When I got home after my NA-034 operation, I wanted to try to understand what happened. I was sure that I'd operated solely off a car battery in the past and as long as I was connected directly to the battery I hadn't had any problems. Based on some testing that I did, I discovered that I may have been mistaken. The short version is that I discovered that by using the battery in my car without the engine running, I was able to reproduce the "strange noise in headset" that I recently wrote about, and that by running the car engine, that problem went away. There's a bit more to it though. Read on if you're interested.
I needed to have some way to measure the voltage from the battery and the amount of current


I crimped a set of the quick disconnects onto the Whattmeter and did a test with the power supply and radio in the shack to ensure that the meter was working and found that it worked perfectly. The shack power supply was putting out just over 13.8 volts with the Icom 756 Pro II drawing around 3 amps while receiving. (Interestingly, this is about 0.6A below what the ARRL reported in their testing, but I've had some repair work done on the radio and it's possible that some of the newer components draw less than the originals.) The next step was to reproduce what I'd set up while in Florida.
As luck would have it, the weather was beautiful this weekend, and as we had no plans on Sunday, I took the 706, the Buddistick, the Whattmeter, the antenna analyzer, and a length of coax outside. I set up the Buddistick on the front lawn (I just had it on the mini-tripod sitting on the lawn, though I did put the radial over a couple of plastic lawn chairs to keep it off the ground) which while not optimal for DX, took me all of 5 minutes to set up with a good match to the radio, as verified by the antenna analyzer. I connected the radio to the antenna then connected the power cables to the car battery, with the Whattmeter in-line. My thoughts were to do a few tests with the engine off, fully expecting that I wouldn't have any issues, then turn the engine on to see what kind of difference it made. I found an empty frequency on 20m and started testing. As soon as I transmitted, without looking at the meter, I knew that, to my surprise, I'd reproduced the problem: That nasty noise in the headphones was back.
What I figured I'd do was to collect data using various levels of transmit power to see the effect on the voltage and current draw. The meter itself also shows power in watts, though of course that's trivial to calculate if you already have current and voltage. (From Ohm's law, P=I×E). I quickly discovered that the car battery wasn't able to supply sufficient voltage unless I was transmitting with about 10 watts or less. The specification for the 706 MkIIG is that it requires 13.8vdc ± 15% meaning the minimum allowable voltage is 11.73vdc. With the car engine turned off, I measured 11.68v with the radio drawing 5.79A when transmitting using 10w. At 40w (the next step I measured; when I was in Florida I was able to "get away" with 40w when testing with KH6ITY), I measured 11.52v while drawing 8.23A. At 60w transmit power and up, the voltage dropped to about 11.4v but the radio was simply unable to draw the current needed.

You can see from the chart that not only couldn't the battery supply sufficient power for the transmit needs, it was only able to supply less as the radio tried to use more, presumably because the battery simply couldn't "keep up". So all my empirical testing seemed to prove that with the setup that I was using, I simply couldn't operation at full power using just a car battery with the engine running.
However, similar to bees who simply don't know that they can't fly, so they do, apparently my radio didn't know that it didn't have enough power to operate, at least during my 2004 and 2006 operations from NA-034, so it worked just fine. During those operations I know that I did not have the car running, yet I had no issues with power. The key for me know will be to find out what has changed. I am using the same power cables and the same radio (the antenna was different, but that shouldn't matter), yet something has changed. The next thing I am going to do is to see if perhaps there is a problem with the power cables that may have occurred over the years.
Although I attempted to measure the resistance in the power cables and came up with a measured 0.1 Ohms, my meter is probably fairly inaccurate at such low resistance, so I did a calculation instead assuming that I've got all 12 AWG wire in place. (Part of it is actually 10AWG, but I'm using 12 to account for any losses due to connectors and splices.) Using a 12v supply with a load of 15A (matching what I saw when the engine was running) and a length of 20 feet, the voltage drop calculator that I used shows an estimated voltage drop of around 8%. Allowing a bit of wiggle room for the length, it appears that the voltage drop would be somewhere between around 6% and 10% which corresponds to a voltage at the load (radio) of between 11.3v and 10.8v. Even the highest end of that range is too low for the radio to operate properly. Dropping the transmit power to 40w results in the voltage to the radio of about 11.5v which is a bit below spec but probably would allow operation, with some minor distortion. That seems to match what I had experienced.
The other factor that I haven't played around with much is temperature, and I'll leave that as an exercise for the reader. At this point, I think the best thing to do is to shorten the power cable as much as I reasonably can, replacing the section that is currently 12AWG wire with 10AWG wire. I suspect that will help ensure sufficient current flow while minimizing voltage drop.
I would be very interested in any feedback from anyone who can shed a bit more light on these issues, as I know that there are other factors that may come into play, such as the battery chemistry and perhaps other parts of the car's electrical system.
I started using Deans connectors when I was into RC. I did a quick search and found this link
ReplyDeletehttp://www.wsdeans.com/products/plugs/ULTRA-PLUG2.jpg
They are small and offer allot of surface area to conduct. The power a radio draws in not nearly as much as a 4wd brushless electric truck with two 7.4V LiPo's I like these connectors because they are easy to solder to, fit snug but no clips to mess with and handle plenty of amps. I use them on my radios as well as airsoft. Not cheap but very good.
AI4NW
Pat
Hi Pat -- A colleague at work who is also into RC told me about the Dean's connectors as well. They look interesting, but I suspect that if I switch to anything it would likely be to the essentially standard Anderson Powerpoles at this point.
ReplyDeleteYou seem to have had better luck with the IC-706 han I had with my FT-100D. The one I had would not operate even on RX at less than 11.3v.
ReplyDeleteSince my interests in portable operations were HF oriented, I sold the FT-100D in favor of my Elecraft K2. Besides the Elecraft radios, I know that a Tentec Scout will operate at 11v, and maybe lower.
The powerpoles are handy, but you may want to make a few adapters. It seems like something different always turns up.
The age of the battery at the time of the operation may have played a part in previous successes. A new battery seems to hold a higher volatage better under load than when they get closer to their end-of-life. i.e., A car battery that works today may not next year.
73 es gl de w4kaz
There are a couple of things on the market made to help stabilize the voltage at the higher level actually required. In my own case it made more sense to change radios rather than buy items for raising or maintaining the voltage level.