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Crazy ham radio install

I figure the hams on this forum will appreciate this.
It's that time of year again where my local club hosts it's fox hunt. Well, unfortunately it got cancelled, but not before I finished this project.

Last year, I picked up a KerberosSDR in the first run on whatever crowdfunding platform it used. It sat on a shelf until last month, when someone said "hey, let's go do this fox hunt". That was the motivation I needed to start putting the KSDR to good use.

I had to find a way to mount four equidistant antennas on the roof of my car. On a typical metal roof vehicle, this is pretty easy, just break out a tape measure and some magnetic mount antennas, easy peasy, call it a day. My Tesla has a glass roof (with sunroof, if that matters at all), making this a much more fun engineering challenge. I made some measurements of the roof rack mounting points and put in an order with an aluminum supplier. The parts took some time to come in, leaving me just 5 days to get it all put together. After a bunch of drilling and a few trips to the local hardware store, I finally completed a roof rack, which holds a square aluminum plate. I ended up opening the rear passenger window a little bit to pass the cables for the antennas into the car. I'm not sure I'll ever be able to use this thing in the rain.

IMG_20190913_172740_LI.jpg IMG_20190913_172650.jpg IMG_20190913_172712.jpg

When the array is stationary, I get good bearings off of it. In the image below, I was DFing the local NOAA weather transmitter. You can ignore the lines on the right-most side of the image, those were before my tablet realized it wasn't at home anymore. I was also able to DF aircraft and the transmitter for the tower at the local airport.

Screenshot_2019-09-13-22-15-33-823.jpg

When the car is moving, the antennas sway too much in the wind, causing the readings to be all over the place. Hopefully I can find a way to work through that problem.
 
Keep an eye on those roof rack mounting holes. They are steel and will rust. I fill mine with wheel bearing grease when the rack is not on the car.

Tie a piece of fishing line from the top of each antenna to the front bar. If they sway side to side, two pieces of fishing line from the top of each antenna to the bar it is mounted on. Use the smallest mono filament line you can find.

73
 
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The GPS in my tablet gave up after 10 minutes or so during yesterday's test. After removing all the points from when the GPS gave up, I actually got decent results.
Screenshot_2019-09-14-12-51-26-178.jpeg

I know what you mean about the rust. I had to clean the threads up a bit when I was putting this together.
I'll have to try the fishing line trick.
 
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Thanks for this. I also have a similar vintage KerberosSDR. It’s fun. I’ve not gotten many good results, but maybe that is because I’ve been putting the antennas on the wrong car. Thanks for the inspiration.
Also, I’ve been using the antenna’s configured as a linear array. I wonder if you’ve done any A B testing with different antenna configurations.

73
 
Thanks for this. I also have a similar vintage KerberosSDR. It’s fun. I’ve not gotten many good results, but maybe that is because I’ve been putting the antennas on the wrong car. Thanks for the inspiration.
Also, I’ve been using the antenna’s configured as a linear array. I wonder if you’ve done any A B testing with different antenna configurations.

73
I've never bothered with the linear array, mainly because I don't see it as practical on a vehicle, plus you don't know if the signal is coming from the front or the back. They claim the linear array is better for beginners, but I only think that is because it's easier to put antennas in a straight line than a square.

One thing that could be happening with your poor results is variations in the lengths of the cables for your antennas. Just because you buy four of the same antenna, each with a "10 ft cable", doesn't mean they're all exactly the same. The cables for the antennas I bought varied by over 6 inches, which is a big deal for applications like this. I ordered some SMA connectors for RG-58 and re-terminated all 4 cables.

Other than that, make sure your spacing factor is correct and that you re-calibrate every time you change frequency.

After some practice and playing with the settings, I'm getting decent results at 70+ MPH.
 
I was poking around the Tesla API, and this gives me ideas!
LatitudeLongitudeHeadingEpoch Time
39.163239-76.8150632321568627237
[tr][/tr]


Now I'm curious, so educate a non ham user here, what is a fox run, and what's the purpose of the antenna configurations that are discussed above?
A fox hunt is when you search for a transmitter in an unknown location. It's typically a competition to see who can find it the fastest. It's good practice for when you need to hunt down someone who is interfering with radio communications.

Edit: I forgot to answer half your question.
That antenna arrangement is used to detect the direction the signal is coming from based on the phase difference of the signal on each antenna. Basically, the signal shows up on each antenna at slightly different times.
 
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