.  Some of the contributing factors were the need to run the display all the way bright because of sunlight and using WiFi and internet.  A screen shade, turning off WiFi and going without internet helped, but the laptop battery was always my limiting factor.  There are laptop power supplies designed to run off 12V.  But the truth is I generally burned out about the same time as the battery went south!  So I never made the investment.


I found a couple of other items that made my back seat ham shack better – a 9” folding step stools for putting the mag mount on and off the roof and a lap desk.  Amazon has a ton of options for both.


Going car portable is easy, inexpensive, and you probably have most of what you’ll need.  Give it a go!  You’ll be glad you did! 73,


Rick – N7WE



Back Seat Shack -- Going Portable

Excellent write up. Here in Virginia it can be sweltering hot or uncomforting cold throughout the year. I found in vehicle set up to be the answer to continue POTA/Portable operations. Everything you mentioned is the same I use. I use a tablet for my logging and my cell phone for the internet connection. They tend to last as long as the Bioenno battery. I use the shark sticks (40/20/17/10). I've picked up Europe, Africa, South America and Alaska. The sticks work amazingly well. The hardest part is getting out of the car in good weather :) Fortunately/Unfortunately Virginia Parks do not like external set ups or wires in a tree/tent stakes in the ground. (with exceptions of a few State Parks). In the car activation is a great solution.

Eric, KN4UDL