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Author Topic: from mobile to portable  (Read 4676 times)

Posts: 113

« on: August 24, 2010, 12:32:15 AM »

hello, I was thinking of going in my mobile set up to a river side park area with lots of tall trees
and operating portable for like an hour or two with the engine on. i could also put up a dipole or an
end feed antenna in those tall trees. i suppose that I could also use a smaller portable battery as well
and turn my car off, of course then i would have to be QRP. please your thoughts. 

Posts: 1626

« Reply #1 on: August 24, 2010, 04:31:12 AM »

I used to do somthing similar.  I operated my IC-706MKIIG from my car during lunch before I retired.  The problem is that the car battery sags pretty quickly to about 12V, and then if you have just a bit of drop in the DC cable the voltage at the radio can be 11.5V or lower.  This can be a problem with some radios (poor linearity, or turning off).  But I just didn't like running my car for an hour just so I could operate.  So I bought a MFJ-4416 battery booster which took care of the problem.

Phil - AD5X

Posts: 18525

« Reply #2 on: August 24, 2010, 08:06:12 PM »

Sure, it is a lot of fun.  I wandered though Australia with a  HF rig in my backpack a few years ago and had
many wonderful QSOs.  A dipole or long wire working against your car frame as earth should work well.  If
you use multiple bands then the end-fed wire and an ATU has advantages.  For only a few bands a dipole
is quite adequate.  My standard dipole kit has feedline and a center insulator, plus dipole wires for 5 bands
that I can add in any combination as I choose each time I set up.  Another solution is a dipole with
jumpers that you can use to change bands.

One of the things that makes it most fun is to minimize the time it takes to put up the antenna.  I rely on
a weight (generally a local rock) and 30m of thin braided cord to throw a rope over a gum tree to hoist the
antenna.  (Once on a beach in Tassie I couldn't find any rocks, so used a sock filled with sand instead.)
Tie the other end of the rope to a plastic bucket then start feeding the rope into the bucket, just laying
the turns on top of each other.  It will come out clean in the reverse order it went in, saving a lot of
time spent untangling things.  I use an under-handed flip and can easily fling a 200g weight up to 15m or
more, but it takes practice.

And if you are on the beach, use vertical polarization for good DX.

Posts: 32


« Reply #3 on: September 12, 2010, 05:40:27 PM »

It may be "expensive", but what about a solar panel to keep the car battery or independent battery charged?  You would also need a charge controller for the solar panel.  My home set-up is a gel-cell with a solar panel.

Posts: 6166


« Reply #4 on: September 12, 2010, 08:26:11 PM »

I think maybe you could compromise your power a bit, somewhere between full and QRP if you wanted to.

An antenna in the trees would be pretty efficient, so lower power would still be effective.  What you could do is measure to see what your drop is over a given period of time at a particular power level.  For instance, if you turned the power down to 50W instead of 100W the current draw is half, but it's only a 1/2 S-unit drop in signal.  I would guess you wouldn't have to go all the way down to 5W before you found a setting where your static battery voltage dropped too far.

Mark K5LXP
Albuquerque, NM

Posts: 117


« Reply #5 on: September 14, 2010, 02:39:50 PM »
A photo of me operating portable from Death Valley. More photos here:

Our van is set up for mobile operation with a Tarheel antenna mounted in the back, and we bring a battery and run off that when we're camping. The van is directional: with the antenna in the back, it's directional along the ground plane toward the front of the van.

No trees for hoisting antennas, as you'll see. :->

Posts: 7995

« Reply #6 on: September 14, 2010, 06:28:33 PM »

I have done the same and even a small automobile battery has run a 100 watt radio for two hours with no problem (and still start the automobile).

Given a transmit duty cycle  of 50% (CW) for 30 minutes of every 120 minutes - with a peak additional transmit current draw of 20 amps - the transmit charge removed from the battery is only 2.5 amp-hours. Add to this a quiescent transceiver draw of 2 amps and the battery must supply 4.5 amp-hours of charge for 2 hours of operation.

If the voltage drops too low (for the transceiver) with the engine off, switch it on.

Posts: 298

« Reply #7 on: September 22, 2010, 10:04:22 PM »

if your car battery can't handle an hour of QRP activity you need to replace it.

I used to run my Icom 2800h in cross-band-repeat @ 50W for hours at a time. (using a legal one-way 440 -> 2m crossband)

Posts: 24

« Reply #8 on: September 30, 2010, 09:51:56 AM »

It may be "expensive", but what about a solar panel to keep the car battery or independent battery charged?  You would also need a charge controller for the solar panel.  My home set-up is a gel-cell with a solar panel.

I might be wrong, but I don't think a car-mounted solar panel would provide enough power to keep the battery charged for an hour or two of operation at 50W or more output power. If you just mean to use the solar panel to trickle-charge the battery when it's not in use, so it is fully recharged the next time he tries to start the car, that might work if it's not a cloudy day, perhaps. But, presumably, after he finishes talking at the park, he's going to want to start the car and drive back to work/home/wherever, and the car alternator will recharge the battery *anyhow* while he's driving, so what's the point of the solar panel again?
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