GPS active antenna conversion.

Discuss commercial and home made antennas.
iScan
Posts: 13
Joined: Wed Sep 09, 2015 10:11 pm

Re: GPS active antenna conversion.

Post by iScan » Mon Oct 12, 2015 6:41 am

Trev,

Surprise, these cables are fairly good.

My two dongles were set with Unitrunker to hear the Newport News, Virginia city-county Project 16 trunked radio system today. I use them like a scanner for these same 852-859 MHz frequencies most of the time. After a few hours I thought about trying to make a DIY antenna and use a cable from one of these active antennas. When I saw http://emcesd.com/tt2006/tt020106.htm this tiny test antenna for 1.8 GHz, I thought back to the 27 MHz version I cut in 1976 or so for the CB Band then. I figured if it didn't work well enough I'd only waste a foot or so of cable to making a 1/2 wave coax antenna. I cut and stripped off the outer insulator at 351 mm to the tip. Then I folded the outer braid back over the outer insulator. Next I stripped the inner shielded braid off the center conductor and threw that out. I left the dielectric and center conductor in place. The outer braid folded back to about 35.1 cm from the tip and I held the cable with needle-nose forceps about that spot. I wound 5 turns around the forceps and secured the coiled cable to itself with a cable tie to prevent the remaining coax from becoming part of this antenna.

Here are pictures of the tiny 36 cm. thing lying on the 20" top element of the planar disk antenna I made.
Image

Image

I am here looking at those two and still not believing this. They are about equally effective!

The 20" Planar disks were so good that I never made it to put them into the attic as I planned. The planar disk connects to the RTL-SDR dongle with a 50 ohm coax and converts to SMA through a pigtail adapter so there are some losses. But, STILL. To be fair I put the feed point of both about 4 feet from the floor and switched one to the other dongle and back so they each took a turn as the control antenna and as the voice antenna. I run them without any units blocked so the control sends the voice units over the whole city, fire, ambulance, police, sheriff, rangers, and all. With either antenna I hear about 2/3 or more of the unit calls. It's a large city the Virginia peninsula. (I am most interested to hear units that are near me. Being mobile they can relocate much closer than they had been when I did not hear any reception.)

It works so well that I might make another, seal them each into its own 30 centimeter length of 1/2-inch PVC and send them into the air outside the house.

trevatxtal
Posts: 6
Joined: Sat Sep 05, 2015 6:27 am

Re: GPS active antenna conversion.

Post by trevatxtal » Sun Oct 18, 2015 6:32 am

Hello Iscan
You really got me thinking there,
Your design attached to a plastic knitting needle pushed in a block of wood would make a very inexpensive and portable antenna.
Now if we can find some old TV's with cathode ray tubes we could remove the ring magnets from the tube neck, with a spot of super glue attach same to bottom of wood block and we have a antenna for a car or truck mobiles.
Trev.

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