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Feed line for ADS-B receiving station

Mar 1st 2017, 18:29

KK4AGH

Joined: Feb 15th 2011, 10:02
Total Topics: 0
Total Posts: 0
Hi,
I originally tried to email this question to Robert Nickels W9RAN who authored the QST article about this, but his email address listed in the article was rejected. Maybe someone else can advise me.

I'm using his instructions to build a coaxial collinear array antenna. He suggests using RG-6U coax to make the antenna elements and the feed line with F-type connectors (all inexpensive and available at Home Depot). RG-6u has 75 ohm characteristic impedance. The ADS-B dongle has an SMA type connector. The necessary adapter is Male SMA to female F-type, which is listed as 50 ohm. Is this mismatch an issue?
Mar 1st 2017, 19:50

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

https://www.microwaves101.com/calculators/872-vswr-calculator
This mismatch loss between 50 and 75 ohms is 0.18 dB. This isn't a whole lot of loss. But, you can eliminate the mismatch loss by using a quarter wave section of 61 ohm transmission line with the appropriate connectors. One technique for doing this is to use square aluminum tubing for the outer conductor and hobby store brass tubing for the inner conductor. Aluminum tubing comes in different wall thicknesses--choose the thickness that gives you the best impedance for your needs. While this seems like a lot of work, this technique can be used with other impedances to create power dividers. This can be a convenient way to connect two collinear arrays together for more gain.

http://fermi.la.asu.edu/w9cf/articles/square/index.html
Here is a derivation of the formula for calculating the impedance of a transmission line with a square shield and a round center conductor.

Zack Lau W1VT
ARRL Senior Lab Engineer


Mar 2nd 2017, 14:43

KK4AGH

Joined: Feb 15th 2011, 10:02
Total Topics: 0
Total Posts: 0
Zack - Thanks for the info. You're right, it does seem like a lot of work to pick up .18dB on a signal that's all one and zeros. But I think I'll experiment with your suggestion just because it's fascinating. Do you have any construction plans for this? How do you keep the conductors separated and supported? Should this be inserted in the line just before the antenna? Don't guess I could buy an adapter Male-F to square tube.
Mar 2nd 2017, 17:42

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
http://www.packratvhf.com/article4.htm
The reason we use square is that we can bolt a standard SMA or N connector to the side of the power divider. The brass tube is soldered to the center pins of the coax connectors. The skin effect allows you to use thin wall brass tubing with no loss in performance.

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