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Author Topic: POTA with the Truck-tenna  (Read 223 times)

K2WPM

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POTA with the Truck-tenna
« on: December 12, 2022, 05:54:09 AM »

Inspired by WK4DS, I built my own Truck-tenna. https://www.davidsaylors.com/wk4ds-amatuer-radio-blog
I took the idea of (a) mounting a vertical antenna near the rear of the pickup truck, and (b) spreading a few radials outward from the back of the truck. I added a bond to the truck body. After several experiments -- radials and their length make a big difference -- I ended up with two 15-foot radials and one 30-foot radial, radiating out from the rear of a 1992 Dodge Dakota (8 foot bed). I constructed a 16-foot vertical element from aluminum poles (dumpster diving). I stripped 30 feet of big box store 14 gauge wire, and wound it on a scrap of PVC pipe. Although designed for down to 80 meters, I only used it on 20 and 40 for a POTA activation, so far.

I constructed the base loading coil from 14 gauge copper wire, wound on a nominal 1.5 inch Schedule 40 PVC form (actual O.D. I learned, is 1.9 inch), and using grommet strips as guides. I calculated 47 turns would allow me to get down to 3.5 Mhz. Grommet adds 3/16 inch to coil diameter, so actual coil diameter is 2.0875 inch. I thought the grommet strips were 8 turns per inch, but I measured them actually to be 7.83 tpi. 47 turns is about 34.7 microhenries -- good for 3.5 Mhz. I did 51 turns 'just in case.' When I measured it on the analyzer, the entire coil gave the antenna a 2.5:1 SWR dip right at 3.5 Mhz, the best I could get on 80 meters, and fine for my IC-7300 ATU. Supported by an umbrella stand (more dumpster diving).

I measured it for 40 meters and it was under 1.5:1 across the band, at about 15 turns (2 inches) of the loading coil. If I was smart, I'd guess either (a) this is a fantastic antenna, or (b) such wide bandwidth suggests lots of loss. With 1:1 form factor on 40, and 1:3 on 80, it may be a little of both.

On 20 meters, the antenna is slightly too long. SWR 2.6 to 2.9, naturally better at lower frequencies. I guess I need to add some capacitance? But still easily tuned by the IC-7300. With only 12 feet of coax, I wasn't too worried about losses at HF.

How did it work?

Dang, a had a three-hour pile-up going. 242 contacts. Okay signal reports on 20 meters. But on 40, lots of "plus 10-20 db" out to around 400 miles. I was surprised there was no directionality in the direction of the three radials; it was pretty much omnidirectional despite the lack of radial symmetry. Worked about 40 states and a little DX. P.S., No outboard tuner needed. Nice.

Next time I go back and try it with low power CW!

Next time improvements: take detailed readings of resistance and reactance at various frequencies; elevate the radials; add capacitance for 20 (and above); try a cap hat on 75/80 meters.

Thanks Dave WK4DS!

Any comments / suggestions would be much appreciated!

David, K2WPM

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WK4DS

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Re: POTA with the Truck-tenna
« Reply #1 on: December 21, 2022, 06:25:05 PM »

This is really awesome! I am really happy to see some fabrication going on. It is cool if you can take some "junk" so to speak and build a new functioning device to serve a new purpose, that is the literally definition of recycling...just cooler...

Thank you for sharing this,
I for one appreciate it.

David
WK4DS
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