1/4 Wave Ground Plane

[Richard Lee]

I'm experimenting with a dongle using a new decoder, and needed a better antenna.
http://forum.planefinder.net/threads/modesdeco2-and-modesmixer2-for-the-dongle.154/

So, I decided to try a 1/4 wave ground plane and feed in the signal with the large hardline on my tower.

 

[Richard Lee]

Here's the close up showing the connection to the hardline.

Tracking a lot of planes close in. So far the max range seen was 160 miles..

 

[stephenkearney]

And how does the 1/4 wave GP antenna compare to the other antennas you've tried? It should have slightly more gain than a dipole but less than a collinear.

 

[Richard Lee]

Right now, that other antenna (the DPD) is running into a 1090 Puck and feeding data to PlaneFinder.net.
I did take some max range charts on the DPD antenna, using the Dongle.
http://www.dpdproductions.com/page_vhf_air.html#adsbout
I think the DPD's average range (for planes at 40k feet) is about 200 miles in most directions.
This little GP is starting to look like it's going to reach out to 120 miles, when using a good LNA at the bottom of the hardline..

I'm currently trying to do a new plot for the GP.. Seems like it's really good close in. Pretty solid out to 50 miles.
The new decoder (modeSdecod2) program is pretty nice. It seems to process and send out displayable data, a lot faster than RTL1090..

I think having the LNA inline is a good way to make of the lack of RF signal capture area of the 1/4~ GP.
I'm going to put the 1-2Ghz filter on the LNA output to see if there is any change in signal levels.

 

[Richard Lee]

VRS fed by modeSdeco2 (fed by dongle & 1/4 wave GP) displaying RF levels 0-255. Once the signal gets below 10-15, drop-out will occur within 10-20 seconds.

 

[Richard Lee]

Checked at midnight and it's getting a little larger..

It's not going to get much larger.. However, it's still pretty impressive considering the size of the antenna..
Not sure, but it looks like it's got about 75% of the range of the factory made 9dBi collinear..

Edit:
I own a A-B coax switch that's good for L-band. So, I plan to connect both antennasto the dongle,
and use the signal readout on VRS to see the difference in signal levels at different ranges and altitudes.
That should give me a good info regarding the good and bad of each antenna.

Right now, I'm using a good LNA and 1-2GHz filter on the dongle radio..
Since 65mm is such a small capture area, I'm not sure if the filter is really needed..
It would be nice to be able to Bypass an in-line device with two A-B switches..
(AKA Change-over relays)..

 

[Richard Lee]

Scanned all day today.. (Sunday 6/29/2014) This looks to be about the max it's going to do.
Rings are 50 statute miles. Distance on plane labels are also in 'miles' from the antenna. (sig level under the miles).

 

[Richard Lee]

Not too much better after a 12 hour run today. I think this is about the max coverage..
What's that look like? About 50,000 square miles?

 

[Mark Daniels]

Performing very well in most directions too.
Is it terrain that is limiting the South East and South West?

 

[Richard Lee]

Yeah, that dead zone down below the islands (Nantucket is 100 miles from here) is likely caused by a nearby hill
It's about 200 feet higher than me and only about 2 miles away. I guess it casts a long tall shadow..

The area around Worcester, to the SW is quite elevated.
Around 600 feet and going up to 1,000 feet, north of Worcester.
I'm amazed that I can reach into central NY state. Between me and Albany NY, there are 1,000 & 2,000 foot hills..

The antenna isn't installed in a very optimum location. It's got a clean shot to the north,
but I can't understand why it would work towards the South at all. All kinds of metal only inches away from it.

Getting high wind and thunderstorms here tonight. Looks like a tiny hurricane coming up the coast.
If the DPD goes down, I'll have the little 1/4 wave ground plane for back-up..

 

[Richard Lee]

We had a pretty good storm here yesterday, but everything stayed on the tower..
http://www.myfoxboston.com/story/25...ts-clean-up-after-storm-leaves-path-of-damage
Damage shown in the video was only about 2 miles from here..

All antennas are still working perfectly.. I don't think there was much chance of the 1/4 wave GP blowing down..
(Unless the whole tower came down)!

 

[Richard Lee]

I've replaced this antenna with a new full-wave antenna. http://forum.planefinder.net/threads/very-small-collinear-antenna-300-mm.177/
This morning I measured the return loss at -13 dB. I think the SWR would be written as 1.56 : 1

This was built before I got the test gear needed to measure return loss.
It's likely that I could have obtained a better return loss, had I been able to tune it during fabrication.